JPH0214925B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a dry water-absorbing resin solid. 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. Known examples of these water-absorbing resins include hydrolysates of starch-acrylonitrile graft polymers, carboxymethyl cellulose, cross-linked polyacrylates, and other polyvinyl alcohol-based, polyethylene oxide-based, and polyacrylonitrile-based resins. Among such water-absorbing resins, hydrolysates of starch-acrylonitrile graft polymers have 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 form a water-absorbing resin product (powder), it is essential to remove water (dry) from the high-viscosity aqueous solution or water-absorbing gel 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. In addition, in the case of reverse-phase emulsion polymerization, reverse-phase suspension polymerization, etc., it is essential to use an organic solvent in the polymerization process, and the use of organic solvents can lead to sudden polymerization and mistakes in polymerization temperature control. This is undesirable because the temperature and pressure of the reaction system will rise abnormally, leading to the risk of explosion or fire, or environmental health problems for workers. In view of the current situation, the present inventors obtained the above-mentioned polyacrylate crosslinked product using aqueous solution polymerization technology, and also removed water using the polymerization heat generated during the polymerization reaction (and crosslinking reaction). That is, the purpose of drying is to easily and quickly obtain a dry solid water-absorbing resin having a low water content and excellent water absorption ability without vaporizing water by external heating or requiring a separate drying process. I did a lot of research. As a result, the vaporization of water due to the heat of polymerization is significantly affected by the type of monomer salt to be polymerized and the concentration of the monomer salt and the compound used as a crosslinking agent in the aqueous solution. When a specific monomer salt is selected and the concentration of its aqueous solution is adjusted within a specific range, the polymerization reaction and crosslinking reaction proceed extremely quickly without external heating, and the desired polymerization rate can be achieved. It has been found that water quickly evaporates from the reaction system due to the polymerization heat of the polymerization reaction, and the desired dry solid having low water content and water absorption can be easily obtained without any subsequent drying step. The present invention was completed based on this new knowledge. That is, in the present invention, a polymerization reaction initiator is added to a heated aqueous solution containing potassium acrylate and a water-miscible or water-soluble divinyl compound, and the concentration of these monomers is in the range of 55 to 80% by weight. It relates to a method for producing a dry solid water-absorbing resin, characterized in that a polymerization reaction is carried out without addition and external heating, and water is vaporized to obtain a dry solid polyacrylate crosslinked product. According to the method of the present invention, as described above, potassium acrylate and a water-miscible or water-soluble divinyl compound are prepared into a heated aqueous solution of a specific concentration, and by polymerizing (and crosslinking) them, The polymerization reaction progresses sufficiently within an extremely short period of time, and the reaction heat of this polymerization reaction quickly evaporates water from the reaction system, which allows direct polymerization without the need for any subsequent drying process. A dry solid water-absorbing resin having a desired low moisture content can be obtained. It can be easily ground into a powder suitable for the desired application. Moreover, the method of the present invention eliminates the risk of non-uniform crosslinking caused by overheating due to the external heating and drying process, or a decrease in water absorption capacity due to this, and the water-absorbing resin has very good quality. Solids can always be obtained stably. Furthermore, the method of the present invention has the advantage that the polymerization reaction and vaporization of water are completed very quickly, usually within a few minutes, without external heating, and that the work efficiency is extremely good and the consumption of thermal energy is extremely low. Furthermore, it has a high polymerization reaction rate and is suitable for industrial implementation. In the method of the present invention, first, a heated aqueous solution of mixed monomers containing potassium acrylate and a water-miscible or water-soluble divinyl compound and having a monomer concentration in the range of 55 to 80% by weight is prepared. do. The aqueous solution is particularly characterized by its solution state (whether the monomer is completely dissolved in water or not), the ease of the polymerization reaction of the monomer,
It was carefully selected in consideration of the scattering of monomer components outside the system during the polymerization reaction. This is usually easily prepared by adding acrylic acid, its neutralizing agent potassium hydroxide, and the above divinyl compound to the above monomer concentration in water.
At this time, a slight heating operation may be employed to ensure sufficient dissolution. The use of potassium hydroxide as the neutralizing agent is an essential requirement in the present invention, and if ammonia, amine, etc. are used instead, it is difficult to obtain the desired dry solid water absorbent resin. , even if obtained, it is only at a very low yield and has a low water absorption capacity. Furthermore, when using an alkali metal compound other than potassium hydroxide as a neutralizing agent, such as sodium hydroxide, lithium hydroxide, cesium hydroxide, potassium carbonate, sodium carbonate, etc., it is difficult to prepare a desired highly concentrated aqueous solution. Therefore, it is not possible to obtain the desired dry solid polyacrylate crosslinked product with low moisture content. Additionally, in the above, potassium hydroxide is 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 or water-soluble divinyl compound 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 it. Examples of such divinyl compounds include bisacrylamides such as N,N-methylenebisacrylamide and N,N-methylenebismethacrylamide;
Typical crosslinking agents include diacrylic (or methacrylic) acid esters represented by the following general formula (); diacrylamides represented by the following general formula (); among these, N,N-methylenebisacrylamide is particularly suitable; , N,N-methylenebismethacrylamide and the like are preferred. (In the formula, X is ethylene, propylene, 2-hydroxypropylene, -(CH ₂ CH ₂ O-) _o CH ₂ CH ₂ - or

