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

Abstract

The present invention relates to a polyvinyl alcohol resin that has a block character (A) of less than 0.4 and, as a 0.1 weight% aqueous solution, an absorbance (B) of at least 0.2 at 320 nm on an ultraviolet absorption spectrum.

Description

Technical Field

[0001] The present invention relates to a polyvinyl alcohol-based resin, and more particularly, to a polyvinyl alcohol-based resin suitable as a dispersant used when suspension-polymerizing a vinyl-based compound during the production of polyvinyl chloride, a method for producing the polyvinyl alcohol-based resin, a dispersant, and a dispersant for suspension polymerization.

Background Art

[0002] A polyvinyl alcohol-based resin (hereinafter, "polyvinyl alcohol" may be abbreviated as "PVA") is obtained by saponifying a polymer obtained by polymerizing a vinyl ester-based monomer such as vinyl acetate, and has a vinyl alcohol structural unit corresponding to the degree of saponification and a vinyl ester structural unit remaining without being saponified. Further, the PVA-based resin undergoes dehydration and deacetic acid by heat treatment, and has a structure having a double bond in the main chain. The PVA-based resin having such a structure is used in applications such as a suspension dispersion stabilizer and a water retention material during the production of polyvinyl chloride. It is also known that the strength can be improved by heat-treating a film or fiber made of a PVA-based resin.

[0003] On the other hand, various heat-treated PVA-based resins have been studied as dispersants for suspension polymerization when producing polyvinyl chloride. For example, a polyvinyl alcohol-based resin having a carbonyl group in the molecule and containing a salt or hydroxide of a divalent to trivalent metal has been proposed (see, for example, Patent Document 1). Further, a PVA-based polymer having an absorbance (a) at 280 nm based on the ultraviolet absorption spectrum of a 0.1 wt% aqueous solution greater than 0.1, an absorbance (b) at 320 nm based on the ultraviolet absorption spectrum of the same aqueous solution of 0.03 or more, an absorbance (b) / absorbance (a) less than 0.3, and a block character of the remaining acetic acid group of 0.4 or more has been proposed (see, for example, Patent Document 2).

[0004] The double bonds in the PVA-based resin act as the starting point for adsorption to vinyl chloride monomers and subsequent graft reactions during the suspension polymerization of vinyl chloride monomers. Therefore, it is generally known that the more such double bonds there are, the better the polymerization stability.

[0005] However, in order to obtain a PVA-based resin having double bonds, as described in Patent Documents 1 and 2, it is necessary to perform heat treatment at about 150°C for 5 to 6 hours, which has the problem of increasing the production cost.

[0006] In order to solve the above problems, it has been proposed to perform heat treatment using a twin-screw extruder (for example, Patent Document 3). The PVA-based resin described in Patent Document 3 is a polyvinyl alcohol-based resin having a carbonyl group in the molecule and a block character of the residual fatty acid ester group of 0.5 or more, and the absorbances at 215 nm, 280 nm, and 320 nm according to the ultraviolet absorption spectrum of a 0.1 wt% aqueous solution of the polyvinyl alcohol resin are each 0.1 or more, and the ratio of the absorbance at 320 nm to the absorbance at 280 nm is 0.3 or more.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0008] However, the above PVA-based resin is obtained by subjecting it to strong heat treatment in order to improve the polymerization stability. In the PVA-based resin obtained by strong heat treatment, the block character of the PVA-based resin becomes high, resulting in a decrease in dispersibility. When the above PVA-based resin is used as a dispersant for suspension polymerization, there is a problem that it is difficult to obtain a porous vinyl chloride resin.

[0009] Therefore, an object of the present invention is to provide a PVA-based resin having a high content of double bonds in the resin and excellent dispersibility in order to enhance the polymerization stability during the polymerization of a suspension polymer (for example, polyvinyl chloride), and to provide a dispersant using the PVA-based resin and a dispersant for suspension polymerization used in the production of polyvinyl chloride.

Means for Solving the Problems

[0010] As a result of intensive studies to solve the above problems, the present inventors have found that a PVA-based resin having a high content of double bonds in the resin and a small block character value can be obtained, and have completed the present invention.

[0011] That is, the present invention relates to the following [1] to [5]. [1] A polyvinyl alcohol-based resin having a block character (A) of less than 0.4 and an absorbance (B) at 320 nm in the ultraviolet absorption spectrum when the polyvinyl alcohol-based resin is made into a 0.1 wt% aqueous solution of 0.2 or more. [2] The polyvinyl alcohol-based resin according to [1], wherein the ratio (B / A) of the absorbance (B) at 320 nm in the ultraviolet absorption spectrum when the polyvinyl alcohol-based resin is made into a 0.1 wt% aqueous solution to the block character (A) is 0.6 or more. [3] A dispersant comprising the polyvinyl alcohol-based resin according to [1] or [2]. [4] A dispersant for suspension polymerization comprising the polyvinyl alcohol-based resin according to [1] or [2]. The method for producing the polyvinyl alcohol-based resin according to [1] or [2] above, which includes a step of polymerizing a monomer composition containing a vinyl ester-based monomer while introducing a gas containing oxygen to obtain a vinyl ester-based polymer.

