Method for producing water-absorbing resin derived from bio-raw materials, and water-absorbing resin obtained using said method

Abstract

[Problem] The present invention addresses the problem of inexpensively obtaining a water-absorbing resin which achieves performance that is at least equivalent to that of conventional water-absorbing resins derived from fossil raw materials and in which the amount of impurities is equivalent to, or lower than, that in conventional water-absorbing resins derived from fossil raw materials despite being produced using bio-raw materials able to be recycled in the production of the water-absorbing resin. [Solution] Provided is a method for producing a water-absorbing resin derived from bio-raw materials, the method having steps (i) to (vii) below. A step (i) for obtaining acetone from bioethanol; a step (ii) for obtaining isopropanol from the acetone; a step (iii) for obtaining propylene from the isopropanol; a step (iv) for obtaining acrylic acid from the propylene; a step (v) for polymerizing an aqueous monomer solution containing the acrylic acid to obtain poly(acrylic acid) and / or a salt thereof; a step (vi) for drying the poly(acrylic acid) and / or salt thereof; and a step (vii) for surface crosslinking the poly(acrylic acid) and / or salt thereof.

Description

Method for producing water-absorbent resin derived from biomaterials, and the resulting water-absorbent resin

[0001] The present invention relates to a method for producing a water-absorbent resin. More specifically, the present invention relates to a method for producing a water-absorbent resin, which exhibits performance equivalent to or superior to that of water-absorbent resins derived from fossil raw materials, and contains fewer impurities, using renewable bio-based raw materials with good productivity and at low cost.

[0002] Water-absorbent resins, also known as superabsorbent resins, superabsorbent polymers, superabsorbent polymers (SAPs), absorbent gelling agents (AGMs), polymeric water absorbents, water-swelling polymers, etc., are widely used in hygiene materials such as disposable diapers, sanitary napkins, and so-called incontinence pads, as well as agricultural and horticultural water-retaining materials, air fresheners, deodorizers, dehumidifiers, and cable waterproofing materials. Examples of water-absorbent resins include partially neutralized crosslinked polyacrylic acid and starch-acrylic acid graft polymers. Typical polymerization methods for producing water-absorbent resins include aqueous solution polymerization, reversed-phase suspension polymerization, and gas-phase polymerization. For example, aqueous solution polymerization involves the steps of preparing an aqueous monomer solution using acrylic acid or the like, a polymerization step, a gel crushing step, a drying step, a granulation step (crushing and classification), a fine powder recycling step, a surface crosslinking step, and the addition of additives (Non-Patent Document 1).

[0003] Water-absorbent resins are typically synthetic polymers, typified by crosslinked polyacrylates. Its monomer, acrylic acid, is generally obtained by oxidation of propylene obtained by cracking naphtha, a fossil fuel derived from petroleum. The primary use of water-absorbent resins is in sanitary materials such as disposable diapers, which are discarded in large quantities after use. From the perspective of sustainability, such fossil-derived polyacrylate water-absorbent resins have recently been recognized as petrochemical products that impact the environment. Therefore, instead of conventional fossil-derived (petroleum) polyacrylate water-absorbent resins, water-absorbent resins in which polyacrylates are grafted or mixed with natural polymers (Patent Document 17) and water-absorbent resins derived from natural polymers in which natural polymers are crosslinked or carboxy-modified (Patent Document 18) have been proposed.

[0004] However, in the case of a water absorbent resin made of a natural polymer (for example, a starch-grafted or starch-mixed polyacrylic acid crosslinked body, a modified starch crosslinked body, a carboxymethyl cellulose crosslinked body, a polyamino acid crosslinked body, etc.), the heat resistance of the natural polymer is low, and therefore, not only are there problems of coloration in the production process of the water absorbent resin (yellowing or browning of the product due to low heat resistance) and processing treatment under high temperature conditions is difficult, resulting in low productivity, but also, due to the use of a natural polymer, the water absorbent resin is significantly inferior in water absorption performance to conventional polyacrylate water absorbent resins.

[0005] Therefore, instead of using natural polymers, which are inferior in performance and heat resistance, in water-absorbent resins, water-absorbent resins have been proposed in which acrylic acid derived from biomaterials (bioacrylic acid) is obtained from biomaterials instead of acrylic acid conventionally obtained from fossil materials (Patent Documents 1 to 16, 19, 20).Water-absorbent resins made of acrylic acid derived from biomaterials can be produced using the same manufacturing process as conventional polyacrylic acid water-absorbent resins, and therefore have the advantage of fewer problems in performance, productivity, and heat resistance (coloration) compared to water-absorbent resins that use natural polymers.

[0006] Specifically, methods for obtaining a water-absorbent resin via bio-acrylic acid include a method using acrylic acid derived from glycerin (Patent Documents 1 to 4), a method using acrylic acid obtained by dehydrating lactic acid or 3-hydroxypropionic acid (Patent Documents 5 to 8, 20), a method using acrylic acid obtained by dehydrating polyhydroxypropionic acid (Patent Document 9), a method for obtaining acrylic acid from β-propiolactone (Patent Document 10), and a method using bio-naphtha based on natural oil and / or fat (Patent Documents 11 and 12). In addition, the content of raw materials derived from bio-based materials and carbon isotope analysis have been used to determine whether these water-absorbent resins are water-absorbent resins using bio-acrylic acid. １４ C amount and １３Methods that define the carbon content are also known (Patent Documents 7, 13, and 14). However, propylene-free bioacrylic acid production methods (Patent Documents 1 to 10, 19, and 20) cannot use existing acrylic acid production facilities that use fossil propylene as a starting material, and require entirely new production facilities, which often result in expensive acrylic acid. Furthermore, despite the use of bio-based feedstocks (Patent Documents 1 to 14 and 20, which use lactic acid, 3-hydroxypropionic acid, glycerin, and bio-naphtha based on natural oils and / or fats) that are more expensive than fossil propylene, impurities are more likely to increase compared to acrylic acid derived from fossil feedstocks. For example, even when biopropylene is used, the thermal decomposition of bio-naphtha contains a large amount of propane, and the increase in propane in biopropylene causes an increase in non-polymerizable propionic acid in the acrylic acid obtained by the oxidation reaction (Patent Document 11). It is also known that biopropane is likely to contain sulfur, phosphorus, and nitrogen derived from bio-based feedstocks (Patent Document 12). Furthermore, bioacrylic acid, whether derived from biopropylene or not, contains relatively high amounts of formic acid (Patent Document 15), bioacrylic acid derived from glycerin contains relatively high amounts of hydroxyacetone, which easily discolors water-absorbent resins (Patent Document 16), and when acrylic acid is obtained from glycerin, organic acids such as propionic acid, acetic acid, and formic acid are often produced as by-products, particularly propionic acid via propionaldehyde. Furthermore, when acrylic acid is obtained from 3-hydroxypropionic acid, there are problems such as contamination with organic acids as fermentation by-products. In particular, the by-products propionic acid and acrylic acid have nearly the same boiling point, 141°C, making separation difficult. Repeated purification is required to remove organic acids such as propionic acid from acrylic acid, but repeated purification at the expense of yield and cost has not necessarily resulted in sufficient removal of organic acids, particularly propionic acid.

[0007] Compared with conventional acrylic acid derived from such fossil raw materials, an increase in impurities in acrylic acid derived from bioacrylic acid or the generation of new impurities not only leads to a reduction in the purification cost and yield of bioacrylic acid, but also causes problems (such as odor, coloration, and performance degradation of the water absorbent resin) when bioacrylic acid containing these impurities is used for a water absorbent resin.

[0008] Therefore, currently, the production cost of any bioacrylic acid is high, and polyacrylic acid (salt) water absorbent resins using the bioacrylic acid are also expensive. Furthermore, when the starting biomaterial is expensive or its production volume is limited, there is a limit to the use of biomaterial-derived water absorbent resins as a substitute for the mass-consumed fossil-derived water absorbent resins. Furthermore, even though the bioacrylic acid-derived polyacrylic acid (salt) water absorbent resins are the same polyacrylic acid (salt) water absorbent resins, they tend to be inferior in odor (e.g., the acidic odor of propionic acid) and coloration due to impurities specific to each bioacrylic acid (e.g., an increase in propionic acid in acrylic acid), compared to water absorbent resins derived from fossil-derived acrylic acid.

[0009] WO2006 / 092272WO2006 / 136336WO2008 / 023040WO2010 / 066513WO2006 / 092271 WO2008 / 023039WO2007 / 109128WO2013 / 155292WO2013 / 185009WO2018 / 085254 WO2014 / 079785 JP2018-083866 WO2011 / 136237 WO2011 / 136238 WO2011 / 040575 WO2009 / 130915 WO2007 / 098932 WO2002 / 096953 WO2010 / 090324 WO2010 / 090322

[0010] Modern Superabsorbent Polymer Technology (1998); pp. 69-117

[0011] The present invention addresses the problem of using renewable bio-based materials in the production of a water-absorbent resin, thereby inexpensively obtaining a water-absorbent resin that has performance equivalent to or superior to that of conventional water-absorbent resins derived from fossil raw materials and has an equivalent or even reduced amount of impurities.

[0012] In order to solve the above problems, the present inventors have focused for the first time on ethanol as a starting material for a monomer that constitutes the main chain of a water absorbent resin, and have investigated a method for producing a water absorbent resin derived from a bio-based material, in place of a method in which a natural polymer that is inferior in performance and heat resistance is used for the water absorbent resin, and in place of a conventional representative method for producing bio-acrylic acid (starting materials: glycerin, bio-naphtha, lactic acid, 3-hydroxypropionic acid, bio-naphtha based on natural oil and / or fat, etc.).

[0013] That is, the present inventors came up with the idea of ​​using mass-produced bioethanol as a starting material instead of conventional expensive biomaterials (which are further limited in production), and synthetically obtaining propylene through a specific process to suppress the production of propane and other impurities in bioacrylic acid (such as the content of propionic acid).

[0014] In the present invention, by using bioacrylic acid obtained from bioethanol through a specific process (bioethanol → bioacetone → bioisopropanol → biopropylene → bioacrylic acid) as a monomer, the present inventors have found that such bioacrylic acid derived from bioethanol that has undergone a specific process does not cause problems in water absorption performance or problems resulting from impurities (e.g., coloration or odor), and is equally or more suitable as a raw material acrylic acid for water absorbent resins than acrylic acid derived from conventional fossil raw materials, thereby solving the above-mentioned problems and completing the present invention.

[0015] That is, the present invention provides a method for producing a water absorbent resin derived from a biomaterial, comprising the following steps (i) to (vii): step (i) of obtaining acetone from bioethanol; step (ii) of obtaining isopropanol from the acetone; step (iii) of obtaining propylene from the isopropanol; step (iv) of obtaining acrylic acid from the propylene; step (v) of polymerizing an aqueous monomer solution containing the acrylic acid to obtain polyacrylic acid and / or a salt thereof; step (vi) of drying the polyacrylic acid and / or a salt thereof; and step (vii) of surface-crosslinking the polyacrylic acid and / or a salt thereof.

[0016] The present invention also relates to a water-absorbent resin obtained by the above-mentioned production method.

[0017] By using renewable bio-based raw materials in the production of water absorbent resins, it is possible to obtain water absorbent resins at low cost that have performance equivalent to or better than that of conventional water absorbent resins derived from fossil raw materials, and that have the same or even reduced amount of impurities. ２ Therefore, by producing a water absorbent resin using bioethanol as a raw material, it is possible to aim for a carbon-neutral, high-performance water absorbent resin. Furthermore, since bioethanol is a bio-raw material that is produced in large quantities at low cost, the water absorbent resin of the present invention produced from such bioethanol can be widely substituted for water absorbent resins derived from fossil raw materials that are consumed in large quantities.

[0018] [Explanation of Terms] (Biomass) In the present invention, biomass refers to any organic resource derived from a living organism, including animal-derived biomaterials (e.g., wool), but renewable plant materials are preferably used, and specifically, biomaterials whose starting materials are plant components containing natural polymers such as sugars, starch, and cellulose are preferably used.

[0019] In the present invention, bioethanol, bioacetone, bioisopropanol, biopropylene, and bioacrylic acid refer to ethanol, acetone, isopropanol (also known as 2-propanol or isopropyl alcohol), propylene, and acrylic acid, which are produced using biomaterials as raw materials or upstream raw materials of the raw materials, and the carbon isotopes １４ Except for the amount of C, there is no difference in chemical structure from known ethanol, acetone, isopropanol, propylene, and acrylic acid.

[0020] Hereinafter, to emphasize that ethanol, acetone, isopropanol, propylene, and acrylic acid are obtained from bio-based raw materials, they may be referred to as bioethanol, bioacetone, bioisopropanol, biopropylene, and bioacrylic acid, respectively.

[0021] In the present invention, the water-absorbent resin derived from a biomaterial means that the water-absorbent resin contains a monomer derived from a biomaterial as a monomer constituting the main chain of the water-absorbent resin.

[0022] In the present invention, terms relating to the performance of the water-absorbent resin are defined by the 2005 edition of WSP (Worldwide Strategic Partners) standards of EDANA Recommended Test Methods, unless otherwise specified. The performances specified by WSP include "pH" (WSP200.2), "residual monomer" (WSP210.2), "particle size distribution" (WSP220.2), "loss on drying" (WSP230.2), "free swelling capacity" or "FSC" (WSP240.2), "centrifuge retention capacity" or "CRC" (WSP241.2), "Amount of absorption under load (AUP)" or "AAP (absorbency under pressure)" (WSP242.2), "liquid absorption under load" or "PDAUP" (WSP243.1), and "Flow "Bulk Specific Gravity" or "Density" (WSP 260.2), "Water Soluble Matter" or "Extractables" (WSP 270.2), Respirable Particles (WSP 280.2), and Dust (WSP 290.2).

[0023] In the present invention, the AAP may be measured under a pressure of 0.7 psi (4.8 kPa), and the AAP at that time may be referred to as AAP 0.7.

[0024] In the present invention, the "moisture content" of the water-absorbent resin or particulate hydrogel is the value measured by "loss on drying" (WSP230.2) when the sample mass is 1 g and the heating temperature is 180°C.

[0025] In the present invention, the weight-average particle diameter (D50) is a value obtained by plotting the proportion R % of particles remaining on a sieve with a predetermined mesh size on a logarithmic probability paper based on the data of "Particle size distribution" (WSP220.2) and reading the mesh size (particle diameter) at which R = 50.

[0026] (Other) In this specification, mass and weight are treated as synonyms. Furthermore, the range "X to Y" means "X or more and Y or less." Furthermore, the "X" in the range "X to Y" can be the basis for the legitimacy of an amendment to "X or more" or "X or less." Furthermore, the "Y" in the range "X to Y" can be the basis for the legitimacy of an amendment to "Y or more" or "Y or less." Furthermore, unless otherwise noted, "%" and "ppm" mean "% by mass" and "ppm by mass." Furthermore, "~acid (salt)" means "~acid and / or its salt," and "(meth)acrylic" means "acrylic and / or methacrylic." Furthermore, unless otherwise noted, measurements of physical properties are performed at room temperature (20-25°C) and a relative humidity of 40-50% RH. Furthermore, when a specific component is described as containing two or more components, the total amount is the total amount.

[0027] [Method for producing water absorbent resin of the present invention] (Bioethanol) The present invention is the first to focus on bioethanol as a raw material for a water absorbent resin, and has a greatest feature in that a water absorbent resin is obtained from bioethanol, which has conventionally been used mainly as a fuel.

[0028] Global bioethanol production is expected to exceed 113 billion liters (approximately 89 million tons) in 2022. Bioethanol accounts for approximately 3% of the approximately 4.6 trillion liters of petroleum production, and as a bio-raw material it can be obtained in large quantities at low cost. Its main uses are as fuel (approximately 85% is used for automobiles and other fuels), as well as industrial uses, primarily as solvents, and in food-related applications such as beverages and disinfectants. While there are examples of bioethanol being used in chemical products such as ethyl esters, the present invention is characterized by its use as a starting material for water-absorbent resins, a use of bioethanol that is primarily used as fuel.

[0029] Bioethanol can be obtained from glucose, sucrose, or the like by known fermentation methods, using molasses (blackstrap molasses) obtained after separating refined sugar as the fermentation raw material, for example, according to the following reaction formula:

[0030] C ６ H １２ O６ →2CH ３ CH ２ OH + 2CO ２ In the first step, one molecule of glucose is broken down into two molecules of pyruvic acid by multiple enzymes in the glycolytic pathway. From the second step onwards, the reactions specific to alcoholic fermentation take place. One molecule of carbon dioxide is removed from one molecule of pyruvic acid to produce acetaldehyde. Afterwards, acetaldehyde is quickly reduced to ethanol by the electrons of reduced NADH.

[0031] Aqueous ethanol solutions obtained by fermentation (ethanol concentrations of several volume percent to several tens of volume percent, particularly about 5 to 20 volume percent) contain a large amount of water, fermentation raw materials (sugars and proteins), and fermentation products other than ethanol (organic acids, alcohols, etc.), and are therefore purified by distillation. However, due to the azeotropic phenomenon between ethanol and water, the resulting aqueous ethanol contains a small amount of water (about 4 to 10% by weight) even after distillation purification. The purity (particularly the concentration) of the aqueous ethanol after distillation can be selected as appropriate, but is typically 90 to 96% by weight. In such highly concentrated aqueous ethanol, the majority of impurities are water, with small amounts of impurities present.