[Formula] represents a group, n and m represent integers of 5 to 40) Compounds of the general formula () include, for example, ethylene glycol, propylene glycol, glycerin,
It is obtained by reacting polyols such as polyethylene glycol and polypropylene glycol with acrylic acid or methacrylic acid. (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 above-mentioned divinyl compound is about 0.001 to 0.3% by weight, preferably 0.005 to 0.1% of the total weight of the potassium acrylate salt or, in some cases, the total weight of this and free acrylic acid in the normally prepared mixed monomer aqueous solution. It is advantageously used in a range of % by weight, whereby a dry solid of a polyacrylate crosslinked product having excellent water absorption ability can be obtained. 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. In the present invention, it is important that the concentration of the above-mentioned potassium acrylate (or in some cases, it and free acrylic acid) and divinyl compound is 55 to 80% by weight, more preferably 65 to 75% by weight. The desired purpose can only be achieved by heating and using an aqueous solution with a concentration within this range. 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. In addition, the above monomer concentration was increased to 80% by weight.
It is difficult and impractical to achieve a concentration exceeding 100% from the viewpoint of the solubility of the monomer. Next, in the present invention, the aqueous solution of the 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). Here, the heating conditions are not particularly limited on the premise that the polymerization reaction is initiated by addition of a polymerization initiator, but it may be generally about 50 to 85°C, preferably about 60 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
-Aminodinopropane) hydrochloride, so-called azo initiators, etc., or a combination of two or more thereof can be appropriately selected and used. Among these, in particular redox initiators consisting of a combination of ammonium persulfate and sodium bisulfite and azo initiators such as azobisisobutyronitrile or 2,2'-azobis(2-aminodinopropane) hydrochloride. Agents etc. are suitable. The polymerization reaction initiator is usually advantageously used in the form of an aqueous solution, but it can also be used diluted with an appropriate solvent, for example. 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 of the total weight of the monomer acrylate (or acrylic acid and acrylic acid) and divinyl compound.
~10% by weight, more preferably 0.5-5% by weight
It is better to set it as a degree. 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 vaporization of water can be carried out without any external heating. The polymerization reaction and vaporization of water by 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 a heated aqueous solution of mixed monomers, and then transferring this mixed solution. It is carried out by flowing down and spreading on a 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 the polymerization reaction initiator is added to and mixed with the heated aqueous solution of mixed monomers, the polymerization reaction starts very 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 approximately 100 to 130℃, and as a result, the moisture in the system is rapidly vaporized without any external heating, and the desired low moisture content polymer (dry solid) is obtained. be done. 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. Thus, according to the present invention, a dry solid crosslinked polyacrylate product having a moisture content of usually 15% or less can be obtained extremely quickly and easily. The amount of monomer remaining in the polymer is only 2000 ppm or less relative to the polymer, and the polymerization rate is also extremely good. 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 various purposes such as sanitary products such as cloths, 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 and comparative examples are given below to explain the present invention in more detail. Example 1 Add 72.1 g of acrylic acid to 22.2 g of deionized water,
Furthermore, 49.5 g of potassium hydroxide with a purity of 85% as a neutralizing agent and 0.01 g of N,N-methylenebisacrylamide as a divinyl compound were sequentially added to this to form a potassium acrylate aqueous solution with a mixed monomer concentration of 70% by weight. (degree of neutralization 75%). The aqueous solution prepared above was kept at 70°C, and 2.9 g of an 18% aqueous solution of ammonium persulfate (0.5% by weight of the total weight of potassium acrylate free acrylic acid and N,N-methylenebisacrylamide, the same hereinafter) and sodium bisulfite
1.7 g (0.5% by weight) of a 30.6% aqueous solution is mixed, and the mixed solution is spread on an endless moving belt in a layer approximately 10 mm thick. After about 30 seconds, the polymerization reaction starts and is completed in about 1 minute. The maximum temperature during this period is approximately 120℃. Thus, the moisture content is 11% and the residual monomer concentration is 1200 ppm.
A strip-shaped dry solid of crosslinked potassium polyacrylate is obtained. This is pulverized by pulverization using a pulverizer. The water absorption capacity of the resulting powder was 450 in deionized water and 60 in 1% saline. This water absorption capacity is based on 1g of the obtained polymer powder.
The powder is evaluated by adding the powder (in terms of absolute dry weight) to test water and then determining the weight (g) of the test water absorbed by the powder. Examples 2 to 9 Concentration of the mixed monomer prepared in Example 1, its insulating temperature conditions, amount of divinyl compound (N,N-methylenebisacrylamide) used, type of neutralizing agent and amount used (degree of neutralization) and ammonium persulfate (abbreviated as APS) and sodium bisulfite (SPS) as polymerization reaction initiators.
Polymers were obtained in the same manner by changing the amount of at least one of the mixed monomers (abbreviated as "1") as shown in Table 1 below. Note that Table 1 also shows the results of Example 1.