Advantages of the Invention

[0012] According to the present invention, a PVA-based resin having a high content of double bonds in the resin and a small block character value can be obtained. Therefore, when such a PVA-based resin is used as a dispersant for suspension polymerization, it is presumed that the polymerization stability of the suspension polymer (for example, polyvinyl chloride) can be improved, and the dispersion stability can also be improved.

Embodiments for Carrying Out the Invention

[0013] Hereinafter, the polyvinyl alcohol-based resin of the present invention will be described in detail, but these are examples of preferred embodiments and are not limited to these contents. In this specification, the ratio on a weight basis (percentage, parts, etc.) is the same as the ratio on a mass basis (percentage, parts, etc.).

[0014] [Polyvinyl Alcohol-Based Resin] The polyvinyl alcohol-based resin (hereinafter referred to as PVA-based resin) of the present invention has a block character (A) of less than 0.4, and an absorbance (B) at 320 nm in the ultraviolet absorption spectrum when the polyvinyl alcohol-based resin is made into a 0.1 wt% aqueous solution is 0.2 or more.

[0015] The PVA-based resin of the present invention has a block character (A) of less than 0.4, preferably 0.25 or more and less than 0.4, more preferably 0.3 or more and less than 0.4, and even more preferably 0.35 or more and less than 0.4. When the block character (A) is less than 0.4, the dispersion stability of the PVA-based resin is improved. If the value of the block character (A) is too large, when the PVA-based resin is used as a dispersant for suspension polymerization, the plasticizer absorbability of the vinyl-based resin obtained by suspension polymerization tends to decrease or the particle size distribution tends to become wider. If the value of (A) is too small, the polymerization stability tends to deteriorate.

[0016] Such a block character (A) is determined by using sodium 3-(trimethylsilyl)-2,2,3,3-d4-propionate as an internal standard substance. 13 It is determined from the absorption intensity ratio of the absorption based on the methylene carbon moiety found in the range of 38 to 49 ppm in 13C-NMR measurement [(absorption of (OH,OH) dyad = 43.5 to 46 ppm, absorption of (OH,OR) dyad = 41.0 to 43.5 ppm, absorption of (OR,OR) dyad = 38 to 40.5 ppm, where R represents an acetyl group (CH3CO-).], and is a value calculated from the following formula.

[0017] Block character (A) = (OH,OR) / 2(OH)(OR) (However, (OH,OR), (OH), and (OR) are all calculated in mole fractions. Also, (OH) is 13 The degree of saponification (mole fraction) calculated by the integration ratio of 13C-NMR. For example, when vinyl acetate is used as the vinyl fatty acid, (OR) represents the mole fraction of the acetoxy group at that time.) The block character indicates the degree of the average chain length of fatty acid ester units in the polyvinyl alcohol-based resin. The larger the value, the shorter the average chain length of the remaining fatty acid ester blocks (the higher the randomness of the fatty acid ester units). Regarding the block character and its measurement method, it is described in detail in Poval (Publisher: Kobunshi Kankokai, 1984) and Macromolecules, 10, 532 (1977).

[0018] When the PVA-based resin of the present invention is made into a 0.1 wt% aqueous solution, the absorbance (B) at 320 nm in the ultraviolet absorption spectrum is 0.2 or more, preferably 0.25 or more, and more preferably 0.30 or more. When the value of such absorbance (B) is equal to or higher than the above lower limit value, the amount of double bonds in the PVA-based resin tends to be sufficient, and the polymerization stability improves when the PVA-based resin is used as a dispersant for suspension polymerization. The upper limit is not particularly limited, but it is about 1.5 from the viewpoint of manufacturability. When the value of the absorbance (B) is too small, the formation of double bonds in the PVA-based resin is small, so when the PVA-based resin is used as various dispersants, the surface activity tends to decrease. Also, when the value of the absorbance (B) is too large, when the PVA-based resin is used as a dispersant for suspension polymerization, the average particle size of polyvinyl chloride or the like obtained becomes too small, and the handleability tends to deteriorate.

[0019] In the present invention, the ratio (B / A) of the absorbance (B) at 320 nm in the ultraviolet absorption spectrum when the PVA-based resin of the present invention is made into a 0.1 wt% aqueous solution to the block character (A) is preferably 0.5 or more, more preferably 0.6 or more. When (B / A) is equal to or higher than the above lower limit value, it is preferable because the polymerization stability improves when used as a dispersant for suspension polymerization. When the above ratio (B / A) is too small, when used as a dispersant for suspension polymerization of vinyl chloride or the like, the surface activity becomes low and the suspension polymerization stability tends to decrease. Also, the upper limit is not particularly limited, but it is about 3 from the viewpoint of productivity.

[0020] Such absorbance is a value obtained by measuring the absorbance of a 0.1 wt% aqueous solution of the PVA-based resin using an ultraviolet-visible near-infrared spectrophotometer (for example, "V-560" (trade name) manufactured by JASCO Corporation). The absorbance is measured using a sample cell (cell) with an optical path length of 1 cm.

[0021] Generally, a PVA-based resin is a resin obtained by saponifying a homopolymer of a vinyl ester-based monomer or a copolymer of a vinyl ester-based monomer and another monomer (hereinafter, these may be referred to as "vinyl ester-based polymers") using an alkali catalyst or the like.