[0032] The hydrous ethanol obtained by distillation is then subjected to azeotropic distillation with a hydrophobic organic solvent such as benzene or hexane for further dehydration and purification, or dehydration using a dehydrating agent such as zeolite or calcium oxide, a reverse osmosis membrane, etc., to obtain anhydrous ethanol (usually with a purity of 99.5% by volume or higher) for general distribution. For example, anhydrous ethanol, which is highly compatible with gasoline, is widely used as a fuel when mixed with gasoline.

[0033] Both hydrous ethanol and absolute ethanol can be used as the bioethanol of the present invention. In step (i) of obtaining acetone from bioethanol (described below), ethanol and water are reacted as shown in Equation 1 (described below), so the presence of water in the ethanol is not a problem. Another feature of the present invention is that hydrous ethanol, which is less expensive than absolute ethanol, can be used as the raw material. On the other hand, the use of absolute ethanol is disadvantageous because it requires higher purification costs. Furthermore, the hydrophobic organic solvent used in the azeotropic distillation during dehydration remains, which may cause an odor in the water-absorbent resin. Furthermore, because hydrous ethanol has a higher flash point than absolute ethanol (13.0°C for absolute ethanol and 22.2°C for 50% ethanol by volume), the use of hydrous ethanol may be preferable in terms of ease of handling.

[0034] (Water Content of Bioethanol) When hydrous ethanol is used in the present invention, the lower limit of the water content of the hydrous ethanol in the present invention may be 3 wt% or more, 4 wt% or more, 5 wt% or more, 6 wt% or more, 7 wt% or more, 8 wt% or more, 10 wt% or more, or 12 wt% or more, and the upper limit of the water content may be 50 wt% or less, 40 wt% or less, 30 wt% or less, or 20 wt% or less. The lower limit of the ethanol concentration is appropriately selected from the range of 40 wt% or more, 50 wt% or more, 60 wt% or more, 70 wt% or more, or 80 wt% or more. Ethanol with a higher water content is preferable because it requires less energy for distillation purification, but the water content of the hydrous ethanol is appropriately selected taking into consideration impurities, cost, and reactivity. For example, hydrous ethanol with an ethanol concentration of about 97 to 50 wt%, 96 to 60 wt%, or 96 to 70 wt%, or even within the above upper and lower limits, can be used appropriately, and acetone can be obtained by reacting the water in the hydrous ethanol with ethanol alone, or with water (water vapor) added as needed. For example, in step (i) of obtaining acetone from bioethanol described below, when acetone is obtained from ethanol according to Equation 1 described below, the water content of the reaction system is about 16 wt% based on the weight ratio of 2 moles of ethanol to 1 mole of water (ethanol / water = 92 / 18) in Equation 1 described below, which is the theoretical value (100% reaction according to Equation 1 described below). Therefore, the ratio of water to anhydrous or hydrous ethanol, if necessary, and the water content of ethanol can be selected by taking into account the reaction rate and recovery rate of ethanol, the acetone yield, energy, etc. If the water content of the ethanol is too high (the ethanol concentration is too low), not only is it unfavorable in terms of energy for acetone synthesis in Equation 1 described below, but it is also unfavorable in terms of the transportation costs of hydrous ethanol containing a large amount of water, and there is a risk that fermentation products other than ethanol (organic acids and alcohols) will not be sufficiently removed during distillation purification from the fermentation liquid. The water content (volume % or weight %) of hydrous ethanol can be measured as appropriate and can also be determined as an intrinsic value from, for example, the density (g / ml) (20°C) of the hydrous ethanol.

[0035] In the present invention, the water content of the hydrous ethanol may be adjusted by the distillation conditions of the aqueous ethanol solution (ethanol concentration after fermentation is several volume % to several tens of volume %, particularly about 5 to 20 volume %), or may be adjusted by adding a predetermined amount of water to the hydrous or anhydrous ethanol after distillation to dilute it, or by mixing multiple anhydrous or hydrous ethanols.

[0036] (Trace Components in Bioethanol) When anhydrous or hydrous ethanol is used in the present invention, the ethanol may contain trace components other than water, such as lower alcohols having 1 or 3 to 5 carbon atoms, such as methanol, 1-propanol, isopropanol, 1-butanol, 2-butanol, 2-methylpropanol, and 2-methyl-1-butanol, as well as lower aldehydes or lower ketones, such as acetaldehyde (having 2 carbon atoms) and acetone (having 3 carbon atoms). The content of the lower alcohols having 1 or 3 to 5 carbon atoms, acetaldehyde, and acetone in the anhydrous or hydrous ethanol is, for example, 1 wt% or less, 0.1 wt% or less, or 0.01 wt% or less, respectively. The content of these trace components can be measured, for example, by gas chromatography.

[0037] In the present invention, acetone and / or isopropanol are obtained from bioethanol, so acetone or isopropanol may be contained or remain in the ethanol. Due to the yield and cost of ethanol, it is not necessary to remove all acetone and isopropanol by ethanol purification. In the present invention, bioethanol (and even hydrous ethanol) containing acetone and / or isopropanol can be used as a suitable starting material. From the perspective of cost and the performance of the resulting water-absorbent resin, the content of acetone and / or isopropanol in ethanol is preferably 1 ppm or more, 5 ppm or more, 10 ppm or more, or 20 ppm or more as the total amount of acetone and isopropanol, and more preferably 1 ppm or more, 5 ppm or more, 10 ppm or more, or 20 ppm or more, respectively. The upper limit of the content of acetone and / or isopropanol in ethanol may be high, but can be selected, for example, from the balance with other impurities within the above range, within the above range of 1 wt. %, 0.1 wt. %, or 0.01 wt. % or less. More preferably, ethanol containing acetone and isopropanol is used, the total content of which is within the above range. Such ethanol containing acetone or isopropanol may be obtained as crude ethanol.

[0038] In the present invention, ethanol obtained from a biological raw material can be used, but preferably ethanol obtained from a plant raw material is used, and more preferably bioethanol obtained by fermenting one or more plant raw materials selected from sugar cane, corn, and sugar beet. The one or more selected from sugar cane, corn, and sugar beet may be further crushed or squeezed before fermentation.

[0039] The plant raw materials may be genetically modified plants (typically genetically modified corn) or non-genetically modified plants, but since the present application goes through the route of bioethanol, acetone, isopropanol, propylene, and acrylic acid, unlike when bioethanol is directly used or eaten, there are no restrictions on genetic modification and a wide range of genetically modified plant raw materials can be used, which is preferable.

[0040] Bioethanol is determined by radiocarbon dating. １４ C / １２ Traceable ethanol can also be obtained. In addition, acetone, isopropanol, propylene, and acrylic acid, which are sequentially obtained from bioethanol in this invention, can also be measured by radiocarbon dating. １４ C / １２ C or records of acquisition and production routes can confirm that the compound is derived from a biological source.

[0041] The bioethanol content can be measured as follows: 1. The ethanol used in the raw gas is burned and all of it is converted into carbon dioxide. 2. The carbon dioxide is separated and purified using a vacuum line. 3. The carbon dioxide produced from the ethanol is completely reduced with hydrogen using iron as a catalyst to produce graphite. 4. １４ Using a C-AMS measurement device (for example, manufactured by NEC), １４ C concentration and １２ C concentration ratio ( １４ C / １２ 5. The same method as above 1 to 4 was used to measure oxalic acid (hereinafter also referred to as the standard sample) from the same year that the raw material ethanol was produced, provided by the National Institute of Standards (NIST). １４ C concentration and １２ C concentration ratio ( １４ C / １２ C) is measured. 6. Graphite derived from raw material ethanol １４ C / １２ The value of C is １４ C / １２The value divided by the C value is multiplied by 100 to obtain the bioethanol content.

[0042] (Step (i) of Obtaining Acetone from Bioethanol) In the method for producing a water-absorbent resin of the present invention, in order to obtain acrylic acid from bioethanol, step (i) of first obtaining acetone from bioethanol is essential. Methods for producing acetone from ethanol are known, and known methods can be applied to obtain acetone from bioethanol. For example, acetone can be synthesized from bioethanol by methods such as WO2022 / 244797, JP2012-240913, Japanese Patent No. 5747326, WO2009 / 110413, and JP2022-178043. The disclosures of these publications are incorporated by reference in their entirety.

[0043] Although these known patent documents describe the synthesis of acetone from bioethanol, acetone is generally used as a solvent and as a raw material for methyl methacrylate, and the above-mentioned documents on the production method of acetone do not suggest the production of a water-absorbent resin from ethanol, nor the production of acrylic acid from ethanol through steps (i) to (iv).The present invention is characterized in that, among the countless uses of acetone obtained from bioethanol, after going through steps (i) to (iv), a water-absorbent resin is further produced through steps (v) to (vii) using bioacrylic acid obtained using ethanol as a raw material.

[0044] In the present invention, more preferably, in the step (i), acetone is produced by the following reaction formula (1) of ethanol and water: By such a constitution, it is possible to more efficiently obtain a water-absorbent resin from renewable natural raw materials.

[0045] Formula 1) 2CH ３ CH ２ OH+H ２ O → CH ３ COCH ３ +CO ２ +4H ２That is, in the present invention, anhydrous or hydrous bioethanol is reacted with water (or water in hydrous ethanol) to obtain a mixed gas containing acetone, water vapor, carbon dioxide, and hydrogen. Furthermore, the bioethanol may contain or contain fermentation by-products such as acetone and isopropanol.

[0046] (Water Used in the Reaction) In the reaction of formula 1 in step (i), the inclusion of water (water vapor) improves acetone selectivity. The molar ratio of water (water vapor) to ethanol (ethanol gas) (water / ethanol) is preferably 0.1 to 10, 0.5 to 10, 0.5 to 5, and 1 to 5, in that order, and may be 1 to 3. When using hydrous ethanol as bioethanol, it is preferable that the total amount of water contained in the hydrous ethanol and water added as needed be in the above molar ratio per 1 mole of ethanol. As an example, 95% by weight (approximately 96% by volume) hydrous ethanol contains 0.14 moles of water per mole of ethanol. To achieve a molar ratio of water to ethanol of 0.1 to 10, either no water can be added, or 9.9 moles or less of water can be added per mole of ethanol.

[0047] The water added as needed in addition to the water in the ethanol is not particularly limited, and tap water, industrial water, pure water (RO water, ion-exchanged water, distilled water), etc. can be used. These waters may be groundwater, river water, or treated versions of these. In addition, water generated in at least one of steps (i) to (iv) may be reused.

[0048] (Catalyst) The catalyst used in step (i) is not particularly limited, but preferably contains at least one metal (Me) selected from the group consisting of magnesium, calcium, manganese, copper, and zinc, iron, and zirconium. The state of the metal element contained in the catalyst used in step (i) is not particularly limited, and may be, for example, a metal oxide containing the metal element, a carrier containing the metal element, a carrier supporting the metal element, or a carrier supporting the metal oxide. The metal oxide may be a composite metal oxide. Examples of composite metal oxides include spinel, perovskite, magnetoplumbite, and garnet types, with spinel being preferred.

[0049] The catalyst used in step (i) preferably contains iron from the viewpoint of catalytic activity, and more preferably contains, in addition to iron (Fe), one or more metals (Me) selected from the group consisting of magnesium (Mg), calcium (Ca), manganese (Mn), and zinc (Zn).

[0050] The catalyst containing, in addition to iron (Fe), one or more metals (Me) selected from the group consisting of magnesium (Mg), calcium (Ca), manganese (Mn) and zinc (Zn) may be a catalyst represented by the following general formula (1): MeO.nFe ２ O ３ (1) (in general formula (1), Me represents one or more metals selected from the group consisting of Mg, Ca, Mn, and Zn, and n represents a number from 1 to 6), is preferred.

[0051] Specific examples of iron composite oxides include MgO.Fe ２ O ３ (MgFe ２ O ４ ), ZnO.Fe ２ O ３ (ZnFe ２ O ４ ) etc.

[0052] When the catalyst used in step (i) is a catalyst in which a metal element or a metal oxide is supported on a carrier, the carrier may be activated carbon, silica (SiO ２ ), alumina (Al ２ O ３ ), silica-alumina, zeolite, silica-calcia, zirconia (ZrO ２ ), ceria (CeO ２ ), magnesia (MgO), and diatomaceous earth. Among these, one or more selected from activated carbon, silica-calcia, zirconia, ceria, and magnesia are more preferred, with zirconia being particularly preferred. The shape of the carrier is not particularly limited, and examples include spherical, pellet, and honeycomb shapes. The BET specific surface area of ​​the carrier is 20 to 200 m ２ / g, and more preferably 40 to 200m ２ The use of a carrier having a large specific surface area is preferred because it makes it easier for the catalyst components to be supported in a dispersed state, thereby increasing the catalytic activity.

[0053] The state of the zirconium element contained in the catalyst is not particularly limited, and may be in the form of a compound containing zirconium as a single metal, or may be in the form of a composite metal oxide formed by containing zirconium with other metal elements, or may be in the form of a carrier. Examples of compounds containing zirconium as a single metal include zirconium oxide (ZrO ２ Examples of composite metal oxides containing other metal elements (Me) include composite metal oxides of zirconium and Sn, Pb, Zn, Cu, Fe, Mn, In, etc. Examples of carriers include zirconium oxide (ZrO ２ ), a composite metal oxide of zirconium, Zn, and Fe is preferred, and from the viewpoint of catalyst performance, zirconium oxide (ZrO ２ ) is more preferable.

[0054] The amount of metal (Me) in the catalyst is preferably 0.4 to 0.7 mol, more preferably 0.4 to 0.6 mol, and even more preferably 0.45 to 0.55 mol per mol of iron (Fe). When the amount of metal (Me) is within the above range, good catalytic activity can be obtained. The amount of zirconium (Zr) in the catalyst is preferably 0.01 to 0.5 mol, more preferably 0.05 to 0.5 mol, and may be 0.1 to 0.4 mol per mol of iron (Fe). When the amount of zirconium is within the above range, the durability of the catalyst can be improved.

[0055] The total amount of the metal (Me), iron and zirconium in the catalyst is preferably 50 to 100 mass %, more preferably 80 to 100 mass %, based on 100 mass % of the catalyst.

[0056] (Reactor / Reaction Conditions) The reaction in step (i) is not particularly limited and may be either a batch or continuous system, but is preferably a continuous system from the viewpoint of productivity. The reaction is preferably a gas phase reaction. Examples of the gas phase reaction system include a fixed bed, a moving bed, and a fluidized bed, but the simpler fixed bed system is preferred.

[0057] When the reaction system is a fixed bed system, ethanol gas and water vapor may be mixed and then supplied to the acetone synthesis reactor to contact the catalyst, or ethanol gas and water vapor may be supplied separately to the acetone synthesis reactor to contact the catalyst. Ethanol gas and water vapor may be produced directly from hydrous ethanol, or water vapor may be added separately. Ethanol gas and water vapor can be obtained by heating hydrous or anhydrous ethanol and water, respectively, in a vaporizer. In addition to ethanol gas and water vapor, an inert gas such as nitrogen or helium may be supplied to the acetone synthesis reactor.

[0058] The concentration of ethanol gas is preferably 3 to 66 mol %, more preferably 5 to 50 mol %, relative to 100 mol % of the total amount of gas supplied to the acetone synthesis reactor. By using such a ratio, acetone can be produced with high productivity.

[0059] The pressure in the reaction of step (i) may be reduced pressure, normal pressure, or increased pressure, but is preferably 0.07 to 2 MPa, and more preferably 0.1 to 1 MPa. The temperature in the reaction of step (i) is preferably 250 to 600°C, more preferably 300 to 550°C, and even more preferably 330 to 500°C. When the reaction is carried out as a gas phase reaction, the space velocity of the raw material gas is preferably 300 to 10,000 (1 / h), more preferably 400 to 8,000 (1 / h), and even more preferably 500 to 6,000 (1 / h).

[0060] (Purification of Acetone) When the purity of the acetone obtained in step (i) is high, it is possible to use the acetone directly in the next step (ii) without purifying or isolating it, or to combine steps (i) and (ii). However, when the acetone-containing mixture obtained from bioethanol described above contains gas, it may be separated into a gas mainly composed of hydrogen, carbon dioxide, etc. and a liquid mixture mainly composed of acetone (sometimes referred to as gas-liquid separation) by a known gas-liquid separation method. The pressure in the gas-liquid separation operation is preferably 0.1 MPa to 2 MPa, more preferably 0.2 MPa to 1 MPa.

[0061] Furthermore, an operation of absorbing acetone from a gas mainly composed of hydrogen, carbon dioxide, or the like may be performed. The method for absorbing acetone is not particularly limited. For example, the gas may be introduced into an absorption tower, and the acetone in the gas may be absorbed by an absorption liquid supplied from the top of the tower, and the acetone may be recovered as an acetone-containing liquid from the bottom of the tower. Water is preferred as the absorption liquid. The water is not particularly limited, and tap water, industrial water, pure water (RO (reverse osmosis) water, ion-exchanged water, distilled water), etc. may be used. Water used or produced in other processes may also be used or reused. The acetone-containing absorption liquid obtained from the bottom of the absorption tower may be combined with the acetone-based liquid mixture obtained by gas-liquid separation. This improves the recovery rate of acetone.

[0062] Next, purified acetone can be obtained by distilling the acetone-containing mixture, which is a liquid mixture mainly composed of acetone. Distillation can be performed by a known method. Known distillation methods include, for example, thin film distillation and rectification. Distillation can be performed continuously or batchwise, but continuous distillation is preferred from the viewpoint of productivity.

[0063] The purification may involve only gas-liquid separation, or may involve both gas-liquid separation and distillation, or may involve only distillation, but it is more preferable to include a gas-liquid separation step and a distillation step in this order, which allows for the production of more sufficiently purified acetone (sometimes referred to as purified acetone).

