JPH0229085B2 - KANSOSHITAKYUSUISEIJUSHINOSEIZOHOHO - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a dried water absorbent resin. In recent years, water-absorbing resins have been used for a variety of purposes, including sanitary products, sanitary products, water retention agents, dehydration agents, sludge coagulants, thickeners, anti-condensation agents, and release control agents for various chemicals. As these water-absorbing resins, hydrolysates of starch-acrylonitrile graft polymers, cross-linked carboxymethylcellulose polyacrylates, polyvinyl alcohol-based, polyethylene oxide-based, polyacrylonitrile-based, etc. are known. Among such water-absorbing resins, hydrolysates of starch-acrylonitrile graft polymers have a relatively high water-absorbing capacity, but because they use starch, which is a natural polymer, they have drawbacks such as heat resistance and rot decomposition. , the manufacturing method is also complicated. Among carboxymethyl cellulose, polyethylene oxide, and polyacrylonitrile, none with satisfactory water absorption ability has yet been obtained. Among conventionally known water-absorbing resins, cross-linked polyacrylates are considered promising as resins with excellent water-absorbing ability and quality stability. Conventionally, various methods such as aqueous solution polymerization, reverse-phase emulsion polymerization, and reverse-phase suspension polymerization have been known as methods for polymerizing polyacrylic acid or acrylates. In order to make them useful water-absorbing resins, they must be crosslinked by some method. However, in the case of aqueous solution polymerization, even if the modification with a crosslinking agent is carried out either during or after the polymerization, the resulting reactant becomes an extremely highly viscous aqueous solution or water-absorbing gel that is difficult to handle. In order to obtain a resin product form (powder), it is essential to remove water (dry) from the high viscosity aqueous solution or water-absorbing gel material and solidify it. However, the above-mentioned drying is difficult to be carried out efficiently using the usual rotating drum roller method or spray drying method, and excessive crosslinking often occurs due to overheating during the drying process, or conversely, if drying is insufficient, the crosslinking density decreases. However, it is extremely difficult to stably obtain a product having a desired moisture content and water absorption capacity. The present inventors, as a means to solve the above problem,
First, a polymerization reaction initiator is added to a heated aqueous solution containing potassium acrylate and a water-miscible polyvinyl monomer, and the concentration of the brain monomer is in the range of 55 to 80% by weight, followed by external heating. Established a method for producing a dry solid water-absorbing resin that allows the polymerization reaction to occur without carrying out the polymerization reaction and evaporates the water (Japanese Patent Application Laid-Open No. 171559-1989).
issue). According to the above method, by preparing a heated aqueous solution of potassium acrylate and water-miscible polyvinyl monomer at a specific concentration and subjecting them to a polymerization reaction (and crosslinking reaction), sufficient The polymerization reaction proceeds, and the reaction heat of this polymerization reaction causes water to quickly evaporate from the reaction system, thereby directly achieving the desired low moisture content without the need for any subsequent drying process. A dry solid water-absorbing resin can be obtained. It can be easily ground into a powder suitable for the desired application. Moreover, this method eliminates the risk of non-uniform crosslinking caused by overheating due to external heating and drying processes, and a decrease in water absorption capacity due to this, and produces a water-absorbing resin solid of very good quality. can always be obtained stably. Although the water absorption capacity of the resin obtained by the above method is excellent, the above method and the resin obtained thereby still have problems that need to be improved. That is, when using the resin as a water-absorbing agent to instantaneously treat a large amount of water or an aqueous solution, the resin is required not only to have a high water-absorbing capacity but also to have a high water-absorbing rate. Furthermore, in the above method, there is a possibility that a polymerization reaction occurs in the monomer aqueous solution at a high temperature side, and solidification occurs at a low temperature side, and therefore, it is necessary to strictly control the temperature. In particular, this temperature control
Problems arise in the conduit leading from the aqueous solution tank to the polymerization zone. The present invention provides a new method for producing a dry water-absorbing resin that also solves the problems of the previously developed methods. That is, the present invention contains an alkali metal acrylate salt, a water-miscible polyvinyl monomer, an organic solvent with a boiling point of 40 to 150°C, and water, and the concentration of these monomers is 55 to 80°C.