[0022] The saponification degree of the PVA-based resin of the present invention is preferably 60 mol% or more, more preferably 65 to 98 mol%, still more preferably 67 to 90 mol%, even more preferably 69 to 88 mol%, and particularly preferably 70 to 82 mol%. Since the PVA-based resin of the present invention has an acetic acid group (hydrophobic group) in addition to a hydroxyl group (hydrophilic group) in the molecule, it has surface activity and can be uniformly dispersed in a dispersion medium. If the saponification degree is too low, the water dispersibility tends to decrease, so the saponification degree is preferably 60 mol% or more. The saponification degree is a value measured in accordance with JIS K 6726:1994.

[0023] The average degree of polymerization of the PVA-based resin of the present invention is preferably 100 to 4000, more preferably 200 to 3000, and particularly preferably 200 to 2000. If the average degree of polymerization is too low, the surface activity tends to be low, and when used as a dispersant for vinyl chloride suspension polymerization, aggregation is likely to occur during suspension polymerization. On the contrary, if the average degree of polymerization is too high, the viscosity of the PVA-based resin aqueous solution increases, and the handleability tends to decrease. The average degree of polymerization can be measured in accordance with JIS K 6726:1994.

[0024] During production, for example, a PVA-based resin heat-treated to have a sufficient amount of double bonds in the resin is likely to undergo yellowing, and when such a PVA-based resin is used as a dispersant for suspension polymerization, the hue of a resin such as vinyl chloride obtained may deteriorate. On the other hand, when obtaining the PVA-based resin of the present invention, since the content of double bonds can be made relatively large without undergoing heat treatment, the PVA-based resin of the present invention is likely to have a relatively small YI value and is excellent in suppressing yellowing.

[0025] [Method for producing polyvinyl alcohol-based resin] As described above, the PVA-based resin of the present invention has a block character (A) of less than 0.4, and an absorbance (B) at 320 nm in the ultraviolet absorption spectrum when the PVA-based resin is made into a 0.1 wt% aqueous solution is 0.2 or more.

[0026] First, a method for obtaining a PVA-based resin having a block character (A) of less than 0.4 and an absorbance (B) at 320 nm in the ultraviolet absorption spectrum of 0.2 or more when made into a 0.1 wt% aqueous solution will be described. Examples of such methods include the following methods (i) to (iv). (i) A method of radically polymerizing a monomer composition containing a vinyl ester-based monomer under conditions containing oxygen and subjecting the obtained vinyl ester-based polymer to a saponification reaction. (ii) A method of radically polymerizing a monomer composition containing a vinyl ester-based monomer in the coexistence of oxygen and aldehyde and subjecting the obtained vinyl ester-based polymer to a saponification reaction. (iii) A method of reacetoxylating a vinyl alcohol-based resin having a double bond and subjecting the obtained vinyl ester-based polymer to a saponification reaction. (iv) A method of radically polymerizing a monomer composition containing a vinyl ester-based monomer in the coexistence of formaldehyde and subjecting the obtained vinyl ester-based polymer to a saponification reaction. Among them, from the viewpoint of productivity, the method (i) or (ii) is preferable.

[0027] Hereinafter, taking the method (i) as an example, the method for producing the PVA-based resin of the present invention will be described.

[0028] The monomer composition as the starting material contains vinyl ester monomers. Examples of the vinyl ester monomers include vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl caprate, vinyl laurate, vinyl palmitate, vinyl stearate, and other linear or branched saturated fatty acid vinyl esters. From a practical perspective, it is preferable to use vinyl acetate as the vinyl ester monomer. For example, it is preferable to use vinyl acetate alone or in combination with a fatty acid vinyl ester compound other than vinyl acetate.

[0029] There are no particular restrictions on polymerizing the monomer composition containing the vinyl ester monomer, and known polymerization methods can be arbitrarily used. For example, solution polymerization using an alcohol having 1 to 3 carbon atoms such as methanol, ethanol, or isopropyl alcohol as a solvent is carried out. Of course, bulk polymerization, emulsion polymerization, and suspension polymerization are also possible. In such solution polymerization, any means such as divided charging or batch charging may be used for charging the vinyl ester monomer. The polymerization reaction is carried out using a known radical polymerization catalyst such as azobisisobutyronitrile, acetyl peroxide, benzoyl peroxide, lauroyl peroxide, azobisdimethylvaleronitrile, or azobismethoxyvaleronitrile. Also, the polymerization reaction temperature is selected from the range of about 40°C to the boiling point.

[0030] The step of polymerizing a monomer composition containing such a vinyl ester monomer to obtain a vinyl ester polymer is preferably carried out under conditions containing oxygen. Specifically, it is preferable to polymerize a monomer composition containing a vinyl ester monomer while introducing a gas containing oxygen or after introducing it before polymerization to obtain a vinyl ester polymer, and it is more preferable to polymerize a monomer composition containing a vinyl ester monomer while introducing a gas containing oxygen to obtain a vinyl ester polymer. The method of introducing the gas containing oxygen is not particularly limited, but for example, introduction by blowing (bubbling) is preferable. By carrying out the polymerization while introducing a gas containing oxygen or after introducing it before polymerization, it is easy to obtain a PVA-based resin having a relatively large double bond content without undergoing heat treatment. This is presumably because the polymerization growth end of the PVA-based resin reacts with oxygen to form an aldehyde group, and then double bond introduction by a deacetic acid reaction occurs.