[0064] (Impurities of bioacetone) When the above obtained acetone is used in the next step (ii), it is preferable that the aldehydes, alcohols, and ketones (excluding acetone) in the acetone are reduced. In particular, since ethanol and acetaldehyde may become acetic acid through steps (i) to (iv), it is preferable to reduce them from the viewpoint of the odor (acid odor) of the water absorbent resin.

[0065] (Purity of bioacetone) The content of acetone contained in the purified acetone obtained by the purification is preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably 98% by mass or more, relative to 100% by mass of purified acetone. When the acetone obtained above is used in the next step (ii), it is preferable that the aldehydes, alcohols, and ketones (excluding acetone) in the acetone have been reduced. In particular, ethanol and acetaldehyde are preferably reduced because they may become acetic acid after steps (ii) to (iv). The total content of ethanol and acetaldehyde in acetone is preferably 20,000 ppm or less, more preferably 10,000 ppm or less, and even more preferably 5,000 ppm or less. A lower total content of ethanol and acetaldehyde in acetone is preferable, but in terms of the relationship between the yield of acetone after purification and the purification cost, it is, for example, 100 ppm or more, 500 ppm or more, or even 1,000 ppm or more. The total content of ethanol and acetaldehyde in acetone is, for example, 100 ppm or more and 20,000 ppm or less. Using high-purity acetone having a purity and impurity content within the above ranges as a raw material, the acetone reduction reaction in the following step (ii) is carried out, and the isopropanol and gas contained in the obtained product are subjected to gas-liquid separation, thereby easily obtaining high-purity isopropanol. If the total content of ethanol and acetaldehyde in the acetone obtained after step (i) is already within the above range, it is preferable not to perform any special reduction operation. Since the acetone obtained with the above purity has very little or no adverse effect on the water-absorbent resin, isopropanol may remain or be contained in the obtained acetone (e.g., 1 ppm or more, or 10 ppm or more) depending on the purification cost and yield of acetone. The content of these trace components can be measured, for example, by gas chromatography.

[0066] (Step (ii) of Obtaining Isopropanol from Acetone) In the method for producing a water-absorbent resin of the present invention, in order to obtain acrylic acid from bioethanol, step (ii) of obtaining isopropanol from acetone is essential. Note that, industrially, isopropanol is obtained from acetone using the cumene method for phenol production, but in the method for producing a water-absorbent resin of the present invention, in order to solve the problem, acetone obtained from bioethanol in step (i) is used in step (ii). Methods for producing isopropanol from acetone are known, and known methods can be applied to obtain isopropanol (also known as 2-propanol, isopropyl alcohol) from the acetone obtained in step (i). For example, isopropanol can be synthesized from acetone by methods such as those described in Japanese Patent Nos. 5197637, 5300392, 4321838, 2762591, 2723621, and WO2022 / 244797. The disclosures of these publications are incorporated by reference in their entireties.

[0067] Although these patent documents describe the synthesis of isopropanol from acetone, isopropanol is generally used as a solvent and as a starting material for glycerin in chemical products, and the above-mentioned isopropanol production method documents do not suggest the production of a water-absorbent resin from bioethanol, nor the production of acrylic acid from bioethanol through steps (i) to (iv).The present invention is characterized in that, among the countless uses of isopropanol obtained from bioethanol, after steps (i) to (iv) are performed, a water-absorbent resin is further produced through steps (v) to (vii) using bioacrylic acid obtained using ethanol as a raw material.

[0068] In the present invention, it is more preferable that in step (ii), isopropanol is produced by hydrogenating acetone according to the following reaction formula (2).

[0069] Formula 2) CH ３ COCH ３ +H ２ →CH ３ CH(OH)CH ３That is, the acetone obtained in step (i) is reacted with hydrogen to obtain a mixed gas containing isopropanol.

[0070] (Catalyst) The catalyst used in step (ii) is not particularly limited, and examples thereof include Raney catalysts. Other catalysts include, for example, solid catalysts containing metal elements such as Ba, Co, Cr, Cu, Fe, Mn, Ni, Pd, Pt, Zn, Zr, Ru, and Rh. Among these, solid catalysts containing at least one metal element selected from the group consisting of Pt, Ru, Ni, Fe, and Co are preferred, and it is more preferable to use at least one solid catalyst selected from the group consisting of Ru catalysts, Ni—Pt catalysts, Ru—Pt catalysts, and Ni—Ru catalysts. By using a solid catalyst containing these metal elements, the activity inhibitory effect of carbon dioxide in the acetone reduction reaction with hydrogen in step (ii) is suppressed, and acetone hydrogenation proceeds efficiently to produce isopropyl alcohol.

[0071] The catalyst may be in the form of a metal element, alloy, oxide, etc. The catalyst may also be in the form of a mixture of metal elements, a mixture of metal elements and metal oxides, a mixture of metal oxides, or a mixed metal oxide.

[0072] The catalyst may be a catalyst containing a metal element, activated carbon, silica (SiO ２ ), alumina (Al ２ O ３ ), titania (TiO ２ ), zirconia (ZrO ２ ), ceria (CeO ２ The catalyst may be supported on a carrier such as silica (SiO), magnesia (MgO), or diatomaceous earth. ２ ) and zirconia (ZrO ２ The above catalysts may be used alone or in combination of two or more.

[0073] The shape of these catalysts is not particularly limited and may be any shape such as ring-like or spherical.

[0074] In the step (ii), the catalyst may be used alone or in combination of two or more.

[0075] (Reactor / Reaction Conditions) The reaction in step (ii) can be carried out either batchwise or continuously, but from the viewpoint of productivity, a continuous reaction is preferred. The reaction in step (ii) is preferably a gas phase reaction. The reaction format of the gas phase reaction is not particularly limited, and examples include a fixed bed and a fluidized bed, but the more convenient fixed bed format is preferred.

[0076] The pressure for the reaction in step (ii) may be any of reduced pressure, normal pressure, and increased pressure, but is preferably 0.1 MPa to 2 MPa, more preferably 0.1 MPa to 1 MPa.

[0077] The reaction temperature in step (ii) is preferably 20°C to 200°C, more preferably 25°C to 150°C. A lower reaction temperature is advantageous in terms of equilibrium, but tends to make it difficult for hydrogenation to proceed. On the other hand, a higher reaction temperature tends to prevent an increase in the acetone hydrogenation conversion rate due to equilibrium constraints, and in addition, hydrogenolysis of acetone and isopropanol occurs simultaneously, resulting in a decrease in yield.

[0078] When the reaction in step (ii) is carried out as a gas phase reaction, the space velocity of the acetone-containing feed is preferably 200 to 50,000 (1 / h), more preferably 1,000 to 20,000 (1 / h), and even more preferably 2,000 to 10,000 (1 / h).

[0079] (Hydrogen) The hydrogen used may be hydrogen extracted from the gas mainly composed of hydrogen and carbon dioxide obtained in step (i), unreacted hydrogen from step (ii) may be reused, or hydrogen obtained separately. The amount of hydrogen used may be at least equimolar to the amount of acetone, and from the viewpoint of separation and recovery, the preferred range is 1 to 10 times by mole, preferably 1 to 5 times by mole, relative to the amount of acetone.

[0080] (Separation of Isopropanol) When the obtained isopropanol is a gas-liquid mixture containing isopropanol and a gas, it may be separated into a gas mainly composed of a gas such as hydrogen and a liquid mixture containing isopropanol by a known gas-liquid separation method, and then the isopropanol may be recovered. Here, the gas in this section refers to a substance that exists as a gas under pressurized and cooled conditions in the gas-liquid separation operation.

[0081] In the separation of isopropanol, the pressure in the gas-liquid separation operation is preferably 0.1 MPa to 2 MPa, more preferably 0.2 MPa to 1 MPa.

[0082] In the separation of isopropanol, the temperature in the gas-liquid separation operation is preferably 0°C to 50°C, more preferably 5°C to 40°C.

[0083] The isopropanol obtained by the separation (gas-liquid separation) may be supplied to the next step (iii) as it is, or may be further purified by distillation, as necessary, and then supplied to the next step (iii).

[0084] (Purity of the obtained isopropanol) The purity of the isopropanol obtained in step (ii) is preferably 85% by mass or more, more preferably 90% by mass or more, and even more preferably 93% by mass or more, from the viewpoint of improving the yield and purity of propylene in step (iii) described below. In addition, the isopropanol obtained in step (ii) preferably has a concentration of water and acetone as impurities of 10,000 ppm or less, and more preferably 5,000 ppm or less. Since ethanol in isopropanol may become acetic acid through steps (iii) to (iv), the content of ethanol in the isopropanol obtained in step (ii) is preferably 20,000 ppm or less, more preferably 10,000 ppm or less, and even more preferably 5,000 ppm or less. Although it is preferable that the ethanol content in isopropanol is small, in view of the relationship between the yield of acetone after purification and the purification cost, the ethanol content is, for example, 100 ppm or more, 500 ppm or more, or even 1000 ppm or more. The ethanol content in isopropanol is, for example, 100 ppm or more and 20000 ppm or less.

[0085] (Step (iii) of Obtaining Propylene from Isopropanol) In the method for producing a water absorbent resin of the present invention, step (iii) of obtaining propylene from isopropanol is essential in order to obtain acrylic acid from bioethanol. Methods for producing propylene from isopropanol are known, and known methods can be applied to obtain propylene from the isopropanol obtained in the step (ii), for example, the methods described in Japanese Patent Nos. 2764058 and 2799004 can be applied. The disclosures of these publications are incorporated by reference in their entirety.

[0086] Although these patent documents describe the synthesis of propylene from isopropanol, there is no mention of bio-based raw materials, and furthermore, propylene is generally used as a raw material for polypropylene (PP), acrylonitrile (ACN), propylene oxide (PO), alcohol, and cumene, and the above-mentioned documents on the production method of propylene do not suggest the production of a water-absorbent resin from bioethanol, nor the production of acrylic acid from bioethanol via steps (i) to (iv). Among the countless uses of propylene obtained from bioethanol, the present invention is characterized in that, after steps (i) to (iv), a water-absorbent resin is produced from bioacrylic acid obtained using ethanol as a raw material via steps (v) to (vii).

[0087] (Catalyst) Examples of the catalyst used in step (iii) include an alumina catalyst, a silica-alumina catalyst, a magnesia catalyst, a zeolite catalyst, and activated clay. ２ ), tungsten oxide, zirconium oxide (ZrO ２ ) and other catalysts supported on the catalyst. Among these, alumina catalysts are preferred, more preferred than γ-alumina catalysts, and more preferably, catalysts in which tungsten oxide is supported on γ-alumina. These catalysts may be used alone or in combination of two or more.

[0088] The catalyst may be any of the above-mentioned catalysts, which have been subjected to an acid treatment and / or calcination treatment as necessary. The acid treatment is carried out by immersing the catalyst (e.g., a γ-alumina catalyst) in an acid to adjust the acid strength of the catalyst. Examples of acids that can be used include aqueous solutions of hydrochloric acid, nitric acid, boric acid, etc., and carboxylic acids such as acetic acid, formic acid, and oxalic acid.

[0089] The form of the catalyst used is not particularly limited as long as it can form a fixed catalyst layer, and examples thereof include tablet type, ring type, spherical type, cylindrical extrusion type, trefoil extrusion type, granular type, etc. Among these, the spherical type, tablet type, and extrusion type are preferred in that they have high catalyst strength and can be uniformly packed into a reaction tube.

[0090] When the catalyst is a γ-alumina catalyst, a catalyst having an average pore diameter of 3 to 15 nm and a standard deviation of 1 to 4 nm, which is determined by statistical calculation based on the relationship between pore diameter and pore volume, is preferably used.

[0091] (Reactor / Reaction Conditions) The reaction in step (iii) can be carried out in either a batch system or a continuous system, but from the viewpoint of productivity, a continuous system is preferred. The propylene synthesis reaction is preferably carried out as a gas phase reaction. Examples of the gas phase reaction include a fixed bed, a moving bed, and a fluidized bed, but the simpler fixed bed system is preferred.

[0092] The reaction temperature is usually 150 to 500° C., preferably 180 to 400° C. The reaction pressure may be reduced, normal or increased, but it is preferable that the reaction system in the catalyst layer is in a gas phase state.

[0093] (Gaseous Substances Other Than Isopropanol) In addition to isopropanol, the raw material mixture used in step (iii) may contain a gaseous substance inert to the dehydration reaction of isopropanol in order to quickly discharge the reaction product containing propylene produced by the dehydration reaction of isopropanol from the reaction system. Examples of such gaseous substances include nitrogen, helium, argon, etc. Furthermore, this gaseous substance includes substances that are liquid before being supplied to the reactor but become gaseous under the reaction conditions in the reactor. Examples of such substances include pentane, hexane, etc.

[0094] When this inert gaseous substance is mixed with isopropanol and fed to the reactor, the amount used is usually preferably in the range of 0.01 to 15 mol or 0.05 to 10 mol per mol of isopropanol. If the amount of gaseous substance used is too large, a large amount of inert gas must be separated from the reaction product mixture of propylene and water and circulated to the reactor, which may result in economic disadvantages such as increased costs for separation and circulation.

[0095] (Impurities and purification of biopropylene) The product obtained by the dehydration reaction of isopropanol (molecular weight 60.1) has the following approximate composition: propylene (molecular weight 42.08): approximately 70 wt%, water (molecular weight 18.0): approximately 30 wt%, isopropanol: 1 wt% or less, acetone: 1 wt% or less, diisopropyl ether: 1 wt% or less, and other (impurities originally contained in the raw isopropanol), where acetone and diisopropyl ether are by-products of the dehydration reaction of isopropanol. Here, approximately X (X is a numerical value) means that in addition to X itself, X×±10% may be contained.

[0096] In the present invention, propylene can be purified. Specifically, by pressurizing and / or cooling a reaction mixture containing propylene and water as the main components, oil-water separation occurs, forming two phases: an upper propylene layer and a lower water layer. The oil-water separation step allows the removal of a large amount of water produced by the dehydration reaction of isopropanol from the dehydration reaction product.

[0097] The pressure during the pressurization is preferably 5 to 50 kg / cm from the viewpoint of separation and purification costs. ２ G. The reaction product (gaseous) can be easily liquefied by simply cooling it to 20 to 50°C. The propylene obtained by separation (gas-liquid separation) may be supplied to the next step (iv) as is, or may be further purified by distillation as necessary and then supplied to the next step (iv).

[0098] The amount of water dissolved in the oil layer is usually as little as 1000 ppm or less, so it can be easily separated from propylene in the subsequent distillation purification step. As a result, high-purity propylene free from water can be produced. If necessary, the propylene fraction flowing out of the distillation column can be passed in a liquid or gaseous state through a packed bed filled with a commonly used desiccant such as a molecular sieve to obtain propylene that is substantially free of water. In addition to propylene, small amounts of impurities may be removed from the oil layer after oil-water separation by distillation purification to obtain high-purity propylene.

[0099] (Step (iv) of Obtaining Acrylic Acid from Propylene) In the method for producing a water-absorbent resin of the present invention, step (iv) of obtaining acrylic acid from propylene is essential in order to obtain acrylic acid from bioethanol. Methods for producing acrylic acid from propylene are known, and known methods can be applied to obtain acrylic acid from the propylene obtained in step (iii). For example, methods such as those described in Japanese Patent Nos. 3,948,837, 4,520,637, 3,938,646, and 4,765, etc., involve appropriately oxidizing propylene to acrolein and further to acrylic acid in the gas phase, collecting the oxidation product, and purifying it by distillation or crystallization. The disclosures of these publications are incorporated by reference in their entirety.

[0100] It should be noted that these patent documents do not describe bio-raw materials, and do not suggest at all the production of a water absorbent resin from bioethanol, nor the production method of the water absorbent resin of the present invention from bioethanol through the steps (i) to (iv) and the steps (v) to (vii).

[0101] In step (iv), bioacrylic acid is obtained by oxidizing biopropylene, and conventional methods and production facilities for producing acrylic acid by oxidizing propylene obtained by cracking naphtha, a fossil raw material, can be applied as is. That is, in the method for producing acrylic acid used in the present invention, conventional acrylic acid production facilities operating around the world (single-stage or multi-stage gas-phase oxidation of propylene derived from fossil raw materials) are used to obtain acrylic acid from biopropylene via acrolein, collect the acrylic acid, and further distill and / or crystallize it to obtain glacial acrylic acid (acrylic anhydride) and acrylic acid for the water-absorbent resin used in the present invention.

[0102] In the reaction of step (iv), propylene is oxidized by contacting it with a molecular oxygen-containing gas such as oxygen or air in the presence of a known catalyst. The oxidation reaction is usually carried out in two stages. The catalyst used in the first stage reaction is one capable of producing acrolein through the gas phase oxidation of propylene gas, and the catalyst used in the second stage reaction is not particularly limited as long as it is capable of producing acrylic acid through the gas phase oxidation of acrolein gas. (Catalyst) The catalyst used in the first stage reaction includes a solid catalyst containing at least one element selected from Fe, Co, Ni, Mo, Bi, Al, and Si. It preferably contains at least one element selected from Fe, Mo, and Bi, and it preferably contains a composite oxide containing at least one element selected from Fe, Mo, and Bi. It more preferably contains at least one of Mo and Bi, and it is even more preferable that it contains Mo and Bi.