A method for producing a dry water-absorbent resin characterized by adding a polymerization reaction initiator to a heated aqueous liquid in a range of % by weight to carry out a polymerization reaction without external heating and vaporizing water. It depends. The method of the present invention is basically based on adding an organic solvent with a boiling point of 40 to 150°C to the monomer aqueous solution in the previously developed invention to form a homogeneous aqueous liquid, which is then subjected to a polymerization reaction. The freezing point of the aqueous liquid is lowered by about 10 to 20°C as compared to the aqueous monomer solution, thereby widening the allowable range of temperature control by about three times or more. Furthermore, the organic solvent used above rapidly evaporates together with water due to the heat of polymerization of the monomer during the polymerization reaction, and this latent heat of vaporization is considerably smaller than that of water, so it functions as a blowing agent in the polymerization reaction system.
The resulting resin becomes porous and has an initial water absorption rate of about 2% compared to when the organic solvent is not used.
It can be improved by ~5 times, and the water absorption capacity remains unchanged. As described above, the present invention is a further improvement on the previously developed method without sacrificing any of the advantages of the method, and is very useful. In the method of the present invention, first, a monomer mixture containing an alkali metal acrylic acid salt, a water-miscible polyvinyl monomer, an organic solvent, and water is added, and the monomer concentration is in the range of 55 to 80% by weight. Prepare a warm aqueous solution.
The aqueous liquid is carefully selected in particular taking into account its state of dissolution, ease of polymerization reaction of the monomer, scattering of monomer components outside the system during the polymerization reaction, and the like. This monomer aqueous liquid is usually easily prepared by mixing and dissolving acrylic acid, an alkali as a neutralizing agent, polyvinyl monomer, water, and an organic solvent. At this time, it is also possible to employ a slight heating operation. The alkali used as the neutralizing agent is a common one such as lithium hydroxide, sodium hydroxide,
Alkali metal hydroxides or carbonates such as potassium hydroxide, cesium hydroxide, sodium carbonate, and potassium carbonate can be used. Therefore, the alkali metal acrylic salt monomers polymerized according to the present invention include lithium salts, sodium salts, potassium salts, and cesium salts of acrylic acid. The above neutralizing agent is normally preferably used in an amount sufficient to neutralize 100% by mole of acrylic acid, but it is not necessary to use the amount to neutralize 100%, and approximately 70% or more is neutralized. Therefore, the aqueous mixed monomer solution prepared may contain free acrylic acid in an amount of up to about 30%. However, if a large amount of free acrylic acid is present in the aqueous solution, there is a risk that it will be scattered and lost to the outside of the system during the polymerization reaction, which is disadvantageous in causing a decrease in the polymerization rate. There is no problem even if the amount of neutralizing agent used is too large, but the excess neutralizing agent itself does not participate in the polymerization reaction and is wasteful. Further, the water-miscible polyvinyl monomer used in the present invention needs to have water-miscibility or water-solubility such that it can be uniformly dissolved or dispersed in the mixed monomer aqueous solution prepared by adding the water-miscible polyvinyl monomer. Such polyvinyl monomers include, for example, bisacrylamides such as N,N-methylenebisacrylamide and N,N-methylenebismethacrylamide; polyacrylic (or methacrylic) acid esters represented by the following general formula (); Common crosslinking agents such as diacrylamides represented by the general formula () can be exemplified, and among these, bisacrylamides such as N,N-methylenebisacrylamide and N,N-methylenebismethacrylamide are particularly preferred. . (In the formula, X is ethylene, propylene, trimethylene, hexamethylene, 2-hydroxypropylene, represents a group, and n and m represent integers of 5 to 40. Further, k represents 1 or 2. ) The compound of the general formula () is a combination of polyols such as ethylene glycol, propylene glycol, trimethylolpropane, 1,6-hexanediol, glycerin, pentaerythritol, polyethylene glycol, peripropylene glycol, and acrylic acid or methacrylic acid. Obtained by the reaction of (In the formula, l represents 2 or 3.) The compound of the general formula () can be obtained, for example, by reacting polyalkylene polyamines such as diethylenetriamine and triethylenetetramine with acrylic acid. The polyvinyl monomer is about 0.001 to 0.3% by weight, preferably 0.005% by weight of the total weight of the alkali metal acrylate salt or, in some cases, the total weight of the alkali metal acrylic acid salt and free acrylic acid in the normally prepared mixed monomer aqueous solution. ~