[0031] That is, by carrying out the polymerization in such a manner, a PVA-based resin can be obtained in which the absorbance (B) is relatively large and the value of the block character (A) is kept relatively small. Furthermore, since the double bond content can be made relatively large without undergoing heat treatment, yellowing of the obtained PVA-based resin and the resin obtained when the PVA-based resin is used as a dispersant for suspension polymerization also hardly occurs. Therefore, according to the method for producing a PVA-based resin of the present invention, a PVA-based resin excellent in polymerization stability, dispersion stability, and yellowing suppression can be obtained when used as a dispersant for suspension polymerization.

[0032] The method of introducing the gas containing oxygen can be arbitrarily selected, but it is preferable to introduce a gas diluted with an inert gas such as nitrogen, argon, or helium so that the oxygen concentration becomes 1% by mass to 9% by mass. If the oxygen concentration is less than 1% by mass, a sufficient amount of oxygen is not introduced into the reaction field, and it is likely to be difficult to obtain PVA having the desired formyl terminal. Also, if it exceeds 9% by mass, the explosive limit oxygen concentration (9 to 10% by mass) of vinyl acetate is reached or exceeded, so safety concerns are likely to arise.

[0033] The method of introducing a gas containing oxygen into the reaction system can be arbitrarily selected. However, a method of performing polymerization using the polymerization solution directly bubbled into the polymerization solution or a method of performing polymerization while directly bubbling into the polymerization solution can increase the contact area between the reaction system and oxygen, so the introduction efficiency is good.

[0034] The amount of oxygen to be introduced can be arbitrarily selected. However, the supply amount of oxygen per minute with respect to the monomer amount is preferably 25 mL or less. If it exceeds 25 mL, it is not preferable in terms of productivity. Also, if it is 0.1 mL or less, the reaction efficiency tends to deteriorate, so the supply amount of oxygen is preferably 0.1 mL or more. When introducing a gas containing oxygen before polymerization, depending on the size of the production equipment and the gas flow rate, the introduction time of the gas containing oxygen is preferably 0 hours to 3 hours, and more preferably 15 minutes to 1 hour.

[0035] In the polymerization of the monomer composition, it is preferable to use a chain transfer agent. Examples of alcohols as the chain transfer agent include ethanol, methanol, 1-propanol, etc. Examples of aldehydes include formaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, benzaldehyde, etc. Examples of ketones include acetone, methyl ethyl ketone, hexanone, cyclohexanone, etc. These may be used alone or in combination of two or more. Among them, it is preferable to use alcohols and / or aldehydes because the structure after polymerization is similar to that of the final product. Particularly, methanol and acetaldehyde are preferable.

[0036] The amount of the chain transfer agent added may vary slightly depending on the chain transfer constant of the chain transfer agent to be added, the degree of polymerization of the target PVA-based resin, etc., but it can be added in any amount. The amount of the chain transfer agent added is usually preferably 0.1 to 200% by weight, more preferably 0.5 to 150% by weight, still more preferably 1.0 to 130% by weight, and particularly preferably 1.3 to 100% by weight based on the vinyl ester monomer. Also, the charging method of the chain transfer agent may be an initial batch charging or may be charged during the polymerization reaction. By charging the chain transfer agent by any method, the molecular weight distribution of the PVA-based resin can be controlled.