[0103] The catalyst used in the second-stage reaction may be a solid catalyst containing at least one element selected from V, Mo, Cu, W, Sb, Al, and Si. Preferably, the catalyst contains at least one element selected from Mo, V, and W, more preferably at least one of Mo and V, and even more preferably Mo and V. (Reaction Conditions) The acrylic acid synthesis reaction can be carried out in either a batch or continuous manner, but from the viewpoint of productivity, a continuous method is preferred. The reaction temperature in the acrylic acid synthesis reaction is usually in the range of 200 to 400°C. When acrylic acid synthesis is carried out in two stages, the temperatures in the first stage and the second stage may be the same or different, and the temperature in the second stage may be set lower than the temperature in the first stage. When the temperatures in the first stage and the second stage are different, the difference may be, for example, 40 to 60°C.

[0104] (Absorption of Acrylic Acid) Typically, the mixed gas obtained by the reaction in step (iv) is contacted with a collecting liquid (typically water) for collecting acrylic acid from the mixed gas to obtain an aqueous solution containing acrylic acid. The mixed gas may contain acrylic acid, molecular oxygen-containing gas, unreacted components (propylene, acrolein), and by-products (e.g., acetone, acrolein, furfural, formaldehyde, etc.). As described above, the liquid for collecting acrylic acid from the mixed gas is typically water, but other liquids can also be used. In such cases, the aqueous solution containing acrylic acid should be read as an acrylic acid-containing solution. At least one of water and an organic solvent is used as the collecting liquid for acrylic acid in the mixed gas. The organic solvent is at least one organic solvent selected from methyl isobutyl ketone, diisopropyl ketone, methyl propyl ketone, methyl isobutyl ketone, methyl t-butyl ketone, n-propyl acetate, n-butyl acetate, diphenyl ether, diphenyl ether, etc., and preferably water, diphenyl ether, or even water is used as the collecting liquid.

[0105] The water used as the absorbent for absorbing acrylic acid is not particularly limited, and tap water, industrial water, pure water (RO water, ion-exchanged water, distilled water), etc. can be used. These waters may be groundwater, river water, or treated versions of these. Furthermore, the water used may be water used for absorbing acrylic acid or water generated or used in other steps, and may be used as is or after purification.

[0106] (Acrylic purification) Acrylic acid is usually obtained by purifying an aqueous solution containing acrylic acid. The aqueous solution of acrylic acid may contain acrylic acid, acetic acid, water, and other impurities (maleic acid, propionic acid, furfural, formaldehyde, etc.).

[0107] The method for purifying acrylic acid is not particularly limited, and known methods such as distillation and crystallization can be appropriately adopted. Purification may involve distillation alone, crystallization alone, or a combination of distillation and crystallization. Furthermore, distillation may be performed only once, or multiple times in combination. Furthermore, crystallization may be performed only once, or multiple times in combination. Furthermore, distillation and crystallization may be performed continuously or batchwise.

[0108] (Impurities in bioacrylic acid) In step (iv), the propionic acid content in the acrylic acid obtained by propylene oxidation and purification is preferably 500 ppm or less, 400 ppm or less, or 300 ppm or less, and the acetic acid content is preferably 1500 ppm or less, 1000 ppm or less, 500 ppm or less, 300 ppm or less, or 200 ppm or less. The propionic acid content in acrylic acid is, for example, 100 ppm or more, or 200 ppm or more. The propionic acid content in acrylic acid is, for example, 100 ppm or more and 500 ppm or less. The acetic acid content in acrylic acid is, for example, 100 ppm or more, or 200 ppm or more. The acetic acid content in acrylic acid is, for example, 100 ppm or more and 1500 ppm or less. Since the amount of propionic acid (more preferably acetic acid) in acrylic acid is small, the odor (acid odor) of the obtained water absorbent resin can be reduced. Also, the yield of the water absorbent resin (ratio of the obtained water absorbent resin to the used acrylic acid) is improved.

[0109] Of the six impurities in the acrylic acid, namely protoanemonin, allyl acrylate, allyl alcohol, aldehydes (particularly furfural), maleic acid, and benzoic acid, preferably one or more, more preferably two or more, even more preferably three or more, even more preferably four or more, particularly preferably five or more, and especially preferably all six impurities are each present at 0 to 20 ppm (by mass, the same applies hereinafter). Preferably, each impurity is present at 0 to 10 ppm, more preferably 0 to 5 ppm, even more preferably 0 to 3 ppm, particularly preferably 0 to 1 ppm, and most preferably N.D. (below the detection limit). Among these impurities, aldehydes may increase in acrylic acid derived from biomaterials, and it is preferable to control them to reduce their content. Control methods include using an aldehyde treatment agent (e.g., hydrazine) or performing crystallization. The total amount of protoanemonin, allyl acrylate, allyl alcohol, aldehyde, maleic acid, and benzoic acid (relative to the mass of acrylic acid) is preferably 100 ppm or less, more preferably 0 to 20 ppm, further preferably 0 to 10 ppm, and particularly preferably 0 to 5 ppm. 0 ppm means N.D. The content of these trace components can be measured, for example, by gas chromatography.

[0110] To avoid solidification of acrylic acid (melting point 14°C) in winter, the obtained acrylic acid may be converted into an aqueous solution (e.g., an 80 wt% aqueous acrylic acid solution) due to its ease of handling as a liquid. However, water in acrylic acid promotes the formation of acrylic acid dimer, and the increase in acrylic acid dimer increases the amount of residual monomer in the water-absorbent resin. Therefore, the preferred water content of acrylic acid is 2 wt% or less, 1 wt% or less, 0.5 wt% or less, 0.3 wt% or less, 0.1 wt% or less, and 0.05 wt% or less, in that order. A small amount of water has little adverse effect on the water-absorbent resin, and the water content of acrylic acid is, for example, 10 ppm or more or 50 ppm or more, taking into account the balance with purification costs. The water content of acrylic acid is, for example, 10 ppm or more and 2 wt% or less. For the same reason, the amount of acrylic acid dimer in acrylic acid is preferably 1000 ppm or less, 500 ppm or less, or 200 ppm or less. The amount of acrylic acid dimer in the acrylic acid to be supplied to the step (v) described below is, for example, 1 ppm or more, for example, 1 ppm or more and 1000 ppm or less, further 500 ppm or less, 200 ppm or less, particularly 100 ppm or less.

[0111] It is desirable that these impurities in acrylic acid are N.D. However, since it is difficult to completely remove them even by the steps (i) to (iv), acrylic acid containing a certain amount of impurities may be used in the step (v), and at least a part of the impurities in the acrylic acid (for example, acetic acid and propionic acid in acrylic acid) may be removed in the step (v) and / or the step (vi) by heating in the production step of a water-absorbent resin. For example, at least a part of one or more selected from acrylic acid, acetic acid, and propionic acid remaining in the reaction system may be removed by the polymerization heat generated in the step (v).

[0112] The acrylic acid obtained from bioethanol of the present invention through steps (i) to (iv) can be suitably used for water-absorbent resins because it can reduce the organic acids (especially acetic acid and propionic acid, especially propionic acid) that tend to increase in conventional bioacrylic acid. In addition, it contains few other impurities, and as described below, it can provide water-absorbent resins that are equivalent to or better than acrylic acid derived from fossil raw materials.

[0113] Therefore, one aspect of the present invention is a method for using bioacrylic acid derived from a biomaterial, obtained by the following steps (i) to (iv), as a monomer for a water-absorbent resin: step (i) of obtaining acetone from bioethanol; step (ii) of obtaining isopropanol from the acetone; step (iii) of obtaining propylene from the isopropanol; and step (iv) of obtaining acrylic acid from the propylene.

[0114] (Polymerization Inhibitor) The obtained acrylic acid may contain a polymerization inhibitor, for example, 1 to 300 ppm, 10 to 200 ppm, or 20 to 80 ppm of a polymerization inhibitor, particularly p-methoxyphenol.

[0115] (Step (v) of obtaining polyacrylic acid (salt) by polymerizing an aqueous monomer solution containing acrylic acid) (Monomer and acrylic acid) In the present invention, the acrylic acid obtained in the above step (iv) is essentially used as the monomer of the water-absorbent resin. The water-absorbent resin of the present invention is a crosslinked polymer obtained by crosslinking polymerizing a monomer containing acrylic acid and / or its salt (hereinafter referred to as "acrylic acid (salt)") as the main component, for example, polyacrylic acid and / or its salt (hereinafter referred to as "polyacrylic acid (salt)") optionally containing a graft component. In terms of the performance of the water-absorbent resin, the proportion of acrylic acid is preferably 50 to 100 mol %, more preferably 70 to 100 mol %, and particularly preferably 90 to 100 mol % of the total monomers.

[0116] In the present invention, in addition to the acrylic acid obtained in step (iv), other acrylic acids may be used in combination. Examples of the acrylic acids used in combination include acrylic acid from fossil raw materials, other bioacrylic acids obtained from sources other than bioethanol (e.g., the bioacrylic acids described in Patent Documents 1 to 14), and even conventional acrylic acids from fossil raw materials. The ratio of the two acrylic acids used in combination can be determined appropriately. However, when the acrylic acid obtained in step (iv) is used in combination with other acrylic acids, the higher the ratio of the acrylic acid obtained in step (iv) is, the more preferable it is in terms of performance, sustainability, and renewability. The acrylic acid obtained in steps (i) to (iv) is preferably 1 mol% or more, 5 mol% or more, 10 mol% or more, 20 mol% or more, 30 mol% or more, 40 mol% or more, 50 mol% or more, 60 mol% or more, and 70 mol% or more of the total acrylic acid. The acrylic acid obtained in steps (i) to (iv) may account for 80 mol% or more, 90 mol% or more, or 95 mol% or more of the total acrylic acid. The upper limit depends on factors such as the production capacity of the acrylic acid obtained in step (iv), but may be less than 100 mol%, 95 mol% or less, or 90 mol% or less. The usage ratio of the acrylic acid obtained in step (iv) is, for example, 1 mol% or more and 100 mol% or less. The usage ratio of the acrylic acid obtained in step (iv) is, for example, 1 mol% or more and less than 100 mol%. In the present invention, an example of a method for using the acrylic acid obtained in step (iv) in combination with another acrylic acid is mixing the acrylic acid obtained in step (iv) with another acrylic acid (acrylic acid derived from a fossil raw material).

[0117] Furthermore, the acrylic acid obtained in the step (iv) is preferably used in the step (v) in a short time after purification in order to prevent an increase in residual monomers in the water absorbent resin, and the time between the step (iv) and the step (v) is preferably within 10 days (particularly including transportation and storage), more preferably within 5 days, 2 days, or 1 day. Furthermore, the acrylic acid until use in the step (v) is preferably stored at room temperature or below (preferably 35°C or below, more preferably 30°C to the melting point or above), and further preferably stored and transported in an oxygen or air atmosphere.

[0118] In the water-absorbent resin of the present invention, the neutralization rate of the acid groups of polyacrylic acid or the like is preferably 10 mol% or more, more preferably 40 mol% or more, even more preferably 50 mol% or more, and particularly preferably 60 mol% or more, in terms of water absorption performance. In the water-absorbent resin of the present invention, the neutralization rate of the acid groups of polyacrylic acid or the like is preferably 90 mol% or less, more preferably 85 mol% or less, even more preferably 80 mol% or less, and particularly preferably 75 mol% or less, in terms of water absorption performance. In the water-absorbent resin of the present invention, the neutralization rate of the acid groups of polyacrylic acid or the like is, for example, 10 mol% or more and 90 mol% or less, in terms of water absorption performance. Neutralization may be performed on the monomer, on the hydrogel after polymerization, or a combination of these. Examples of neutralization salts include alkali metal salts such as sodium, potassium, and lithium, ammonium salts, and amine salts.

[0119] The monomer for obtaining the water-absorbent resin of the present invention may be substantially acrylic acid (salt) alone. Furthermore, the water-absorbent resin may be obtained by using other unsaturated monomers in combination with acrylic acid (salt) (for example, 0 to 50 mol %, more than 0 mol % but not more than 48 mol %, or even 5 to 45 mol %, when the total amount of monomers is taken as 100 mol %). Monomers (other unsaturated monomers) other than acrylic acid (salt) are not particularly limited, but specific examples include methacrylic acid, maleic acid, itaconic acid, 2-(meth)acrylamido-2-methylpropanesulfonic acid, (meth)acrylamide, and the like. These other unsaturated monomers may be used alone or in a suitable mixture of two or more types. Among these, itaconic acid is preferred when used in combination with other monomers because it can be obtained by a fermentation method and therefore contributes to the use of bio-based raw materials.

[0120] Specific examples of the internal crosslinking agent include N,N'-methylenebis(meth)acrylamide, (poly)ethylene glycol di(meth)acrylate, (poly)propylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, glycerin tri(meth)acrylate, glycerin acrylate methacrylate, ethylene oxide-modified trimethylolpropane tri(meth)acrylate, pentaerythritol hexa(meth)acrylate, triallyl cyanurate, triallyl isocyanurate, triallyl phosphate, triallylamine, poly(meth)allyloxyalkane, (poly)ethylene glycol diglycidyl ether, glycerol diglycidyl ether, ethylene glycol, polyethylene glycol, propylene glycol, glycerin, pentaerythritol, ethylenediamine, ethylene carbonate, propylene carbonate, polyethyleneimine, and glycidyl (meth)acrylate. According to a preferred embodiment, the average number of polyethylene glycol units in the (poly)ethylene glycol di(meth)acrylate is 7 to 11. From the viewpoint of physical properties, the amount of these internal crosslinking agents used is preferably within the range of 0.001 to 2 mol %, more preferably 0.005 to 0.5 mol %, even more preferably 0.01 to 0.2 mol %, and particularly preferably 0.03 to 0.15 mol %, relative to the monomer (excluding the crosslinking agent).

[0121]

[0044] In the above polymerization, 0 to 50% by weight (relative to the monomer) of a hydrophilic polymer (particularly a fine powder of a water-absorbent resin) such as starch / cellulose, a starch / cellulose derivative, polyvinyl alcohol, a linear polyacrylic acid (salt), or a crosslinked polyacrylic acid (salt) may be added to the reaction system, and 0 to 10% by weight, or further 0 to 1% by weight (relative to the monomer) of various foaming agents such as (hydrogen) carbonates, carbon dioxide, azo compounds, and inert organic solvents; various surfactants; chelating agents; chain transfer agents such as hypophosphorous acid (salts); etc. may be added. Note that, since natural polymers have an adverse effect on the heat resistance and performance of a water-absorbent resin as described above, it is preferable to use a small amount (for example, 10% by weight or less) or not use any natural polymers, even when they are used optionally.

[0122] According to a preferred embodiment, the chelating agent is an amino polycarboxylic acid or an amino polyphosphate. Suitable amino polycarboxylic acids include compounds having 2 to 100, preferably 3 to 20, 4 to 10, or 5 to 8 carboxyl groups. Specific examples include iminodiacetic acid, hydroxyethyliminodiacetic acid, nitrilotriacetic acid, nitrilotripropionic acid, ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid, triethylenetetraminehexaacetic acid, trans-1,2-diaminocyclohexanetetraacetic acid, N,N-bis(2-hydroxyethyl)glycine, diaminopropanoltetraacetic acid, ethylenediaminedipropionic acid, N-hydroxyethylethylenediaminetriacetic acid, glycoletherdiaminetetraacetic acid, diaminopropanetetraacetic acid, N,N'-bis(2-hydroxybenzyl)ethylenediamine-N,N'-diacetic acid, 1,6-hexamethylenediamine-N,N,N',N'-tetraacetic acid, and salts thereof. Examples of amino polyphosphates include ethylenediamine-N,N'-di(methylenephosphinic acid), ethylenediaminetetra(methylenephosphinic acid), nitriloacetic acid-di(methylenephosphinic acid), nitrilodiacetic acid-(methylenephosphinic acid), nitriloacetic acid-β-propionic acid-methylenephosphonic acid, nitrilotris(methylenephosphonic acid), cyclohexanediaminetetra(methylenephosphonic acid), ethylenediamine-N,N'-diacetic acid-N,N'-di(methylenephosphonic acid), ethylenediamine-N,N'-di(methylenephosphonic acid), ethylenediaminetetra(methylenephosphonic acid), polymethylenediaminetetra(methylenephosphonic acid), diethylenetriaminepenta(methylenephosphonic acid), 1-hydroxyethylidene diphosphonic acid, and salts thereof. The use of a chelating agent (preferably at 1 ppm or more, further 10 ppm or more relative to the monomer, with an upper limit of, for example, 1 wt % or less, further 0.5 wt % or less) is preferable because it further stabilizes the polymerization of acrylic acid and allows a water-absorbent resin with better performance to be obtained. Such a chelating agent may be added in or after step (v) as described later to improve performance.

[0123] The monomers may be bulk polymerized (solvent-free polymerized), but are preferably polymerized as an aqueous solution in terms of water absorption performance. From the viewpoint of the physical properties of the water-absorbent resin, the total concentration of the monomer components in the aqueous monomer solution is preferably 10% by weight or more, more preferably 20% by weight or more, and even more preferably 30% by weight or more. The concentration is preferably 80% by weight or less, more preferably 75% by weight or less, and even more preferably 70% by weight or less. The total concentration of the monomer components in the aqueous monomer solution is, for example, 10% by weight or more and 80% by weight or less. To promote polymerization, the aqueous monomer solution during polymerization may be heated or an inert gas may be introduced to reduce the dissolved oxygen (preferably to 5 ppm or less, or even 2 ppm or less).