It is advantageously used in a range of 0.1% by weight, thereby making it possible to obtain the desired dry solid crosslinked polyacrylate product with excellent water absorption ability. If the amount used is too large, the resulting polymer will have excessive gel strength upon water absorption, resulting in decreased water absorbency; if it is too small, a sol-like polymer with low gel strength will be obtained. This also has low water absorption, and both are unfavorable. Examples of the organic solvent with a boiling point of 40 to 150°C used in the present invention include methanol, ethanol, n-propanol, acetone, methyl ethyl ketone, cyclohexane, n-hexane, n-heptane, benzene, toluene, and tetrahydrofuran. may be used in combination of two or more. The amount of the organic solvent used is preferably 0.5 to 15% by weight, preferably 1 to 10% by weight, based on the monomer. If the amount used is less than 0.5% by weight, the foaming effect and freezing point depression of the monomer solution will be small, and if it is used more than 15% by weight, the water absorption ability of the resin may decrease even though the initial water absorption rate increases. Furthermore, the monomer may precipitate, which is not preferable. As mentioned above, the boiling point of the organic solvent is particularly between 55 and 120, because the monomer solution is heated in advance and it evaporates together with water.
It is preferable to set it as the range of °C. In the present invention, the above alkali metal salt of acrylic acid (or in some cases, it and free acrylic acid)
It is important to adjust the concentration of polyvinyl monomer to 55 to 80% by weight, more preferably 65 to 75% by weight, and by heating and using an aqueous liquid at a rate in this range, the desired result can be achieved for the first time. You can achieve your goals. If the above-mentioned mixed monomer concentration is less than 55% by weight, the desired dry solid with low moisture content cannot be obtained, and the resulting polymer will continue to be complicated, increase product cost, and reduce product quality. Unless a drying process is carried out, which may cause oxidation, it cannot be made into a powder form suitable for the intended use. Further, it is difficult to increase the monomer concentration to more than 80% by weight from the viewpoint of solubility of the monomer, and is not practical. Next, in the present invention, the aqueous solution of mixed monomers prepared as described above is heated in advance, and then a polymerization reaction initiator is added thereto to start the polymerization reaction (and crosslinking reaction). The heating conditions here are not particularly limited on the premise that the polymerization reaction is initiated by the addition of a polymerization initiator, but are usually about 50 to 85°C, preferably about
The temperature may be about 90 to 75°C. Furthermore, as the polymerization reaction initiator, any of the various types known to be used in the production of polyacrylates can be used. A specific example thereof is a redox initiator which is a combination of a reducing agent such as an alkali metal sulfite or bisulfite, ammonium sulfite, or ammonium bisulfite, and an initiator such as an alkali metal persulfate or ammonium persulfate. or azobisisobutyronitrile, 4-t-
Butylazo-4'-cyanovaleric acid, 4,4'-azobis(4-cyanovaleric acid), 2,2'-azobis(2
-amidinopropane) hydrochloride and other so-called azo initiators, or a combination of two or more of them can be appropriately selected and used. The amount of such a polymerization initiator to be used is not particularly different from the usual one, and the solid content is usually 0.1 to 10% of the total weight of the monomer acrylate (or acrylic acid and acrylic acid) and polyvinyl monomer. %, more preferably about 0.5 to 5% by weight. Further, depending on the type and amount of the polymerization reaction initiator used, a common chain transfer agent such as isopropyl alcohol or alkyl mercaptan may be used together with the polymerization reaction initiator to control the molecular weight of the polyacrylate to be produced. By adding a polymerization reaction initiator to the heated aqueous solution of mixed monomers, the polymerization reaction of the present invention and the vaporization of water and organic solvent can be carried out without any external heating. The polymerization reaction and the vaporization of water