[0037] As the monomer composition, a vinyl ester monomer may be used alone, but if necessary, it may be combined with a vinyl ester monomer and a monomer copolymerizable with the vinyl ester monomer. That is, the PVA-based resin of the present invention may be a modified PVA-based resin obtained by using a vinyl ester-based polymer obtained by copolymerizing a vinyl ester monomer and a monomer copolymerizable with the vinyl ester monomer. Examples of the monomer copolymerizable with the vinyl ester monomer include monomers having a vinyl group and an epoxy group such as glycidyl (meth)acrylate, glycidyl (meth)allyl ether, 3,4-epoxycyclohexyl (meth)acrylate, and allyl glycidyl ether; monomers having two or more allyl groups such as triallyloxyethylene, diallyl maleate, triallyl cyanurate, triallyl isocyanurate, tetraallyloxyethane, and diallyl phthalate; allyl ester monomers such as allyl acetate, vinyl acetoacetate, allyl acetoacetate, and diallyl acetoacetate; acetoacetoxyalkyl (meth)acrylates such as acetoacetoxyethyl (meth)acrylate and acetoacetoxypropyl (meth)acrylate; acetoacetoxyalkyl crotonates such as acetoacetoxyethyl crotonate and acetoacetoxypropyl crotonate; 2-cyanoacetoacetoxyethyl (meth)acrylate; divinylbenzene; alkylene glycol (meth)acrylates such as ethylene glycol di(meth)acrylate, 1,2-propylene glycol di(meth)acrylate, 1,3-propylene glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, and neopentyl glycol di(meth)acrylate; trimethylolpropane tri(meth)acrylate; allyl (meth)acrylate; hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate (the alkyl moiety is a C1-C10 alkyl group, preferably a C1-C6 alkyl group).); nitrile monomers such as (meth)acrylonitrile; styrene monomers such as styrene and α-methylstyrene; olefins such as ethylene, propylene, 1-butene, and isobutene; halogenated olefins such as vinyl chloride, vinylidene chloride, vinyl fluoride, and vinylidene fluoride; olefin monomers such as ethylenesulfonic acid; diene monomers such as 1,3-butadiene, 2-methylbutadiene, 1,3- or 2,3-dimethyl-1,3-butadiene, and 2-chloro-1,3-butadiene; hydroxy group-containing α-olefins such as 3-buten-1-ol, 4-penten-1-ol, 5-hexene-1,2-diol, and glycerin monoallyl ether, and derivatives thereof such as their acylates; hydroxymethylvinylidene diacetates such as 1,3-diacetoxy-2-methylenepropane, 1,3-dipropionyloxy-2-methylenepropane, and 1,3-dibutyryloxy-2-methylenepropane; unsaturated acids such as itaconic acid, maleic acid, and acrylic acid, their salts or mono- or dialkyl esters; nitriles such as acrylonitrile, amides such as methacrylamide and diacetoneacrylamide, olefin sulfonic acids such as ethylenesulfonic acid, allylsulfonic acid, methallylsulfonic acid, and AMPS (2-acrylamido-2-methylpropanesulfonic acid) or their salts and other compounds, vinyltriethoxysilane, vinyltrimethoxysilane, vinyltripropoxysilane, vinyltributoxysilane, vinylmethyldimethoxysilane, vinylmethyldiethoxysilane, and other vinylalkyldialkoxysilanes; γ-(meth)acryloxypropyltrialkoxysilanes such as γ-(meth)acryloxypropyltrimethoxysilane and γ-(meth)acryloxypropyltriethoxysilane; γ-(meth)acryloxypropylalkyldialkoxysilanes such as γ-(meth)acryloxypropylmethyldimethoxysilane and γ-(meth)acryloxypropylmethyldiethoxysilane; vinyltris(β-methoxyethoxy)silane, and hydroxymethylvinylidene diacetate. Specific examples of hydroxymethylvinylidene diacetate include 1,3-diacetoxy-2-methylenepropane, 1,3-dipropionyloxy-2-methylenepropane, 1,3-dibutyryloxy-2-methylenepropane, and the like. Also included are compounds having diols such as 3,4-dihydroxy-1-butene, 3,4-diasyloxy-1-butene, 3-asyloxy-4-hydroxy-1-butene, 4-asyloxy-3-hydroxy-1-butene, 3,4-diasyloxy-2-methyl-1-butene, 4,5-dihydroxy-1-pentene, 4,5-diasyloxy-1-pentene, 4,5-dihydroxy-3-methyl-1-pentene, 4,5-diasyloxy-3-methyl-1-pentene, 5,6-dihydroxy-1-hexene, 5,6-diasyloxy-1-hexene, glycerin monoallyl ether, 2,3-diacetoxy-1-allyloxypropane, 2-acetoxy-1-allyloxy-3-hydroxypropane, 3-acetoxy-1-allyloxy-2-hydroxypropane, glycerin monovinyl ether, glycerin monoisopropenyl ether, vinyl ethylene carbonate, 2,2-dimethyl-4-vinyl-1,3-dioxolane, and the like. These monomers may be used alone or in combination of two or more.

[0038] Note that “(meth)acrylate” means “acrylate and / or methacrylate”, and the same applies to “(meth)allyl” and “(meth)acrylo”.

[0039] The content of the monomer polymerizable with the vinyl ester monomer is preferably 20 mol% or less, more preferably 10 mol% or less in the monomer composition.

[0040] Saponification can be carried out by a known method, usually by dissolving a vinyl ester polymer in an alcohol and an ester and performing the reaction in the presence of an alkali catalyst or an acid catalyst. Examples of the alcohol include alcohols having 1 to 6 carbon atoms such as methanol, ethanol, and butanol. Examples of the ester include esters having 3 to 8 carbon atoms such as methyl acetate, ethyl acetate, and butyl acetate. The alcohol and the ester used during saponification can be used in any combination, but it is preferable to use methanol and methyl acetate from the viewpoint of productivity.

[0041] From the viewpoint of the dissolution rate, the concentration of the vinyl ester polymer in the alcohol and the ester is preferably selected from the range of 1 to 70% by weight.

[0042] As the alkali catalyst, for example, alkali metal hydroxides and alcoholates such as sodium hydroxide, potassium hydroxide, sodium methylate, sodium ethylate, and potassium methylate can be used. As the acid catalyst, for example, aqueous solutions of inorganic acids such as hydrochloric acid and sulfuric acid, and organic acids such as p-toluenesulfonic acid can be used. The amount of such a catalyst used is preferably 1 to 100 millimole equivalents based on the vinyl ester monomer, more preferably 1 to 40 millimole equivalents, and still more preferably 1 to 30 millimole equivalents. If the amount of the catalyst used is too small, it tends to be difficult to proceed with saponification to the desired degree of saponification. Also, if the amount of the catalyst used is too large, it is not preferable because it is difficult to observe an improvement in the reactivity of saponification.

[0043] The reaction temperature during saponification is not particularly limited, but for example, 10 to 70°C is preferable, and more preferably, it is selected from the range of 20 to 50°C.

[0044] The PVA-based resin of the present invention may be a modified PVA-based resin obtained by post-modifying the obtained PVA-based resin. Examples of methods for producing a modified PVA-based resin by post-modification include, for example, methods of acetacetic acid esterifying, acetalizing, urethanizing, etherifying, grafting, phosphoric acid esterifying, oxyalkylenating the PVA-based resin, and the like.