[0124] (Polymerization method) The polymerization mode applicable to the embodiment of the present invention is not particularly limited, but from the viewpoint of water absorption characteristics, ease of polymerization control, etc., preferred examples include spray droplet polymerization, aqueous solution polymerization, reversed-phase suspension polymerization, droplet polymerization, bulk polymerization, and precipitation polymerization. More preferred is aqueous solution polymerization or reversed-phase suspension polymerization, even more preferred is aqueous solution polymerization, and even more preferred is continuous aqueous solution polymerization. Continuous aqueous solution polymerization is particularly preferred, and either continuous belt polymerization or continuous kneader polymerization can be applied. The polymerization may be neutralization polymerization of the monomer, or acid polymerization of the monomer followed by post-neutralization.

[0125] Representative examples of the acid polymerization followed by neutralization method include, but are not limited to, JP-A-10-101735, JP-A-01-103606, JP-A-62-054751, JP-A-2002-527547, and JP-A-03-174414. Representative examples of the reverse phase suspension polymerization method include, but are not limited to, JP-A-57-158209, JP-A-61-087702, JP-A-03-227301, JP-A-11-005808, WO2004 / 083284, WO2009 / 025235, WO13 / 018571, WO2022 / 265459, and WO2022 / 265459. The disclosures of these publications are incorporated herein by reference in their entirety.

[0126] The polymerization initiator used in the embodiment of the present invention is appropriately selected depending on the type of monomer to be polymerized, polymerization form, etc., and is not particularly limited, as one or more types may be selected and used from those used in the production of ordinary water-absorbent resins.

[0127] Examples of the polymerization initiator include a thermally decomposable polymerization initiator, a photodecomposable polymerization initiator, or a redox-based polymerization initiator used in combination with a reducing agent that promotes the decomposition of these polymerization initiators. Specifically, one or more of the polymerization initiators disclosed in U.S. Pat. No. 7,265,190 are used. From the viewpoint of ease of handling of the polymerization initiator and the physical properties of the water-absorbent resin, preferably a peroxide or an azo compound is used, more preferably a peroxide, and even more preferably a persulfate. The peroxide is preferably selected from potassium persulfate, ammonium persulfate, sodium persulfate, t-butyl hydroperoxide, and hydrogen peroxide. The temperature at the start of polymerization is preferably, for example, about 50 to 100°C.

[0128] The amount of the polymerization initiator used is preferably 0.001 mol % or more, more preferably 0.01 mol % or more, and preferably 1 mol % or less, more preferably 0.5 mol % or less, and even more preferably 0.1 mol % or less, based on the total moles of the monomers excluding the internal crosslinking agent. The amount of the reducing agent used is preferably 0.0001 to 0.02 mol % based on the total moles of the monomers excluding the internal crosslinking agent.

[0129] Instead of using the above polymerization initiator, the polymerization reaction may be carried out by irradiation with active energy rays such as radiation, electron beams, and ultraviolet rays, or these active energy rays may be used in combination with the polymerization initiator.

[0130] The reaction temperature in the polymerization reaction is not particularly limited, but the temperature range from the lowest temperature to the highest temperature (peak temperature) of the polymerization reaction is preferably within the range of 15 to 130°C, and more preferably within the range of 20 to 120°C. The reaction time and polymerization pressure are also not particularly limited and may be set appropriately depending on the type of monomer and polymerization initiator, the reaction temperature, etc. The polymerization rate is usually 95% or higher, preferably 98% or higher, and particularly 99% or higher. The upper limit of the polymerization rate is 100%, but to avoid long polymerization times in consideration of productivity, the upper limit of the residual monomer may be 0.05% by weight, or even 0.1% by weight or 0.5% by weight.

[0131] Furthermore, the polymerization temperature and pressure can also be selected appropriately, with the minimum and maximum temperatures being selected within the range of 20°C to the boiling point, further 50°C to the boiling point, or 70°C to the boiling point.

[0132] In the present invention, high-purity acrylic acid suitable for water-absorbent resins is obtained from bioethanol through steps (i) to (iv). Furthermore, preferably, the maximum polymerization temperature in step (v) is set to a temperature at which water easily volatilizes from the reaction system, for example, 105°C or higher, or 110°C or higher. The upper limit of the maximum polymerization temperature in step (v) is, for example, 130°C or 120°C. The maximum polymerization temperature in step (v) is set to, for example, 105°C or higher and 130°C or lower. By doing so, during polymerization (step (v)), at least a portion of the impurities (e.g., acetic acid and propionic acid) in the acrylic acid can be further volatilized and removed as water volatilizes. Here, "at least a portion" refers to, for example, 1% by weight or higher, 5% by weight or higher, or 10% by weight or higher of the impurities. By setting the maximum polymerization temperature in step (v) to a temperature at which water easily volatilizes from the reaction system, excessive purification of the impurities in the acrylic acid in step (iv) is not necessary. From the viewpoints of performance and impurity removal, the rate of water loss due to volatilization of water during polymerization (the rate of reduction in the water content (wt%) of the obtained hydrogel relative to the water content (wt%) of the monomer) is preferably 1 to 20 wt%, more preferably 2 to 15 wt%, and even more preferably 3 to 10 wt%. Furthermore, in the present invention, acrylic acid of a purity equivalent to or higher than that of acrylic acid derived from fossil raw materials can be obtained compared to conventional acrylic acid. Therefore, excessive purification of the obtained acrylic acid is not required, and the amount of further impurities removed in at least one of steps (iv), (v), and (vi) can be reduced. For example, when impurities in acrylic acid are removed in at least one of steps (iv), (v), and (vi), the amount removed is, for example, 0.1 wt% or less, or 0.01 wt% or less, based on the mass of the acrylic acid. This results in improved yields of acrylic acid and water-absorbent resin. The water content of the obtained hydrogel may be adjusted by evaporating a portion of the water during polymerization. In this case, the water content is preferably in the range of the water content of the above-mentioned monomer, for example, about 20 to 80% by weight, more preferably about 30 to 75% by weight, from the viewpoint of performance.

[0133] (Optional Aging Step) An aging step may be carried out as an optional step after the polymerization step. In the aging step, the hydrogel after polymerization is removed from the polymerizer, and the hydrogel having the above polymerization rate is stored under heating, preferably at a temperature of 40 to 100°C or 50 to 90°C (for example, for 1 minute to 5 hours), to improve the polymerization rate and molecular weight.

[0134] (Optional Gel Crushing Step) The gel crushing step is a step of kneading or crushing the hydrogel (hydrogel-like crosslinked polymer) obtained in the polymerization step to obtain a particulate hydrogel granulated to a predetermined size. For example, the hydrogel is crushed (also referred to as gel crushing) with a gel crusher such as a kneader, a screw extruder such as a meat chopper, or a cutter mill to obtain a particulate hydrogel (hereinafter referred to as "particulate hydrogel"). When the polymerization step is kneader polymerization, the polymerization step and the gel crushing step are carried out simultaneously. In addition, when the particulate hydrogel is obtained directly during the polymerization process, such as gas phase polymerization or reverse phase suspension polymerization, the gel crushing step may not be carried out. Note that, before the gel crushing step, a step of cutting the hydrogel to an appropriate size may be optionally provided. In addition, the performance of the water-absorbent resin may be improved by mixing the additives used in the polymerization or various additives described below with the hydrogel in the gel crushing step.

[0135] For uniform and efficient drying (and further removal of impurities in acrylic acid during drying), the average particle size of the particulate hydrogel may be 5 mm or less, further 2 mm or less, and particularly 1 mm or less. The water content of the particulate hydrogel is preferably 30 wt % or more, more preferably 45 wt % or more. The water content of the particulate hydrogel is preferably 70 wt % or less, more preferably 55 wt % or less. The water content of the particulate hydrogel may be, for example, 30 wt % to 70 wt %, 30 wt % to 55 wt %, or 45 wt % to 55 wt %.

[0136] (Optional Acrylic Acid Recycling Step) Acrylic acid (boiling point 141°C) may volatilize in the polymerization step (step (v)) or the drying step (step (vi)) described later. The volatilized acrylic acid may be discarded, but this may be harmful from an environmental standpoint and may also result in CO ２From the viewpoint of reducing CO₂ emissions and achieving carbon neutrality, the volatilized acrylic acid is preferably recycled, for example, by being collected. Acrylic acid may be collected, for example, by using water or alkaline water, or by cooling. In the present invention, a method for recycling acrylic acid involves using the collected bioacrylic acid or its aqueous solution (or alkaline aqueous solution) for polymerization in step (v). The amount of acrylic acid recycled is determined appropriately, but is, for example, 0 to 20%, or even 0.01 to 10%, of the acrylic acid used in polymerization.

[0137] (Step (vi) of drying polyacrylic acid (salt)) The hydrogel crosslinked polymer obtained in the above step (v) can be dried to a desired water content. Note that both steps (v) and (vi) may be carried out by continuously carrying out polymerization and drying by evaporating a portion of the water in the hydrogel crosslinked polymer using the heat of polymerization in step (v). However, from the viewpoint of performance, it is preferable to separately provide a step (vi) of drying after the completion of step (v). The preferred water content (%) at the start of drying in step (vi) is in the above-mentioned range. That is, the water content of the particulate hydrogel at the start of drying may be 30 to 70% by weight, or 45 to 55% by weight, etc. In terms of reducing coloration, reducing residual monomers, and removing impurities in acrylic acid, drying is preferably initiated (put into a dryer) within 2 hours, more preferably within 1 hour, after the end of polymerization (after discharge from the polymerizer, or after discharge from the gel crusher if a gel crushing step has been performed), and may be initiated (put into a dryer) within 0.5 hours or even 0.2 hours. Drying is usually carried out at a temperature range of 60 to 250°C, preferably 100 to 220°C, more preferably 120 to 200°C, and even more preferably 150 to 190°C. The drying time is preferably about 0.1 to 5 hours. The temperature, air volume, and dew point during drying may be constant or may be varied in multiple stages. Suitable drying methods include azeotropic dehydration in a hydrophobic organic solvent, hot air drying (particularly ventilated band drying), agitation drying in a rotary agitator vessel (e.g., steam tube dryer, rotary kiln), agitation drying in a heat transfer dryer with agitating blades (e.g., paddle dryer), and fluidized bed drying. When a plurality of drying methods are used in combination, an intermediate step of crushing the dried material (semi-dried material) may be provided to promote drying, or a portion of the undried material may be removed.

[0138] According to one embodiment, the drying time depends on the surface area of ​​the polymer, the moisture content, the type of dryer, etc., and is selected so as to achieve a target moisture content, but from the viewpoint of physical properties such as reduction of residual monomers and removal of impurities in acrylic acid, hot air drying is preferably carried out for 0.1 to 5 hours using hot air containing water vapor and having a dew point of 50 to 100° C., more preferably hot air containing water vapor and having a dew point of 60 to 90° C. The moisture content of the water absorbent resin in the present invention (defined as the amount of moisture contained in the water absorbent resin / measured as loss on drying at 180° C. for 3 hours) is not particularly limited, but from the viewpoint of physical properties of the obtained water absorbent resin product, it is preferably a powder that exhibits fluidity even at room temperature, more preferably a powder state of 0.2 to 30 wt %, still more preferably 0.3 to 15 wt %, and particularly preferably 0.5 to 10 wt %. The surface cross-linking step (vii) described later may be carried out after the completion of drying, or may be carried out simultaneously with the start of the drying step (vi) or during the drying step (vi) (for example, when the water content is 10 to 40% by weight, further 15 to 30% by weight). In addition, the water may be removed by polymerization heat in the polymerization step (v) to perform a part or all of the drying step (vi), but preferably, the drying step (vi) is separately provided after the polymerization step (v), and more preferably, the surface cross-linking step (vii) is separately provided after the drying step (vi).

[0139] In the drying, the heat for heating the hot air and the heat transfer part is preferably the heat of oxidation in step (iv) for obtaining acrylic acid from propylene. Preferably, a heat medium heated with the heat of oxidation, typically steam, is supplied to the dryer to heat the hot air and the heat transfer part. If the heat medium used in the drying still has heat, it may be reheated as necessary and reused as a heat medium for drying, or may be used to heat and keep warm the equipment in step (v) and thereafter. In addition, the heat medium may be distributed between the drying and other steps in step (v) and thereafter.

[0140] (Removal of the above-mentioned acrylic acid impurities in the drying step) In the present invention, high-purity acrylic acid suitable for water-absorbent resins is obtained from bioethanol through steps (i) to (iv). Furthermore, preferably, by drying at high temperature and high dew point (for example, 120 to 200 ° C., 150 to 190 ° C., with a dew point of 50 to 100 ° C.) in step (vi), at least a portion (1 wt % or more, or even 5 wt % or more) of impurities in the acrylic acid (for example, acetic acid (boiling point 118 ° C.) or propionic acid (boiling point 141 ° C.)) can be removed by volatilization during drying. Therefore, there is no need to excessively purify the impurities in the bioacrylic acid in step (iv).

[0141] Furthermore, in the present invention, bioacrylic acid of a purity equivalent to or higher than that of acrylic acid derived from fossil raw materials can be obtained compared to conventional bioacrylic acid, and therefore excessive purification of the bioacrylic acid is not required. Furthermore, the amount of further impurities removed in steps (iv) to (vi) can be reduced. For example, the amount of removed impurities is 0.1 wt % or less, 0.05 wt % or less, or 0.01 wt % or less of the bioacrylic acid. As a result, CO ２ This also leads to a reduction in the amount of wastewater and an improvement in the yield of acrylic acid and the resulting water-absorbent resin.

[0142] In some applications of water-absorbent resins, the step (vii) of surface-crosslinking polyacrylic acid (salt) described below may not be carried out, and the final product may be obtained up to the step (vi) of drying the polyacrylic acid and / or its salt. In such cases, one aspect of the present invention is a method for producing a water-absorbent resin derived from biomaterials, comprising the following steps (i) to (vi): step (i) of obtaining acetone from bioethanol; step (ii) of obtaining isopropanol from the acetone; step (iii) of obtaining propylene from the isopropanol; step (iv) of obtaining acrylic acid from the propylene; step (v) of polymerizing an aqueous monomer solution containing acrylic acid to obtain polyacrylic acid and / or its salt; and step (vi) of drying the polyacrylic acid and / or its salt.

[0143] (Optional pulverization step) The production method according to the present invention may include a pulverization step of pulverizing the hydrogel-like crosslinked polymer obtained in the polymerization step in the drying step, and then pulverizing the hydrogel-like crosslinked polymer in a pulverizer to form a particulate water-absorbent resin, as necessary. In particular, when aqueous solution polymerization is performed in the polymerization step, it is preferable to include the pulverization step.

[0144] (Optional Classification Step Before or After Surface Crosslinking) In the present invention, preferably, classification is further performed in a classification step to adjust the powder particle size according to the purpose. The classification step is preferably performed after the drying step, more preferably after the pulverization step. Furthermore, when a surface crosslinking step is included, the classification step is preferably performed before the surface crosslinking step, and it is more preferable that a second classification step is also performed after the surface crosslinking step.

[0145] The particle size of the water-absorbent resin obtained in the present invention varies depending on the application, but for example, when used in disposable paper diapers, which is a main application of the water-absorbent resin, the weight-average particle size (defined by sieve classification) after classification or as a final product is preferably in the range of 200 to 700 μm, more preferably in the range of 250 to 600 μm, particularly preferably in the range of 300 to 500 μm.

[0146] The water-absorbent resin obtained in the present invention varies depending on the application, but for example, when used in disposable paper diapers, which are a major application of water-absorbent resins, it is preferable that the water-absorbent resin contains 95 to 100 wt % of water-absorbent resin powder having a size of 850 to 150 μm (passing an 850 μm standard sieve but not passing a 150 μm standard sieve / standard sieve is JIS or an equivalent) after classification or as a final product. The water-absorbent resin obtained in the present invention preferably has a low proportion of fine powder (for example, preferably less than 100 μm, more preferably less than 150 μm), specifically less than 5.0 wt %, further less than 3.0 wt %, and particularly less than 1.0 wt %. For example, the proportion of fine powder polyacrylic acid and / or a salt thereof in the polyacrylic acid and / or a salt thereof provided to step (iv) is less than 5.0 wt %, less than 3.0 wt %, or less than 1.0 wt %. Furthermore, the water absorbent resin obtained in the present invention preferably has a low proportion of coarse particles (for example, preferably substantially 1000 μm sieve or larger, more preferably 850 μm sieve or larger), specifically preferably 5.0 wt % or less, more preferably 1.0 wt % or less. For example, the proportion of coarse particles of polyacrylic acid and / or a salt thereof in the polyacrylic acid and / or a salt thereof supplied to the step (iv) is 5.0 wt % or less, or 1.0 wt % or less.

[0147] (Suitable Fine Powder Recovery Step) The present invention may include a step of recovering fine powder of a water absorbent resin as a production step not described in Patent Document 1 and the like. Fine powder recovery can result in a more carbon-neutral production method. In the fine powder recovery step, fine powder is removed from the water absorbent resin before surface cross-linking and / or the water absorbent resin after surface cross-linking, and recycled to the production step of a water absorbent resin. That is, after the step (v), a part of the water absorbent resin is separated and can be recycled to the step (v) and / or the step (vi). The fine powder recovery is preferably recycled before the drying step, and is recycled to at least one step of the polymerization step (step (v)), the gel crushing step, and the drying step (step (vi)). The recycled fine powder can be recycled to the production step of a water absorbent resin as a dry powder and / or as a water-swellable gel after hydration.