and organic solvent by the addition of the polymerization initiator are more advantageously carried out by adding and mixing a predetermined amount of the polymerization initiator or its aqueous solution to the heated aqueous solution of the mixed monomers, and then mixing the mixture. It is carried out by spreading the liquid down on a moving belt. When the polymerization reaction initiator is used in the form of an aqueous solution, the acrylate concentration in the resulting mixed solution must not fall below the above-mentioned range of 55 to 80% by weight. Further, when the mixed liquid flows down onto the belt, a polymerization reaction initiator may be further placed in the part of the belt where the mixed liquid flows down. When a polymerization reaction initiator is added to the heated aqueous solution of mixed monomers and mixed, the polymerization reaction starts extremely quickly after mixing, and the reaction is completed in a short time, usually about 30 seconds to 10 minutes. . Moreover, this reaction is an exothermic reaction, and the reaction system quickly changes to approximately
The temperature rises to about 100 to 130℃, and as a result, the moisture and organic solvent in the system are rapidly vaporized without any external heating, resulting in the desired low water content polymer (dry solid). ) is obtained. The water content of the obtained polymer varies slightly depending on the reaction conditions, etc., but is usually about 15% or less, more preferably about 10% or less, and this can be achieved by simply crushing or grinding according to a conventional method without any subsequent drying step. The desired powder product can be obtained by simply performing the following steps. The polymer dry solid obtained in this way can be easily powdered using a conventional grinder, etc. The powder thus obtained has excellent water absorbing ability and can be used for example in sanitary napkins, disposable diapers, disposable diapers, etc. It can be used for a variety of purposes, including sanitary products such as kitchen utensils, water retention agents for agriculture, forestry, and horticulture, dehydration agents for various industries, sludge coagulants, thickeners, anti-condensation agents for building materials, and release control agents for various chemicals. Examples will be given below to explain the present invention in more detail. Examples 1 to 4 72.1 g of acrylic acid was mixed with 18.0 g of deionized water, 40.9 g of solid potassium hydroxide (4% water content), and 5.2 g of various solvents listed in Table 1 (equivalent to 5% of monomer).
Mix and heat to 75℃. This aqueous monomer liquid and 4.0 g of a 10% aqueous solution of 2,2'-azobis(2-amidinopropane) hydrochloride were mixed and immediately spread on an endless moving belt to a thickness of 5 mm. The polymerization reaction started after about 15 seconds and was completed in about 30 seconds. The maximum temperature during that time is 130-135℃.
It was hot. A dried strip of crosslinked potassium polyacrylate was thus obtained. This was ground into a powder of 20 to 100 meshes. In all cases, the moisture content was 4 to 6%. 0.1 g of each of these powders was accurately weighed, and the water absorption capacity was measured after immersing them in deionized water or 1% saline for 10 seconds, 30 seconds, and 15 minutes. The results are shown in Table 1. Also the first
For comparison, the table shows the measurement results for a dry solid powder obtained by polymerizing a monomer aqueous solution prepared using only deionized water (23.2 g) without using any organic solvent in the above example. Also listed.

[Table] Examples 5 to 7 In Example 1, 5.2 g of acetone was replaced with methanol in amounts corresponding to 3, 5, and 10% by weight relative to the monomer, respectively, and deionized water was added in the total amount with the methanol above. Change it to 23.2g,
A water absorbent resin solid was obtained in the same manner. Table 2 shows the water absorption capacity of powdered powders having the same moisture content and immersion in 1% saline and deionized water for a predetermined period of time. Table 2 also shows the same test results for the resin solids as described above when only 23.2 g of deionized water was used without using methanol.

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Claims (1)

[Claims] 1. A heated aqueous liquid containing an alkali metal acrylic acid salt, a water-miscible polyvinyl monomer, an organic solvent with a boiling point of 40 to 150°C, and water, with a concentration of these monomers in the range of 55 to 80% by weight. A method for producing a dry water-absorbing resin, which comprises adding a polymerization reaction initiator to carry out a polymerization reaction without external heating, and vaporizing moisture.