[0045] As described above, the PVA-based resin obtained by saponification is then dried, and the PVA-based resin obtained by such saponification may contain at least one of salts and hydroxides of divalent to trivalent metals.

[0046] Examples of divalent to trivalent metals include magnesium, calcium, zinc, aluminum, and the like. Specific examples of salts or hydroxides of these metals include, for example, magnesium acetate tetrahydrate, calcium acetate, calcium propionate, magnesium butyrate, magnesium carbonate, magnesium hydroxide, zinc acetate, aluminum hydroxide, and the like. One kind may be used alone, or two or more kinds may be used in combination. Among them, magnesium acetate tetrahydrate and calcium acetate are preferable in that they are soluble in water and / or methanol and are easy to handle industrially.

[0047] The method of containing salts and / or hydroxides of divalent to trivalent metals is not limited. For example, the above compounds may be directly added to the paste before saponification or the slurry after saponification. Preferably, it is dissolved in an alcohol such as methanol, ethanol, propanol, or water to form a solution with a concentration of about 3 to 15% by weight and added to the slurry of the PVA-based resin after saponification and distributed to the PVA-based resin.

[0048] The PVA-based resin obtained as described above is dried after saponification to obtain a powdery PVA-based resin. Examples of drying methods include vacuum drying, normal pressure drying, hot air drying, and the like. Such drying time is usually 10 minutes to 20 hours, preferably 1 hour to 15 hours, and the drying temperature is usually 40 to 140 °C, more preferably 40 to 120 °C, and particularly preferably 50 °C or higher and lower than 100 °C.

[0049] [Use] The PVA-based resin of the present invention obtained as described above is excellent in hue because coloring (yellowing) is suppressed, and can be suitably used for various applications. Examples of the applications of the PVA-based resin of the present invention include the following. (1) Related to molded articles: fibers, films, sheets, pipes, tubes, leak-proof membranes, temporary membranes, for chemical lace, water-soluble fibers, etc. (2) Related to adhesives: adhesives for wood, paper, aluminum foil, plastics, etc., pressure-sensitive adhesives, rewetting agents, binders for non-woven fabrics, binders for various building materials such as gypsum boards and fiberboards, binders for various powder granulation, additives for cement and mortar, hot-melt adhesives, pressure-sensitive adhesives, fixing agents for anionic paints, etc. (3) Related to coating agents: clear coating agents for paper, pigment coating agents for paper, internal sizing agents for paper, sizing agents for fiber products, warp sizing agents, fiber processing agents, leather finishing agents, paints, anti-fogging agents, metal corrosion inhibitors, brightening agents for zinc plating, antistatic agents, conductive agents, temporary paints, etc. (4) Related to blend agents for hydrophobic resins: antistatic agents for hydrophobic resins, hydrophilicity-imparting agents, composite fibers, additives for films and other molded articles, etc. (5) Related to dispersants: dispersants for developers of coating liquids for heat-sensitive coloring layers, pigment dispersion stabilizers for paints, inks, aqueous colors, adhesives, etc., dispersion stabilizers for suspension polymerization of various vinyl compounds such as vinyl chloride, vinylidene chloride, styrene, (meth)acrylate, vinyl acetate, etc. (6) Related to emulsion dispersion stabilizers: emulsifiers for emulsion polymerization of various acrylic monomers, ethylenically unsaturated compounds, butadienic compounds, post-emulsifiers for hydrophobic resins such as polyolefins, polyester resins, epoxy resins, paraffin, bitumen, etc. (7) Related to thickeners: thickeners for various aqueous solutions, emulsions, and oil drilling fluids, etc. (8) Related to flocculants: flocculants for suspended solids and dissolved substances in water, drainage aids for pulp and slurries, etc. (9) Related to ion exchange resins, etc.: ion exchange resins, chelating exchange resins, ion exchange membranes, etc. (10) Others: soil conditioners, photosensitizers, photosensitive resist resins, etc. Among these, in particular, the PVA-based resin of the present invention is useful as a dispersion stabilizer for suspension polymerization of various vinyl compounds such as vinyl acetate and vinyl chloride, and is particularly useful as a dispersion stabilizer for suspension polymerization of vinyl chloride-based compounds.

[0050] [Dispersant] When the PVA-based resin of the present invention is used as a dispersant, examples of the dispersion include polymerizable monomers, powders, and the like. The PVA-based resin of the present invention is preferably used as a dispersant for suspension polymerization, particularly with a polymerizable monomer as the dispersion. Examples of the polymerizable monomer to be subjected to suspension polymerization include vinyl chloride, vinylidene halide, vinyl ether, vinyl acetate, vinyl benzoate, acrylic acid, methacrylic acid, maleic acid or its anhydride, ethylene, propylene, styrene, and the like. Among them, the PVA-based resin of the present invention is preferably used for homopolymerization of vinyl chloride or copolymerization of vinyl chloride with a monomer copolymerizable with vinyl chloride.

[0051] [Dispersant for suspension polymerization] The case where the PVA-based resin of the present invention is used as a dispersant for suspension polymerization will be described in detail below. The amount of the PVA-based resin used may be appropriately adjusted according to the monomer to be suspension polymerized. For example, when used for suspension polymerization of a vinyl chloride-based monomer, it is preferably used in an amount of 5 parts by weight or less, more preferably 0.01 to 1 part by weight, and even more preferably 0.02 to 0.2 part by weight, based on 100 parts by weight of the vinyl chloride-based monomer. If the amount used is too large, there is a tendency for an increase in the PVA-based resin that does not act as a dispersant.