[0148] The recovery of fine powder in the polymerization process is described in WO92 / 01008, WO92 / 020723, WO10 / 046267, WO11 / 101188, etc., in which fine powder is mixed with a monomer and polymerized. The recovery of fine powder in a hydrogel after polymerization is described in JP-A-03-152104, JP-A-04-227934, JP-A-04-041532, etc., in which fine powder or its hydrate is mixed with the hydrogel obtained in the polymerization process and dried. The recovery of fine powder in the granulation process is described in EP0885917A2, WO2015 / 088242A1, WO2017 / 010660A1, WO2019 / 194399A, etc., in which the granulated fine powder is further dried (generally recycled to the drying process). These fines recovery processes generally recycle the fines prior to the drying step, the disclosures of which are incorporated by reference in their entirety.

[0149] The fine powder to be recovered is the fine powder removed in the above classification step, and is preferably fine powder of less than 150 μm (defined by a standard sieve), and is fine powder containing 50% by weight or more, further 70% by weight or more, particularly 90% by weight or more of particles of less than 150 μm. The amount of the fine powder is appropriately determined in the range of 1 to 40% by weight, further 2 to 35% by weight, and further 5 to 30% by weight of the water absorbent resin to be produced.

[0150] (Step (vii) of surface cross-linking polyacrylic acid (salt)) In the present invention, preferably, surface cross-linking is further carried out by the surface cross-linking step (vii). As described above, the surface cross-linking step (vii) may be carried out after the completion of the drying step (vi), or the drying step (vi) and the surface cross-linking step (vii) may be carried out simultaneously at the start of the drying step (vi) or during the drying step (vi).

[0151] Surface cross-linking is an operation for improving various physical properties by increasing the cross-linking density near the surface of a water-absorbent resin compared to the interior of the particle. In addition to internal cross-linking, various surface cross-linking agents (which act as second cross-linking agents for internal cross-linking agents) are added to the water-absorbent resin to cross-link only the surface or the surface layer. This process improves the water absorption performance under pressure, which is necessary for use in disposable paper diapers, which are the main application of water-absorbent resins. The surface cross-linking agent is not particularly limited, but a cross-linking agent that reacts with a carboxyl group, and in particular, a dehydration-reactive cross-linking agent, is preferably used.

[0152] More specifically, examples of the dehydration-reactive crosslinking agent include polyhydric alcohol compounds such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, 1,3-propanediol, 2-methyl-1,3-propanediol, glycerin, 1,4-butanediol, 1,5-pentanediol, and 1,6-hexanediol; aminoalcohol compounds such as ethanolamine, diethanolamine, and triethanolamine; alkylene carbonate compounds such as 1,3-dioxolan-2-one (ethylene carbonate) and 4-methyl-1,3-dioxolan-2-one; oxetane compounds such as 3-methyl-3-oxetanemethanol, and polyhydric oxetane compounds. Among these, in order to maximize the effects of the present invention, one or more dehydration-reactive crosslinking agents selected from polyhydric alcohols, alkylene carbonates, oxazolidinone compounds, and (polyhydric)oxetane compounds are preferred, with polyhydric alcohols and alkylene carbonates being particularly preferred.

[0153] Examples of the surface crosslinking agent include, in addition to these dehydration-reactive crosslinking agents, non-dehydration-reactive crosslinking agents, for example, epoxy compounds such as ethylene glycol diglycidyl ether and γ-glycidoxypropyltrimethoxysilane; polyvalent isocyanate compounds such as 2,4-tolylene diisocyanate; polyvalent oxazoline compounds such as 1,2-ethylenebisoxazoline; silane coupling agents such as γ-aminopropyltrimethoxysilane; polyvalent aziridine compounds such as 2,2-bishydroxymethylbutanol-tris[3-(1-aziridinyl)propionate]; and polyvalent metals such as beryllium, magnesium, calcium, strontium, zinc, aluminum, iron, chromium, manganese, titanium, and zirconium.

[0154]

[0223] In consideration of physical properties, the amount of use of a surface crosslinking agent (when two or more kinds are used, the total amount) is preferably in the range of 0.001 to 10 parts by mass, more preferably in the range of 0.01 to 8 parts by mass, still more preferably in the range of 0.05 to 5 parts by mass, and most preferably in the range of 0.1 to 2 parts by mass, relative to 100 parts by mass of a water absorbent resin (preferably, 100 parts by mass of a water absorbent resin in a dry form).

[0155] When mixing the surface cross-linking agent with the water-absorbent resin, water and / or a hydrophilic organic solvent may be used. The amount of water used is preferably 0.1 to 10 parts by mass, more preferably 0.5 to 8 parts by mass, and even more preferably 1 to 5 parts by mass, relative to 100 parts by mass of the water-absorbent resin (preferably 100 parts by mass of the water-absorbent resin in a dried form). Examples of the hydrophilic organic solvent include alcohols such as ethyl alcohol and isopropanol; ketones such as acetone; and ethers such as dioxane, alkoxy(poly)ethylene glycol, and tetrahydrofuran. The amount used is preferably in the range of 0 to 10 parts by mass, more preferably 0 to 5 parts by mass, and even more preferably 0 to 3 parts by mass, relative to 100 parts by mass of the water-absorbent resin.

[0156] When heat treatment is performed, the treatment time is preferably 1 to 180 minutes, more preferably 3 to 120 minutes, and particularly preferably 5 to 100 minutes. The heat treatment temperature (defined as the heat medium temperature or material temperature) is preferably in the range of 100 to 250°C, more preferably in the range of 140 to 220°C, even more preferably in the range of 150 to 220°C, and particularly preferably in the range of 160 to 220°C. After the heat treatment, in order to stop the reaction, the surface-crosslinked water-absorbent resin is preferably forcedly cooled, particularly to 40 to 100°C, or 50 to 90°C.

[0157] (Step of Adding Optional Additive) In the production method according to the present invention, before, during or after the surface cross-linking step, an additive such as an inactive surfactant, an inactive deodorant or an inactive inorganic fine particle powder, which does not cause surface cross-linking and does not substantially react with a water absorbent resin, may be added to a surface of the water absorbent resin.

[0158] In the production method according to the present invention, various additives may be further added to the aqueous monomer solution and / or the polyacrylic acid (salt) in any step after step (v). That is, the method may include an addition step for imparting various functions to the water-absorbent resin, such as adding a deodorant, an antibacterial agent, a fragrance, a foaming agent, a pigment, a dye, a hydrophilic short fiber, a plasticizer, an adhesive, a surfactant, a fertilizer, an oxidizing agent, a reducing agent, water, a salt, a chelating agent, a disinfectant, a hydrophilic polymer such as polyethylene glycol, paraffin, a hydrophobic polymer, a thermoplastic resin such as polyethylene or polypropylene, or a thermosetting resin such as a polyester resin or a urea resin, preferably to the surface of the water-absorbent resin. For example, the chelating agent added in the polymerization step (v) or after step (v) can further improve the polymerization of the bioacrylic acid obtained in steps (i) to (iv) and the performance of the water-absorbent resin.

[0159] The amount of these additives used is preferably in the range of 0 to 30 parts by mass, more preferably in the range of 0 to 10 parts by mass, and further preferably in the range of 0 to 1 part by mass, relative to 100 parts by mass of the water absorbent resin. Note that even after surface crosslinking to the water absorbent resin and / or addition of an additive, the water absorbent resin is collectively referred to as the water absorbent resin in the present invention, within the range in which the water absorbent resin is the main component and is substantially integrated.

[0160] (Optional Granulation Step) Simultaneously with or separately from the surface cross-linking step, water or an aqueous binder solution may be added to the surface of the water absorbent resin to granulate the water absorbent resin, and dust may be reduced. As the binder for the water absorbent resin, 0.1 to 5 parts by weight of water, or about 0.1 to 5 parts by weight of a water-soluble polymer or polyhydric alcohol or an aqueous solution thereof is used.

[0161] (Transportation or storage of optional intermediates) The method for producing a water-absorbent resin of the present invention includes a polymerization step, a drying step, and a surface cross-linking step, and further includes an optional aging step, an optional gel crushing step, an optional crushing step, an optional classification step, an optional fine powder recovery step, an optional classification step after surface cross-linking, and an optional step of adding an additive, and these steps may be performed in the same device (for example, gel crushing and drying in a polymerization device, and drying and surface cross-linking in a drying device), or may be performed in different devices. When each of the above steps is performed in a different device, a transport step of the hydrogel or its dried product is provided between each step to connect the steps, and in this case, a storage step (for example, an intermediate hopper) of the intermediate hydrogel or its dried product may be provided between each step.

[0162] (Step of Removing Optional Foreign Matter) The water absorbent resin that has been subjected to the above-mentioned production step is optionally subjected to a step of removing foreign matter. Examples of foreign matter in the water absorbent resin production step include metallic foreign matter such as metal pieces that are fragments of a classification net, and further, colored particles (black particles or brownish-red particles) that are part of the water absorbent resin particles that have been burned by overheating in the production step. Metallic foreign matter can be removed with an iron remover (magnet), and colored foreign matter (colored particles of the water absorbent resin) can be removed by color sorting.

[0163] The magnetic flux density of the magnetic field in the iron remover is preferably 0.05 Wb / m ２ (500 gauss) or more, more preferably 0.5 Wb / m ２ More preferably, 1.0 Wb / m ２ As described above, preferably a permanent magnet and / or an electromagnet is used, and more preferably, the magnets are arranged in a lattice pattern and the water-absorbent resin is passed through the spaces between the lattices.

[0164] (Final Storage Step) The production method according to the present invention may include a storage step of storing the dried water-absorbent resin in a storage tank. Examples of storage tanks used in this storage step include silos, hoppers, etc., and preferred examples include those equipped with a means for heating the inner wall surface. From the viewpoint of the abrasion and electrostatic charge of the water-absorbent resin, a storage tank having a metallic inner surface, for example, an iron or stainless steel inner surface, is preferred. Furthermore, aside from the storage of the final product, a storage step (intermediate hopper) may be separately provided between each step, and each step may be connected by a buffer hopper and a constant-feed hopper for continuous production.

[0165] (Combined use of acetone, isopropanol, and propylene derived from fossil raw materials) As the acrylic acid used in the above step (v), other bioacrylic acids derived from fossil raw materials or from sources other than bioethanol may be used in combination. As long as at least a portion of the monomers constituting the main chain of the water absorbent resin contains a bioethanol-derived monomer, at least one of acetone, isopropanol, and propylene derived from fossil raw materials or from biomaterials other than bioethanol may be included in the acetone in step (ii), the isopropanol in step (iii), and the propylene in step (iv), respectively. For example, in the case of a poor harvest of crops used as raw materials for bioethanol, or when treating surplus or by-products of compounds derived from fossil raw materials generated in the production of other compounds, at least one of acetone, isopropanol, and propylene derived from fossil raw materials or from biomaterials other than bioethanol can be used in combination. In addition, depending on the type of plant used as the raw material, or the difference between fossil raw materials and non-fossil raw materials, the carbon isotope ratio may be increased. １３ C amount and １４ By utilizing the different C amounts, multiple raw materials can be used in various ratios to produce various carbon isotopes. １３ C amount and １４ It becomes possible to produce a water-absorbent resin having a high C content. １３ C amount and １４By measuring the C amount, it is possible to realize the traceability (identifiability) of the water absorbent resin after production. When at least one selected from acetone, isopropanol, propylene, and acrylic acid derived from fossil raw materials and / or biomaterials other than bioethanol are used in combination, the proportion of bioethanol-derived acrylic acid among the monomers constituting the main chain of the final water absorbent resin is preferably 1 mol% or more, 5 mol% or more, 10 mol% or more, 20 mol% or more, 30 mol% or more, 40 mol% or more, 50 mol% or more, 60 mol% or more, and 70 mol% or more in this order. The proportion may be 80 mol% or more, 85 mol% or more, 90 mol% or more, or 95 mol% or more.

[0166] (Method of Connecting Steps (i) to (vii)) The steps (i) to (vii) may be connected and carried out continuously, or may be carried out individually. Furthermore, the individual purification of at least one of acetone, isopropanol, propylene, and acrylic acid described in the steps (i) to (iv) may be omitted. Furthermore, the steps (i) to (vii) may be carried out by the same producer, or some or all of them may be carried out by different producers. For example, the tasks may be shared as follows:

[0167] Company A: (i) a process for obtaining acetone from bioethanol; Company B: (ii) a process for obtaining isopropanol from bioacetone; Company C: (iii) a process for obtaining propylene from bioisopropanol; Company D: (iv) a process for obtaining acrylic acid from biopropylene; Company E: (v) a process for obtaining polyacrylic acid and / or a salt thereof by polymerizing an aqueous monomer solution containing bioacrylic acid; Company F: (vi) a process for drying polyacrylic acid and / or a salt thereof; Company G: (vii) a process for surface cross-linking polyacrylic acid and / or a salt thereof.

[0168] The steps (i) to (vii) may be carried out at the same location or at different locations. The term "same location" refers to a location within an industrial site that is close enough to be connected by a pipeline. If the steps are carried out at different locations, transportation methods other than pipelines, such as long-distance transportation by tanker, truck, rail, etc., are required.

[0169] As described above, the bioethanol may be absolute ethanol, hydrous ethanol, or ethanol containing acetone or isopropanol (crude ethanol, particularly crude hydrous ethanol).

[0170] Before step (i), the step of obtaining bioethanol (referred to as step (0)) involves fermentation, distillation, and any other purification steps, as described above, to adjust the water content and trace components (acetone, isopropanol, etc.) of the ethanol used. Step (i) of obtaining acetone from bioethanol and step (0) of obtaining ethanol (with the water content and trace component amounts adjusted as necessary) may be carried out at the same location or at different locations, and step (0) and step (i) may be connected by a pipeline, as described below.

[0171] From the viewpoint of the performance of the water-absorbent resin, it is preferable that the step (v) and the step (vi) are carried out at the same location, and further, the step (v) to the step (vii) are carried out at the same location, and the locations of each of the steps (i) to (vii) are at most four locations, further at most three locations, or two locations, and it is particularly preferable that all the steps are carried out at one location.

[0172] Furthermore, since step (iv) is an exothermic reaction accompanied by oxidation, by carrying out step (iv) and steps (v) and (vi) at the same location, the heat generated in step (iv) can be used for polymerization in step (v), drying in step (vi), and / or heating for surface crosslinking in step (vii), etc., thereby reducing CO ２ This can reduce CO emissions, resulting in an environmentally friendly method for producing a water-absorbent resin. For example, heat can be supplied as high-pressure steam through a pipeline to at least one of steps (v), (vi), and (vii). In addition, since the dehydration in step (iii) and the oxidation in step (iv) are both gas-phase reactions, and the dehydration reaction of isopropanol in step (iii) also produces a high yield of biopropylene, it is possible to omit the purification of biopropylene in step (iii) and perform steps (iii) and (iv) continuously. ２ From the viewpoint of cost reduction, it is preferable to carry out the steps (iii) and (iv), and further the steps (iii) to (vii) continuously at the same location.

[0173] On the other hand, acetone is easier to transport because its weight has decreased compared to the consumed ethanol, and isopropanol is easier to handle and transport.

[0174] The ethanol (boiling point 78°C), acetone (boiling point 56°C), isopropanol (boiling point 82°C), propylene (boiling point -47°C), and acrylic acid (boiling point 141°C) used or produced in the above steps (i) to (v) are handled as liquids or gases, and of these, propylene may be cooled and liquefied or handled as a gas. The process may involve transporting the liquid or gas (e.g., pipeline transport) and storing the liquid (e.g., storage in a tank having a liquid cooling or circulation mechanism) as appropriate. When propylene is liquefied and transported, when the liquefied propylene is used in step (iv), a cooling medium may be produced by recovering latent heat from the propylene and used for cooling in steps after step (iv). Furthermore, since the polyacrylic acid obtained in step (v) from liquid acrylic acid is a gel, transport and storage methods suitable for the gel are selected between step (v) and step (vi). That is, in steps (v) to (vii), the products are gel-like or solid (particularly powder), and therefore are transported appropriately by various conveyors or by air transport.

[0175] Furthermore, transportation between adjacent steps among the steps (i) to (vii) depends on the location of each step and the manufacturing company, but in order to solve the above problem, transportation of the bio-raw material between at least one step among the steps (i) to (vii) (preferably between at least one step among the steps (i) to (v)) is performed by long-distance transportation of 10 km or more by tanker, truck, or railroad, and other transportation is also used in combination between at least one other step ((preferably between the steps (i) to (iv), or between the steps (v) to (vii))). Such a configuration enables production of a more optimal water-absorbent resin.

[0176] In addition, to solve the above problems, the transport of the bio-raw material between at least one of the steps (i) to (vii) is performed through a pipeline connecting the steps, and optionally, transport between other steps is performed by means other than a pipeline. Such a configuration enables more optimal production of a water-absorbent resin. Among these, in steps (i) to (iv), the product is a liquid or gas, so when each step is performed by one company or multiple companies, it is preferable that each step is connected by a pipeline, and one or more steps, further two or more steps, and particularly all steps are connected by a pipeline. The length of the pipeline for all steps is appropriately adjusted to 100 km or less, further 10 km or less, particularly 1 km or less, etc.

[0177] Similarly, the above-mentioned step (0) of producing bioethanol and the step of obtaining bioacetone from bioethanol may be carried out by one or more companies, and the steps (0) and (i) are transported between them by the same means as the above-mentioned steps (i) to (vii), or are connected by a pipeline.

[0178] Furthermore, from the viewpoint of reducing residual monomers in a water absorbent resin, as described in the above step (v), it is preferable that the step (iv) and the step (v) are carried out within a certain time (particularly including transportation and storage) of 10 days or less, further 5 days or less, 2 days or less, or 1 day or less. Here, the temperature and conditions for storing and transporting acrylic acid are as described above.