[Table] The amount of the divinyl compound used in the table indicates the weight% in the mixed monomer (potassium acrylate, free acrylic acid, and the divinyl compound), and the initiator concentration is the amount of the divinyl compound used in the above mixed monomer and the divinyl compound. It shows the weight% of the initiator in the total weight (solid content equivalent) with the initiator,
The same shall apply hereinafter. The moisture content of the dry solid of each obtained polymer (potassium polyacrylate crosslinked product) was determined by adding the powder after powdering to deionized water and 1% saline in the same manner as in Example 1. The water absorption capacity is shown in Table 2 below.

[Table] Comparative Example 1 A polymer salt was obtained in the same manner as in Example 1 except that the monomer concentration was changed to 40% by weight. The moisture content of the obtained polymer salt was 42%, which meant that it was not possible to powder it directly because it required subsequent drying. Comparative Example 2 A polymer salt was obtained in the same manner as in Example 1 except that the monomer concentration was changed to 50% by weight. The moisture content of the obtained polymer salt was 33%, which meant that it was not possible to powder it directly because it required subsequent drying. Comparative Example 3 In Example 1, an attempt was made to prepare a sodium acrylate aqueous solution with a mixed monomer concentration of 50% by weight (neutralization degree of 100%) by using sodium hydroxide instead of potassium hydroxide as a neutralizing agent. A homogeneous aqueous solution was not obtained, and some precipitate was deposited. Even when this was heated to 65°C, it still did not become homogeneous and became an aqueous solution when heated above 70°C. After keeping it at the same temperature, the same polymerization reaction initiator was added and mixed to the liquid in the same manner as in Example 1, and the same operation was repeated thereafter, and a polymer salt with a water content of 30% was obtained. This product could not be subsequently directly pulverized, and pulverization required a heating drying operation. Comparative Examples 4 and 5 Polymers were obtained in the same manner as in Example 1 except that the amount of N,N-methylenebisacrylamide used as the divinyl compound was changed as shown in Table 3 below. The water content and water absorption capacity of the obtained polymer were determined in the same manner and the results are also shown in Table 3.

[Table] Note that the polymer obtained in Comparative Example 4 had a sol-like property upon water absorption, and no gel-like material was obtained as in the polymer according to the present invention. Examples 10 to 13 Polymers were obtained in the same manner as in Example 1 by changing at least one of the amount of N,N-methylenebisacrylamide used and the type and amount of polymerization initiator as shown in Table 4 below. Ta. Table 4 also shows the results of determining the water content and water absorption capacity of each of the obtained polymers.

[Table] Examples 14 to 21 Polymers were obtained in the same manner as in Example 1 using predetermined amounts of each compound shown in Table 5 below as the divinyl compound. The water content and water absorption capacity of each of the obtained polymers are also listed in Table 5 below.

【table】

[Table] The polyethylene glycol diacrylate* used in Examples 20 and 21 is represented by the following formula.

Claims (1)

[Claims] 1 Contains potassium acrylate and a water-miscible or water-soluble divinyl compound, and the concentration of these monomers is
A polymerization reaction initiator is added to a heated aqueous solution in the range of 55 to 80% by weight, the polymerization reaction is carried out without external heating, and water is vaporized to obtain a dry solid polyacrylate crosslinked product. A method for producing a dry solid water-absorbing resin. 2. The method according to claim 1, wherein the monomer concentration is 65 to 75% by weight. 3. The method according to claim 1, wherein the temperature of the heated aqueous solution is in the range of 50 to 85°C. 4 The water-miscible or water-soluble divinyl compound is N,
2. A method according to claim 1, wherein the N-methylenebisacrylamide and N,N-methylenebismethacrylamide are selected.