[0052] When performing suspension polymerization, for example, it is preferable to add the PVA-based resin of the present invention as a dispersant to water or a heated water medium, disperse the vinyl chloride-based monomer, and carry out polymerization in the presence of an oil-soluble catalyst.

[0053] As a method for adding the PVA-based resin, there are a method of adding it in the form of a powder, in water, or in an organic solvent such as alcohol, ketone, ester, or in the state of a solution obtained by dissolving the PVA-based resin in a mixed solvent of these organic solvents and water, or a method of adding it in the state of a dispersion obtained by dispersing the PVA-based resin in the above-mentioned solvent. As for the timing of addition, it may be added all at once at the initial stage of polymerization, or added in portions during the polymerization.

[0054] As other additives, it is also possible to use known stabilizers, for example, in combination with a polymer substance. Examples of the polymer substance include PVA-based resins other than the PVA-based resin of the present invention. As such PVA-based resins, unmodified PVA, the modified PVA-based resins described above, etc. can be used.

[0055] Examples of the polymerization aid include various surfactants or inorganic dispersants, etc., and it is also possible to use the PVA-based resin of the present invention as a polymerization aid.

[0056] The polymerization catalyst may be any oil-soluble catalyst. For example, benzoyl peroxide, lauroyl peroxide, diisopropyl peroxydicarbonate, α,α'-azobisisobutyronitrile, α,α'-azobis-2,4-dimethyl-valeronitrile, acetylcyclohexylsulfonyl peroxide or a mixture thereof is used.

Examples

[0057] Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to the following examples as long as the gist thereof is not exceeded. Note that "parts", "%", etc. are based on weight.

[0058] (Example 1) [Production of the PVA-based resin (PVA-1) of the present invention] 100 parts by weight of vinyl acetate and 100 parts by weight of methanol were charged into a polymerization kettle, and while heating while supplying an oxygen / nitrogen (5:95, volume ratio) mixed gas to the liquid phase at 120 mL / min, 0.4% by weight of azobisisobutyronitrile (AIBN) with respect to vinyl acetate was charged into the polymerization kettle at a temperature below the boiling point to initiate polymerization. Polymerization was stopped when the polymerization rate reached 73.2% by weight after about 7 hours of reaction time. Then, unreacted vinyl acetate was removed and saponified by a conventional method to obtain a PVA-based resin (degree of polymerization 590, degree of saponification 71.8 mol%).

[0059] <Measurement of block character> By the above method, the value of the block character (A1) of the PVA-based resin (PVA-1) was measured. The results are shown in Table 1.

[0060] <Measurement of ultraviolet absorption spectrum> A 0.1% aqueous solution of the PVA-based resin (PVA-1) was prepared. Using an ultraviolet-visible near-infrared spectrophotometer ("V-560" (trade name) manufactured by JASCO Corporation), the absorbance (B1) of the 0.1% aqueous solution of the PVA-based resin at 320 nm was measured. A sample cell (cell) with a thickness of 1 cm was used. The results are shown in Table 1. Furthermore, the ratio (B1 / A1) of the absorbance (B1) at 320 nm in the ultraviolet absorption spectrum when the PVA-based resin (PVA-1) was a 0.1% aqueous solution to the block character (A1) was calculated. The results are shown in Table 1.

[0061] <Measurement of yellow index (YI value)> A 0.1% aqueous solution of the PVA-based resin (PVA-1) was prepared. The YI value of such an aqueous solution was measured using a colorimeter "CM-3600A" (trade name) manufactured by Konica Minolta Inc. The results are shown in Table 1.

[0062] (Example 2) [Production of the PVA-based resin (PVA-2) of the present invention] 100 parts by weight of vinyl acetate, 2.0 parts by weight of acetaldehyde, and 4.9 parts by weight of methanol were charged into a polymerization kettle. While supplying an oxygen / nitrogen (5:95, volume ratio) mixed gas at 120 mL / min to the liquid phase, it was heated. At a temperature below the boiling point, 0.01% by weight of azobisisobutyronitrile (AIBN) with respect to vinyl acetate was charged into the polymerization kettle to initiate polymerization. Polymerization was stopped when the polymerization rate reached 61.9% by weight after about 11.5 hours of reaction time. Subsequently, unreacted vinyl acetate was removed and saponified by a conventional method to obtain a PVA-based resin (degree of polymerization 760, degree of saponification 75.8 mol%). For the obtained PVA-based resin (PVA-2), in the same manner as in Example 1, the block character (A2), the ultraviolet absorption spectrum at 320 nm (B2), and the YI value were measured. The results are shown in Table 1.