[0179] Some or all of the steps (i) to (vi) may be carried out at multiple locations, by multiple manufacturing companies, or by methods under different conditions within the scope of the present invention. Note that, within the scope of the present invention, methods under different conditions correspond to, for example, a case where raw materials that have been purified or not are used together in the next step, or a case where raw materials obtained using different catalysts are used together in the next step.

[0180] (Differences from Prior Art) As described above, in the method for producing a water absorbent resin of the present invention, as a method for producing a water absorbent resin derived from a biomaterial, instead of a method in which a natural polymer which is inferior in performance and heat resistance is used as a raw material for the water absorbent resin, and instead of a conventional representative method for producing acrylic acid derived from a biomaterial (raw materials: glycerin, bio-naphtha, lactic acid, 3-hydrocypropionic acid, and bio-naphtha based on natural oil and / or fat), a water absorbent resin is obtained for the first time from inexpensive bioethanol as a starting raw material for a water absorbent resin, and it is possible to inexpensively obtain a water absorbent resin which has performance equivalent to or better than that of a conventional water absorbent resin derived from a fossil raw material and has an equivalent or even reduced amount of impurities.

[0181] The above-mentioned Patent Documents 1 to 20 and Non-Patent Document 1, which are related to water-absorbent resins, do not suggest the production of a water-absorbent resin from bioethanol through the present steps (i) to (vii). Furthermore, the patent documents that describe the above steps (i) to (iv) do not suggest the production of a water-absorbent resin from bioethanol, nor the production of acrylic acid from bioethanol through steps (i) to (iv). Conventional methods for producing water-absorbent resins derived from biomaterials, such as those in Patent Documents 1 to 20, not only require expensive biomaterials (and even biomaterials with limited production volumes), but also result in insufficient water-absorbent resins, even after further purification than conventional methods, due to an increase in impurities in the acrylic acid, particularly impurities such as organic acids (e.g., propionic acid). The above-mentioned patent documents and non-patent documents do not suggest the production of a water-absorbent resin from bioethanol through steps (i) to (vii) as a means of solving the problem.

[0182] [Water-absorbent resin of the present invention] In the present invention, there is provided a water-absorbent resin obtained by the above-mentioned production method.

[0183] The water-absorbent resin obtained through the above-mentioned production method (production process) has the following target properties.

[0184] The target performance varies depending on the water-absorbent resin, and particularly includes water absorption capacity without load, water absorption capacity under load, particle size distribution, water absorption rate, liquid permeability, fluidity, color, dust amount, deodorizing performance, antibacterial performance, etc. As for the representative performance of the water-absorbent resin, WSP (Worldwide Strategic Partners) standards of EDANA Recommended Test Methods can be referred to, and the following are listed: pH (WSP 200.2), residual monomer (WSP 210.2), particle size distribution (WSP 220.2), loss on drying (WSP 230.2), FSC (WSP 240.2), CRC (WSP 241.2), AAP (WSP 242.2), PDAUP (WSP 243.1), Flow Rate (WSP 250.2), bulk specific gravity (WSP 260.2), water-soluble matter (WSP 270.2), suction particles (WSP 280.2), dust (WSP 290.2), and other than WSP, examples of the properties that can be considered include liquid permeability (SFC and GBP), coloring (YI / WB), and water absorption rate (Vortex / FSR / DW), but are not limited to these.

[0185] As an example, the water-absorbing resin of the present invention has the following properties.

[0186] ( １４ The ratio of bio-based raw materials to the amount of polyacrylic acid obtained is １４ C (radioactive carbon) / １２ It can be identified by C (carbon). Conventional acrylic acid (salt)-based water absorbent resins obtained from fossil raw materials (especially petroleum, and more specifically propylene) １４ C / １２ C is 1.0 x 10 －１４ whereas the water-absorbent resin of the present invention is １４ C / １２ C is preferably 1.0 × 10 －１４ More preferably, 1.0 × 10 －１３ Above, further 5.0 x 10 －１３ More preferably, 1.0 × 10 －１２ If nearly 100% by weight of the raw material is non-fossil, the upper limit is 1.25 × 10 －１２ is. １４ C / １２C can be measured by isotope mass spectrometry or the like, as described, for example, in U.S. Patent Nos. 3,885,155, 4,427,884, 5,438,194, and 5,661,299. The specific measurement procedure is as follows.

[0187] 1. Acrylic acid (salt)-based water-absorbing resin is burned and converted into carbon dioxide. 2. The carbon dioxide is separated and purified using a vacuum line. 3. The carbon dioxide produced from the acrylic acid (salt)-based water-absorbing resin is reduced with hydrogen using iron as a catalyst to produce graphite. 4. １４ Using a C-AMS measuring device, graphite derived from acrylic acid (salt)-based water-absorbent resin １４ C concentration and １２ C concentration ratio ( １４ C / １２ C) is measured.

[0188] １４ C (radioactive carbon) can be adjusted by the ratio of bio-based raw materials (especially bioethanol) used.

[0189] ( １３ C content) Carbon stable isotope ratio (δ １３ C) can be adjusted appropriately within the range of 0 to -40‰ (per mille). １３ C) can be adjusted depending on the type of plant material, and δ １３ C3 plants (wheat, potato, rice, etc.) with C ≦-20‰ and δ １３ This can be adjusted appropriately by adjusting the raw material to C4 plants (such as corn) with a C≧-20‰. For measurement methods, see Patent Documents 13 and 14.

[0190] ​(CRC (WSP241.2)) The CRC (absorption capacity without load) of the water absorbent resin obtained by the present invention is preferably 10 [g / g] or more, more preferably 20 [g / g] or more, even more preferably 25 [g / g] or more, and particularly preferably 27 [g / g] or more. The upper limit of the CRC is not particularly limited, but in view of the balance of other physical properties, it is preferably 50 [g / g] or less, more preferably 45 [g / g] or less, even more preferably 42 [g / g] or less, and most preferably 35 [g / g] or less. The CRC can be appropriately controlled by the amount of crosslinking agent during polymerization and subsequent surface crosslinking (secondary crosslinking). The CRC (absorption capacity without load) of the water absorbent resin obtained by the present invention is, for example, 10 [g / g] or more and 50 [g / g] or less.

[0191] (AAP (WSP242.2)) The AAP (absorbency under pressure) of the water-absorbent resin obtained by the present invention is as high as or higher than that of conventional fossil raw materials, so there is no decrease in AAP. In order to prevent leakage in disposable diapers, as an example of a means for achieving the above-mentioned polymerization, the AAP under a pressure of 2.1 kPa or 4.8 kPa is preferably 17 [g / g] or more, more preferably 20 [g / g] or more, even more preferably 22 [g / g] or more, even more preferably 23 [g / g] or more, and most preferably 24 [g / g] or more. The upper limit of AAP is not particularly limited, but in terms of the balance with other physical properties, it is preferably 35 [g / g] or less, more preferably 30 [g / g] or less, and even more preferably 28 [g / g] or less. The AAP can be improved (adjusted) by surface crosslinking after the drying step (iv), preferably after particle size control. The AAP value may change depending on the process carried out after the surface cross-linking process. The AAP (absorbency under pressure) of the water absorbent resin obtained in the present invention is, for example, 17 [g / g] or more and 35 [g / g] or less as AAP under a pressure of 2.1 kPa or 4.8 kPa.

[0192] (Water-soluble content (WSP270.2)) The water-soluble content of the water-absorbent resin obtained by the present invention does not increase because the purity of the bioacrylic acid is equal to or higher than that of conventional fossil raw materials. In order to prevent stickiness during use in paper diapers due to the influence of liquid elution, the water-soluble content is preferably 35% by weight or less, more preferably 25% by weight or less, even more preferably 15% by weight or less, even more preferably 10% by weight or less, even more preferably less than 10% by weight, and particularly preferably less than 8.3% by weight. The water-soluble content can be appropriately controlled by controlling the amount of crosslinking agent during polymerization and, preferably, by severing chemical bonds by mechanical action during subsequent gel crushing. The lower limit of the water-soluble content of the water-absorbent resin obtained by the present invention is, for example, 4.0% by weight or more.

[0193] (SFC (Saline Flow Conductivity)) The SFC (saline flow conductivity) of the water absorbent resin obtained in the present invention can be improved by surface crosslinking after the above-mentioned production method, particularly after the gel pulverization of the present invention, preferably after the above-mentioned particle size control, in order to prevent leakage in disposable diapers. As an example of a means for achieving surface crosslinking to the above-mentioned AAP range, the SFC (Saline Flow Conductivity) of a 0.69% aqueous sodium chloride solution, which is the liquid permeability property of a liquid under pressure, can be improved to 10 [×10 －７ ・cm ３ ・s・g －１ ] or more is preferable, and 20 [× 10 －７ ・cm ３ ・s・g －１ ] or more is more preferable, and 30 [× 10 －７ ・cm ３ ・s・g －１ ] or more is more preferable, and 50 [×10 －７ ・cm ３ ・s・g －１ ] or more is more preferable, and 70 [× 10 －７ ・cm ３ ・s・g －１ ] or more is particularly preferred, and 100 [×10 －７ ・cm ３ ・s・g －１ ] or more is most preferable.

[0194] (Residual Monomer (WSP210.2)) The water absorbent resin obtained by the present invention has the advantage that the residual monomers are small because bioacrylic acid is obtained with high purity. From the viewpoint of safety, the residual monomer is controlled to usually 500 ppm or less, preferably less than 500 ppm, more preferably 0 to 450 ppm, even more preferably 0 to 400 ppm, particularly preferably 0 to 300 ppm, and particularly preferably 0 to 200 ppm, as an example of a means for achieving the above polymerization. The residual monomer can be appropriately controlled by the polymerization initiator during polymerization and the subsequent drying conditions, etc.

[0195] (Amount of impurities other than residual monomers) The water-absorbent resin obtained by the present invention does not have an increased amount of impurities because the purity of the acrylic acid used in the present invention is equal to or higher than that of conventional fossil raw materials. Furthermore, there are no problems with coloration or odor. Typical impurities of water-absorbent resins other than residual monomers include acetic acid and propionic acid, and the total content thereof is preferably 1000 ppm or less, 800 ppm or less, 600 ppm or less, 500 ppm or less, 400 ppm or less, 300 ppm or less, or 250 ppm or less in the water-absorbent resin. The total content of acetic acid and propionic acid in the water-absorbent resin obtained by the present invention is preferably low, but considering the balance with reduction costs and excessive reduction, performance may be reduced. For example, the total content may be 100 ppm or more, or 200 ppm or more. The total content of acetic acid and propionic acid in the water-absorbent resin obtained in the present invention is, for example, 100 ppm or more and 1000 ppm or less, or 100 ppm or more and 250 ppm or less.

[0196] The total content of acetic acid, propionic acid, and residual monomers (particularly acrylic acid), which are the cause of the acid odor of the water-absorbent resin, is preferably 1500 ppm or less, 1200 ppm or less, 1000 ppm or less, 900 ppm or less, 800 ppm or less, 700 ppm or less, or 685 ppm or less. The total content of acetic acid, propionic acid, and residual monomers (particularly acrylic acid) in the water-absorbent resin is preferably low, but performance may be reduced in some cases due to a balance with costs or an excessive reduction, and may be, for example, 100 ppm or more, or 200 ppm or more.

[0197] (FSR (Water Absorption Rate)) In order to prevent leakage in disposable diapers, the FSR (water absorption rate) of the water-absorbent resin obtained by the present invention is, as an example of a means for achieving the above-mentioned polymerization, usually 0.20 [g / (g·s)] or more, preferably 0.25 [g / (g·s)] or more, more preferably 0.30 [g / (g·s)] or more, even more preferably 0.35 [g / (g·s)] or more, particularly preferably 0.40 [g / (g·s)] or more, and most preferably 0.45 [g / (g·s)] or more. The upper limit of FSR is 1.00 [g / (g·s)] or less. The method for measuring FSR is specified in WO 2009 / 016055. The FSR can be adjusted by the production method of the present invention and the above-mentioned particle size control after drying.

[0198] (Particle size distribution) The water absorbent resin of the present invention may be in the form of a sheet or fiber, but is preferably adjusted to have the particle size (preferably 850 to 150 μm, etc.) described above (optionally in the classification step before or after the surface crosslinking).

[0199] (Coloring) The YI (Yellow Index) indicates white of 20 or less, 15 or less, or 10 or less.

[0200] (Representative Performance of Water Absorbent Resin) As an example, the present invention provides a water absorbent resin derived from a bio-based raw material, which satisfies the following performance, and further satisfies the above-mentioned performance, has performance equivalent to that of a water absorbent resin derived from 100% fossil raw materials, and has an equivalent or even reduced amount of impurities.

[0201] CRC = 10 to 50 g / g, AAP 2.1 kPa ≥ 17 g / g, AAP 4.81 kPa ≥ 17 g / g, water solubles ≤ 35%, residual monomer ≤ 500 ppm, FSR ≥ 0.20 g / g / sec.

[0202] Preferably, the following performance is achieved:

[0203] CRC = more than 27.5 g / g and not more than 50 g / g, AAP 2.1 kPa = more than 20.5 g / g or AAP 4.81 kPa = more than 20.5 g / g, water soluble content = less than 10%, residual monomer = less than 500 ppm.

[0204] CRC = more than 30.0 g / g and not more than 50 g / g, AAP 4.81 kPa = more than 20.5 g / g and not more than 4050 g / g, water soluble matter = 4.0% or more and less than 8.3%, residual monomer = 50 ppm or more and less than 500 ppm.

[0205] [Uses of the Water-Absorbent Resin of the Present Invention] The use of the water-absorbent resin is not particularly limited, but is directed to sanitary materials, that is, the present invention provides sanitary materials containing the water-absorbent resin. Preferred uses of the sanitary materials include absorbent materials for absorbent articles such as disposable diapers (for infants and adults), sanitary napkins, and incontinence pads. In particular, the water-absorbent resin can be used as an absorbent material for high-concentration disposable diapers. Examples of other absorbent articles include drip absorbents, freshness-preserving materials, portable toilets for emergencies, pet sheets, and cat litter. Other uses of water-absorbent resins include soil water retention agents, seedling raising sheets, seed coating materials, anti-condensation sheets, disposable warmers, cooling bandanas, ice packs, medical waste liquid solidifying agents, soil surplus solidifying materials, dehydrating agents for wet loose materials, water-loss preventing waste liquid gelling agents, water-absorbing sandbags, compresses, thickeners for cosmetics, water-stopping materials for electrical and electronic materials and communication cables, gasket packing, sustained-release agents for fertilizers, various sustained-release agents (space disinfectants, air fresheners, etc.), wound protection dressings, anti-condensation building materials, oil moisture removers, paints, adhesives, anti-blocking agents, light diffusing agents, matting agents, additives for decorative panels, additives for artificial marble, additives for toner, and other resin additives.

[0206] Production Example 1 The bioethanol used was "Specific Alcohol Traceable 95 Grade 1" manufactured by Japan Alcohol Sales Co., Ltd. (quality standard: ethanol content 95.2 to 95.4% by volume (equivalent to approximately 92.6 to 92.8% by weight (calculated from specific gravity at 20°C)), the majority of the remainder being water; evaporation residue 0.5 mg or less / 100 ml; 2-propanol 30 mg or less / L; methanol, 1-propanol, 1-butanol, 2-butanol, 2-methyl-1-propanol, 2-methylbutanol, and acetone each 1 mg or less / L; other organic impurities 2 mg or less / L; acetaldehyde less than 1 mg / L; and 1,4-dioxane 0.01 mg / L or less).

[0207] In step (i) of the present invention, acetone was synthesized by reacting a gas of bioethanol / water vapor / nitrogen (molar ratio: 2 / 8 / 1) at 400°C in the presence of a composite metal oxide catalyst with a molar composition of Fe / Zn / Zr = 1 / 0.5 / 0.5. The acetone was then purified to obtain acetone with a purity of 95% by mass or more. The impurities contained ethanol and acetaldehyde at a total of 4000 ppm. The majority of the remainder was water.

[0208] Next, in step (ii), isopropanol was synthesized by reacting a gas of hydrogen / acetone having a purity of 95% by mass or higher at a molar ratio of 2.7 / 1 in the presence of a catalyst comprising spherical silica (particle size 1.7 to 4 mm) carrying 5% by weight of nickel and 5% by weight of ruthenium, at 0.5 MPa and 100°C, to obtain isopropanol with a purity of 98% by mass. The ethanol content in the isopropanol was 2,200 ppm. The majority of the remainder was water and acetone.

[0209] Furthermore, in step (iii), the gas of isopropanol having a purity of 98 mass% and having a composition of isopropanol / oxygen / nitrogen=6.8 / 12.5 / 80.7% by volume was reacted at 325°C in the presence of a catalyst in which 10% by weight of tungsten oxide was supported on spherical γ-alumina having a particle size of 2 to 4 mm, to obtain propylene.

[0210] Next, in step (iv), the propylene was oxidized at 325°C in the presence of a bismuth molybdenum catalyst (acrolein catalyst) with a particle size of 5 to 7 mm to obtain acrolein, and the acrolein was subsequently reacted at 275°C in the presence of a molybdenum vanadium catalyst with a particle size of 5 to 7 mm to obtain acrylic acid. This was purified to obtain acrylic acid (bioacrylic acid) with a purity of 99% by mass or more. The bioacrylic acid contained 140 ppm of acetic acid and 200 ppm of propionic acid. The water content was 1500 ppm. The amount of acrylic acid dimer was 80 ppm.

[0211] Comparative Production Example 1 Production of Acrylic Acid Derived from Fossil Feedstock Instead of the method of obtaining biopropylene from bioethanol in steps (i) to (iii) of Production Example 1 above, acrylic acid was produced by a conventional method in step (iv) using propylene derived from fossil feedstock and catalytic gas-phase oxidation.