[0063] (Comparative Example 1) [Production of PVA-based resin (PVA-3)] 100 parts of vinyl acetate, 1.6 parts of acetaldehyde, 4.7 parts of methanol, and 0.0092% of acetyl peroxide (APO) with respect to vinyl acetate were charged into a polymerization kettle and purged with nitrogen. Then, it was heated to initiate polymerization at a temperature below the boiling point. Polymerization was stopped when the polymerization rate reached 80.0% after about 7 hours of reaction time. Next, unreacted vinyl acetate was removed, and the obtained polymer was saponified with sodium hydroxide by a conventional method. Sodium acetate was added to the saponification slurry of the PVA-based resin (degree of polymerization 630, degree of saponification 71.7 mol%) with a resin content of 12% so that the amount of sodium acetate after decanting was 1% by weight with respect to the PVA-based resin, and decanting was performed. Next, a 20% methanol solution of magnesium acetate tetrahydrate as a metal compound was added to the PVA-based resin prepared above so that the magnesium acetate content was 2% by weight with respect to the PVA-based resin. Then, it was dried to obtain a PVA-based resin containing 2% by weight of magnesium acetate and 1% by weight of sodium acetate. Further, the obtained resin was heat-treated at 140 °C to obtain a PVA-based resin (PVA-3). For the obtained PVA-based resin (PVA-3), in the same manner as in Example 1, the block character (A3), the ultraviolet absorption spectrum at 320 nm (B3), and the YI value were measured. The results are shown in Table 1.

[0064] (Comparative Example 2) [Production of PVA-based resin (PVA-4)] A PVA-based resin (PVA-4) was obtained in the same manner as in Comparative Example 1, except that the heat treatment was carried out at 110°C. For the obtained PVA-based resin (PVA-4), the block character (A4), the ultraviolet absorption spectrum at 320 nm (B4), and the YI value were measured in the same manner as in Example 1. The results are shown in Table 1.

[0065] (Comparative Example 3) [Production of PVA-based resin (PVA-5)] 100 parts of vinyl acetate, 1.6 parts of acetaldehyde, 4.7 parts of methanol, and 0.0092% of acetyl peroxide (APO) with respect to vinyl acetate were charged into a polymerization kettle and purged with nitrogen. Then, it was heated to initiate polymerization below the boiling point, and the polymerization was stopped when the polymerization rate reached 80.0% after about 7 hours of reaction time. Next, unreacted vinyl acetate was removed, and the obtained polymer was saponified by a conventional method with sodium hydroxide. Sodium acetate was added to the saponification slurry of the PVA-based resin (degree of polymerization 630, degree of saponification 71.7 mol%) with a resin content of 12% so that the amount of sodium acetate after decantation was 1% by weight with respect to the PVA-based resin, and decantation was performed. Next, a 20% methanol solution of magnesium acetate tetrahydrate as a metal compound was added to the PVA-based resin prepared above so that magnesium acetate was 2% by weight with respect to the PVA-based resin, and then it was dried to obtain a PVA-based resin (PVA-5) containing 2% by weight of magnesium acetate and 1% by weight of sodium acetate. For the obtained PVA-based resin (PVA-5), the block character (A5), the ultraviolet absorption spectrum at 320 nm (B5), and the YI value were measured in the same manner as in Example 1. The results are shown in Table 1.

[0066]

Table 1

[0067] From the results in Table 1, the PVA-based resins of Examples 1 and 2 can achieve both a high double bond content and a small block character value compared to the PVA-based resins of Comparative Examples 1 to 3. In addition, the PVA-based resins of Examples 1 and 2 have a lower YI value compared to the PVA-based resin of Comparative Example 1, which has a relatively close double bond content. Thus, in Examples 1 and 2, PVA-based resins excellent in polymerization stability, dispersibility, and yellowing suppression during polymerization were obtained.

[0068] Although the present invention has been described in detail with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention. This application is based on a Japanese patent application filed on January 16, 2020 (Japanese Patent Application No. 2020-005372), the content of which is incorporated herein by reference.

Industrial Applicability

[0069] Since the PVA-based resin of the present invention has a large amount of double bonds, when used as a dispersant for suspension polymerization, it is excellent in suspension polymerization stability, and further, when used as various dispersants, it is excellent in polymerization stability during polymerization. In addition, since the value of the block character is small, the PVA-based resin of the present invention has high surface activity and excellent dispersibility when used as various dispersants. The PVA-based resin of the present invention is particularly useful as a dispersant for suspension polymerization of vinyl chloride-based monomers.

Claims

1. A polyvinyl alcohol-based resin, wherein the block character (A) is less than 0.4, and the absorbance (B) at 320 nm in the ultraviolet absorption spectrum when the polyvinyl alcohol-based resin is a 0.1 wt% aqueous solution is 0.25 or more and 1.5 or less.

2. The polyvinyl alcohol-based resin according to Claim 1, wherein the ratio (B / A) of the absorbance (B) at 320 nm in the ultraviolet absorption spectrum when the polyvinyl alcohol-based resin is a 0.1 wt% aqueous solution to the block character (A) is 0.6 or more and 3 or less.

3. A dispersant comprising the polyvinyl alcohol-based resin according to Claim 1 or 2.

4. A dispersant for suspension polymerization comprising the polyvinyl alcohol-based resin according to Claim 1 or 2.

5. A method for producing a polyvinyl alcohol-based resin according to Claim 1 or 2, comprising a step of solution-polymerizing a monomer composition containing a vinyl ester-based monomer while introducing a gas containing oxygen to obtain a vinyl ester-based polymer.

6. while introducing a gas having an oxygen concentration of 1% by mass to 9% by mass so that the supply amount of oxygen per minute with respect to the monomer amount is 0.1 mL or more and 25 mL or less, solution-polymerizing a monomer composition containing a vinyl ester-based monomer to obtain a vinyl ester-based polymer The method for producing a polyvinyl alcohol-based resin according to Claim 5.

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