[0212] When the resulting reaction gas was absorbed in water in an absorption tower, 300 ppm of hydroquinone was added to the absorbing solution relative to the resulting aqueous acrylic acid solution, resulting in an aqueous acrylic acid solution. Next, low-boiling components such as acetic acid and propionic acid were removed by distillation from this aqueous acrylic acid solution using the azeotropic solvent toluene, and high-boiling components such as maleic acid and acrylic acid dimers were further removed to obtain crude acrylic acid. This crude acrylic acid was purified using a simple distillation apparatus to obtain acrylic acid (2). The acrylic acid contained 170 ppm of acetic acid and 210 ppm of propionic acid.

[0213] Comparative Production Example 2: Production of acrylic acid from glycerin. Naturally-derived glycerin was dehydrated under a strong acid solid catalyst to obtain acrolein containing by-products such as propanal. The acrolein containing propanal was then oxidized in the gas phase to produce gaseous acrylic acid, which was then collected with water to produce an aqueous acrylic acid solution. This was then distilled to obtain acrylic acid containing 3% by weight of propionic acid. To further purify the acrylic acid, simple distillation and crystallization were performed to obtain acrylic acid (3) derived from non-fossil raw materials containing 2000 ppm of propionic acid and 300 ppm of acetic acid.

[0214] [Example 1] Production of water-absorbent resin from bioethanol-derived acrylic acid Step (v) of polymerizing a monomer aqueous solution containing acrylic acid and its salt to obtain a polyacrylate salt In a 2 L polypropylene container, 439.4 parts by weight of bioacrylic acid obtained through steps (i) to (iv) in Production Example 1 (containing 80 ppm of acrylic acid dimer, and separately containing 70 ppm of p-methoxyphenol), 181.1 parts by weight of 48.5 wt% aqueous sodium hydroxide solution, 1.9 parts by weight of polyethylene glycol diacrylate (average number of polyethylene glycol units (average n number); 9), 1.35 parts by weight of 2.0 wt% aqueous diethylenetriamine pentacetate trisodium solution, and 351.7 parts by weight of deionized water were added and mixed to prepare an aqueous solution. The deionized water was preheated to 40 ° C.

[0215] Subsequently, while stirring the aqueous solution, 196.1 parts by weight of a 48.5 wt % aqueous sodium hydroxide solution was added to the aqueous solution over a period of about 30 seconds in an open-to-air state, and mixed to prepare an aqueous monomer solution. Note that the temperature of the aqueous monomer solution rose to about 80° C. due to the heat of neutralization and heat of dissolution generated during the mixing process.

[0216] Thereafter, when the temperature of the aqueous monomer solution reached 78° C., 28.45 parts by weight of a 3% by weight aqueous solution of sodium persulfate was added as a polymerization initiator, and the mixture was stirred for about 5 seconds to obtain a reaction liquid.

[0217] Next, the reaction solution was poured into a stainless steel bat-shaped container in an open-air state. The bat-shaped container had a bottom dimension of 200 mm × 260 mm, an upper dimension of 460 mm × 560 mm, and a height of 140 mm, and the cross section of the central part was trapezoidal. A silicone sheet was attached to the inner surface. Before pouring the reaction solution into the bat-shaped container, it was placed on a hot plate heated to 50°C to preheat it.

[0218] After the reaction solution was poured into the bat-shaped container, the polymerization reaction began within one minute. The reaction solution expanded and foamed upward while generating steam, and then shrunk to a size slightly larger than the bottom of the bat-shaped container. The polymerization reaction (expansion and contraction) was completed within approximately one minute. The maximum polymerization temperature reached 112°C due to the heat of polymerization, and some of the acrylic acid, acetic acid, and propionic acid remaining in the reaction system during polymerization were volatilized and removed along with the generated steam. A hydrogel-like cross-linked polymer (hereinafter referred to as "hydrogel") was obtained through the polymerization reaction.

[0219] Next, the hydrogel was cut into appropriate sizes and then fed into a screw extruder for gel pulverization (gel pulverization step) to obtain particulate hydrogels having particle diameters of 0.1 to 2 mm. The water content of the particulate hydrogels was 52% by weight.

[0220] (Step (vi) of drying the hydrogel polymer after polymerization, and step (vii) of further surface cross-linking) Next, in step (vi), the particulate hydrogel was spread and placed on a wire mesh with an opening of 300 μm (50 mesh) and placed in a hot air dryer. Thereafter, the particulate hydrogel was dried by passing hot air at 190° C. for 30 minutes to obtain a dried polymer. The water content of the dried polymer was 2 wt %. The particulate hydrogel discharged from the screw extruder was subjected to step (vi) of drying within 1 hour. Subsequently, the dried polymer was put into a roll mill and pulverized, and then classified using two types of JIS standard sieves with openings of 850 μm and 150 μm to obtain an irregularly pulverized water absorbent resin powder (1). The weight average particle diameter (D50) of the water absorbent resin powder (1) was 390 μm.

[0221] Next, in step (vii), 3.5 parts by weight of a surface cross-linking agent solution (1) consisting of 0.4 parts by weight of ethylene carbonate, 0.6 parts by weight of propylene glycol, and 2.5 parts by weight of deionized water was added to 100 parts by weight of the water absorbent resin powder (1) and mixed until uniform, thereby obtaining a humidified mixture (1). Subsequently, the humidified mixture (1) was heat-treated at 200°C for 40 minutes, cooled to 60°C, and passed through a mesh of 850 μm. Particles that did not pass through the mesh of 850 μm were crushed by lightly pressing with a spatula on a mesh and passed through, and particles that could not be crushed were removed. In this way, an irregularly crushed surface cross-linked water absorbent resin (1) was obtained. Table 1 shows the physical properties of the water absorbent resin powder (1) and the water absorbent resin (1).

[0222] [Comparative Example 1] Production of a water absorbent resin with acrylic acid derived from a fossil raw material

[0141] The same operation as in Example 1 was carried out, except that the acrylic acid was changed to that produced from propylene derived from a fossil raw material in Comparative Production Example 1, in the step (v) of Example 1, so as to obtain an irregularly pulverized comparative water absorbent resin powder (1) and a surface cross-linked comparative water absorbent resin (1).

[0223] [Comparative Example 2] Production of water absorbent resin with acrylic acid derived from glycerin

[0123] The same operation as in Example 1 was carried out, except that in the step (v) of Example 1, the acrylic acid was changed to the acrylic acid produced in Comparative Production Example 2, so as to obtain an irregularly pulverized water absorbent resin powder (2) for comparison and a surface cross-linked water absorbent resin (2) for comparison.

[0224] [Comparative Example 3] Production of a water-absorbent resin using a natural polymer In Comparative Example 1 (production of acrylic acid derived from fossil raw materials), in order to make a part of the water-absorbent resin derived from bio-raw materials, an operation was carried out in the same manner as in Comparative Example 1, except that solubilized starch (25% by weight based on the monomer solid content) was mixed with the hydrogel after polymerization in step (vi) of Comparative Example 1, to obtain a comparative water-absorbent resin (3) (starch content: 20% by weight). The water-absorbent resins of Example 1 and Comparative Examples 1 and 2 were white, whereas the comparative water-absorbent resin (3) containing 20% ​​by weight of starch as shown in Comparative Example 1 had a reduced water absorption capacity and was colored ochre by drying (hot air at 190°C for 30 minutes) due to the low heat resistance of the starch, and the water-absorbent resin was further colored brown by surface cross-linking after drying (heat treatment at 200°C for 40 minutes).

[0225] [Comparative Example 4] In Comparative Example 3 (coloration occurred when starch was 20%), in order to suppress coloration and a decrease in water absorption capacity during drying and surface crosslinking, the drying conditions (hot air at 190°C for 30 minutes) were changed to hot air at 100°C for 4 hours in step (vi) of Comparative Example 3. Although coloration during drying was suppressed, the drying time was extended, and a significant decrease in productivity was confirmed.

[0226] Furthermore, in order to suppress coloration during surface crosslinking after drying (heat treatment at 200°C for 40 minutes), the surface crosslinking temperature was changed to 100°C, but even after heating for 1 hour, surface crosslinking did not proceed and the AAP was low. Note that "Comparative Example 4" shown in Table 1 shows the results of surface crosslinking performed under the condition of 100°C for 1 hour.

[0227] [Comparative Example 5] In Comparative Example 3, in order to suppress coloration during surface crosslinking after drying (heat treatment at 200°C for 40 minutes), 0.1 parts of ethylene glycol diglycidyl ether having high low-temperature reactivity as a surface crosslinking agent was added to 0.4 parts by weight of ethylene carbonate and 0.6 parts by weight of propylene glycol, and surface crosslinking was performed at 100°C for 1 hour.

[0228]

[0229] [Example 2] Recycling of water-absorbent resin In Example 1, the dried polymer obtained in the drying step of step (vi) of Example 1 was classified using two types of JIS standard sieves with mesh sizes of 850 μm and 150 μm, and the fine powder that passed through the 150 μm mesh was mixed at 1 wt % (relative to the monomer used in step (v)) in the gel crushing step of Example 1, thereby recycling the water-absorbent resin fine powder derived from biomaterials. A water-absorbent resin almost the same as that of Example 1 was obtained.

[0230] [Example 3] Recycling of acrylic acid In the polymerization step of step (v) of Example 1 (the maximum polymerization temperature was 112°C due to the heat of polymerization, and some of the acrylic acid, acetic acid, and propionic acid remaining in the reaction system during polymerization volatilized together with the generated water vapor), the volatilized acrylic acid was collected by cooling. The collected and recovered acrylic acid was purified and used in the polymerization of Example 1 at 1% by weight (relative to the monomer used in step (v)), thereby recycling bioacrylic acid. A water-absorbent resin almost identical to that of Example 1 was obtained.

[0231] (Summary) When Example 1 (the water absorbent resin (1) made of acrylic acid obtained from bioethanol in this case) is compared with Comparative Example 1 (the comparative water absorbent resin (1) made of acrylic acid obtained from conventional fossil raw materials), it is found that the production method in this case has performance equivalent to that of the water absorbent resin derived from conventional fossil raw materials, and impurities (acetic acid and propionic acid, which cause odors) are reduced to the same level or more.

[0232] A comparison of Example 1 (the present water-absorbent resin (1) using acrylic acid obtained from bioethanol) and Comparative Example 2 (the comparative water-absorbent resin (2) using acrylic acid obtained from bioglycerin) shows that the production method of the present invention has excellent water absorption performance (the relationship between water absorption capacity and soluble content) and further reduces impurities (acetic acid and propionic acid). With the water-absorbent resin of Comparative Example 2 (residual acetic acid / propionic acid of 1,400 ppm), even if the acrylic acid is highly purified at the expense of yield and cost in Comparative Production Example 2, it is difficult to sufficiently purify and remove impurities such as propionic acid (boiling point 141°C) from acrylic acid (boiling point 141°C). In addition to the problems of the purification cost and yield of acrylic acid, the resulting water-absorbent resin was confirmed to emit an acidic odor during storage.

[0233]

[0123] Furthermore, when Example 1 (residual acetic acid / propionic acid in acrylic acid was 340 ppm in total) is compared with Comparative Example 2 (residual acetic acid and propionic acid in acrylic acid was 2300 ppm in total), it is understood that Example 1 is also superior in the yield of a water absorbent resin (ratio of acrylic acid used and water absorbent resin obtained) because acetic acid and propionic acid are more volatilized in Comparative Example 2 in which a large amount of impurities is present in acrylic acid.

[0234] When comparing Example 1 (water absorbent resin (1) using acrylic acid obtained from bioethanol in this case), Comparative Example 1 (comparative water absorbent resin (1) using acrylic acid obtained from a conventional fossil raw material), and Comparative Examples 3 to 5 (comparative water absorbent resins (3) (bio raw material ratio 20 wt%) of starch graft obtained from acrylic acid and starch (relative to monomer solid content 25 wt%) obtained from a fossil raw material, it is found that the use of starch in addition to polyacrylate reduces the water absorption capacity (CRC) of the water absorbent resin, increases the amount of residual monomer, and furthermore, the water absorbent resin is colored from yellow to brown. Since the main use of water absorbent resins is as sanitary materials such as diapers, in addition to the reduction in water absorption capacity (CRC), coloration of the water absorbent resin and increase in residual monomer as in Comparative Examples 3 to 5 are not preferable.

[0235] As in Examples 2 and 3, by recycling bio-acrylic acid or a water-absorbent resin derived from a bio-raw material, a carbon-neutral, high-performance water-absorbent resin can be provided.

[0236] Furthermore, it can be seen from the water-absorbent resin of Example 1, which uses acrylic acid obtained from aqueous ethanol containing the specified trace components of Production Example 1, that the residual water or isopropanol in the ethanol used as the bioethanol does not adversely affect the acrylic acid used in the water-absorbent resin. As the ethanol used in the present invention, rather than absolute ethanol, which requires a hydrophobic solvent for advanced purification, relatively inexpensive aqueous ethanol free of hydrophobic solvent contamination, particularly aqueous ethanol containing fermentation by-products (isopropanol, acetone) or water (and even crude ethanol), can be appropriately used, and it can be confirmed that results similar to those of Example 1 can be obtained.

[0237] That is, in the production method of the Examples of the present invention, substantially 100% by weight of the main chain is derived from a bio-based raw material, but compared with the comparative water-absorbent resins (3) to (5) derived from 20% starch, the water-absorbent resin is superior in water-absorption performance (CRC / AAP) and residual monomers, and further, is a white water-absorbent resin even when produced at a high temperature.

[0238] By producing a water absorbent resin using bioethanol as a raw material, it is possible to provide a carbon-neutral, high-performance water absorbent resin. Because bioethanol is a bio-raw material that is inexpensive and mass-produced, the water absorbent resin produced from bioethanol by the present production method can be widely used in the application fields of water absorbent resins, replacing the conventional water absorbent resins derived from fossil raw materials that are mass-consumed.

[0239] This application is based on Japanese Patent Application No. 2023-198629 filed on November 22, 2023, and Japanese Patent Application No. 2023-223208 filed on December 28, 2023, the disclosures of which are incorporated by reference in their entirety.

Claims

1. A method for producing a water-absorbing resin derived from bio-based raw materials, comprising the following steps (i) to (vii); (i) Steps to obtain acetone from bioethanol; (ii) Steps to obtain isopropanol from the acetone; (iii) Steps to obtain propylene from the isopropanol; (iv) The step of obtaining acrylic acid from the propylene; (v) A step of polymerizing the monomer aqueous solution containing the acrylic acid to obtain polyacrylic acid and / or its salts; (vi) drying the polyacrylic acid and / or its salt; (vii) A step of surface crosslinking the polyacrylic acid and / or its salt.

2. The manufacturing method according to claim 1, wherein the total content of ethanol and acetaldehyde in the acetone is 20,000 ppm or less.

3. The manufacturing method according to claim 1 or 2, wherein the ethanol content in the isopropanol is 20,000 ppm or less.

4. The method for producing bioethanol according to claim 1 or 2, wherein the bioethanol is aqueous ethanol with a water content of 3% by weight or more.

5. The manufacturing method according to claim 1 or 2, wherein the content of the bioethanol is 1% by weight or less each of the lower alcohols having 1, 3 to 5 carbon atoms, acetaldehyde, and acetone.

6. The method for producing bioethanol according to claim 1 or 2, wherein the bioethanol is ethanol containing isopropanol and / or acetone.

7. The method for producing the bioethanol according to claim 1 or 2, wherein the bioethanol is obtained by fermenting one or more genetically modified or non-genetically modified plant materials selected from sugarcane, corn, and sugar beet.

8. The manufacturing method according to claim 1 or 2, wherein the propionic acid content in the acrylic acid is 500 ppm or less.

9. The manufacturing method according to claim 1 or 2, wherein the content of acetic acid in the acrylic acid is 1500 ppm or less.

10. The manufacturing method according to claim 1 or 2, wherein at least a portion of the impurities in the acrylic acid is removed in step (v) and / or step (vi).

11. The manufacturing method according to claim 1 or 2, wherein the acrylic acid that volatilizes in step (v) and / or step (vi) is recycled back to step (v).

12. The manufacturing method according to claim 1 or 2, wherein in step (v) and thereafter, a portion of the water-absorbing resin is separated and recycled to step (v) or step (vi).

13. The manufacturing method according to claim 1 or 2, wherein in step (v), in addition to the acrylic acid obtained in step (iv), another acrylic acid is used in combination with the monomer, and the acrylic acid obtained in step (iv) is 1 mol% or more of the total acrylic acid.

14. The manufacturing method according to claim 1 or 2, wherein the total content of acetic acid and propionic acid in the water-absorbent resin is 1000 ppm or less.

15. The manufacturing method according to claim 1 or 2, wherein the total content of acetic acid, propionic acid, and acrylic acid in the water-absorbent resin is 1500 ppm or less.

16. The water-absorbent resin is measured by radiocarbon dating. １４ C / １２ C is 1.0 × 10 －１４ The manufacturing method according to claim 1 or 2.

17. The manufacturing method according to claim 1 or 2, wherein in step (v) and thereafter, a chelating agent is added to the monomer aqueous solution and / or the polyacrylic acid and / or its salt.

18. The manufacturing method according to claim 6, wherein the total content of acetone and isopropanol in the bioethanol is 1 ppm or more.

19. A water-absorbing resin obtained by the manufacturing method described in Claim 1 or 2.

20. A sanitary material comprising the water-absorbent resin described in claim 19.