JPS6343410B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field of the Invention] The present invention relates to a method for producing a water-absorbing resin with improved gel strength. [Prior art] Water-absorbing resins are used in sanitary products such as sanitary products, diapers, and disposable rags, as well as in agricultural and horticultural products such as water retention agents, as well as for solidifying sludge, preventing condensation on building materials,
It is used for purposes such as dehydrating oils. Examples of this type of water-absorbing resin include carboxymethylcellulose crosslinked products, polyethylene oxide partially crosslinked products, starch-acrylonitrile graft copolymer hydrolysates, polyacrylate partially crosslinked products, vinyl alcohol-acrylate copolymers, etc. However, in either case, the water absorption capacity is low, or even if the water absorption capacity is high, the gel strength after water absorption is weak, or the gel after water absorption becomes sticky. It has drawbacks such as: As a method for increasing the gel strength of a water-absorbing resin after water absorption, there is a method of increasing the crosslinking density of the water-absorbing resin, but this is not preferred because the water-absorbing ability, which is the inherent performance of the water-absorbing resin, decreases. Another method for increasing the gel strength of a water-absorbing resin after water absorption is to add water to the water-absorbing resin in the presence of a hydrophilic organic solvent such as a lower monohydric alcohol, so that water is substantially absorbed into the water-absorbing resin. There is a method in which the material is cross-linked after being uniformly absorbed, and then dried. In this method, crosslinking with a large amount of water absorbed is considered preferable from the viewpoint of the water absorption ability of the water absorbent resin, but when this method is adopted, the amount of water used is limited, and the water absorption Even in a small amount, resin particles in a swollen state after absorbing water tend to aggregate and form lumps, and the workability is poor, so it cannot be said to be suitable for industrial use. Therefore, in this method, a small amount of water is added in the presence of a large amount of a hydrophilic organic solvent to make the water-absorbing resin particles absorb water and swell, thereby making it difficult for agglomeration to occur between the resin particles and allowing the crosslinking reaction to occur. As a result, there are problems such as high manufacturing costs and low productivity. [Summary of the Invention] In view of the above-mentioned circumstances, the present inventors maintained the water absorption ability and water absorption rate of the water absorbent resin, and created a gel that has high gel strength after water absorption and does not feel sticky after water absorption. As a result of extensive research aimed at efficiently, easily, and inexpensively producing a water-absorbing resin with improved gel strength after water absorption, we found that inert By using an inorganic powder, spraying a crosslinking agent and water, and crosslinking, the above objects can be achieved without using a hydrophilic organic solvent, which was an indispensable component in the conventional technology. They discovered this and completed the present invention. That is, the present invention involves stirring a water-absorbing resin containing a monomer unit having a carboxylate as a constituent component of the polymer and an inert inorganic powder,
Crosslinking agent and water (70 to 70% based on the solid content of the water-absorbing resin)
200% (corresponding to % by weight, hereinafter the same)) is added by spraying, then heated to cause a crosslinking reaction, and then the water is distilled off. Regarding the manufacturing method. Note that the term "carboxylate" as used herein is a concept that includes carboxyl groups and salts of carboxyl groups. [Embodiments of the invention] The water-absorbing resin whose gel strength after water absorption is improved according to the present invention (hereinafter referred to as the "water-absorbing resin improved according to the present invention") is a polymer or copolymer containing carboxylic acid in its constituent components. It can be used without particular limitation as long as it contains a monomer unit having a certain rate. As the water-absorbing resin, (meth)
Crosslinked products of acrylic acid polymers (meaning acrylic acid polymers or methacrylic acid polymers, hereinafter the same descriptions have the same meanings), crosslinked products of polysaccharide-(meth)acrylic acid graft copolymers, ( meth)Acrylic acid-acrylamide-crosslinked product of sulfonated acrylamide terpolymer or alkali metal salt or alkaline earth metal salt thereof, such as acrylic acid (salt) polymer, acrylic acid (salt)-
Crosslinked products such as methacrylic acid (salt) copolymer, starch-acrylic acid (salt) graft copolymer; crosslinked product of saponified polysaccharide-(meth)acrylic acid alkyl ester graft copolymer, polysaccharide-acrylonitrile Crosslinked products of saponified graft copolymers, crosslinked products of saponified polysaccharide-acrylamide copolymers, such as saponified products of starch-ethyl acrylate graft copolymers, and saponified products of starch-methyl methacrylate graft copolymers. Crosslinked products such as saponified products of starch-acrylonitrile graft copolymers, saponified products of starch-acrylamide graft copolymers; Crosslinked products of saponified products of (meth)acrylic acid alkyl ester-vinyl acetate copolymers, such as methacrylate Crosslinked products such as saponified products of ethyl acid vinyl acetate copolymer, saponified products of methyl acrylate-vinyl acetate copolymer; starch-acrylonitrile-acrylamide-2-
Crosslinked product of saponified methylpropane sulfonic acid graft copolymer; starch-acrylonitrile-
Crosslinked products of saponified vinyl sulfonic acid graft copolymers; crosslinked products of sodium carboxymethylcellulose, etc. may be mentioned, but are not limited to these. These may be used alone,
Two or more types may be mixed and used. Preferred water-absorbing resins improved by the present invention are crosslinked products of (meth)acrylic acid polymers, crosslinked products of polysaccharide-(meth)acrylic acid graft copolymers, and (meth)acrylic acid-acrylamide- These are crosslinked products of sulfonated acrylamide terpolymer or alkali metal salts or alkaline earth metal salts thereof. The particle size of the water absorbent resin improved by the present invention is not particularly limited, and as long as it is in powder or particulate form, there are no particular restrictions on its shape or size, but the particle size is usually about 10 to 600 mesh. It is preferable to have the following. The inert inorganic powder used in the present invention includes:
Examples include hydrated silicon dioxide powder, hydrated aluminum oxide powder, hydrated titanium oxide powder, anhydrides of these, or powders containing these as main components, and these may be used alone or in combination of two or more. May be used. There is no limit to the crystal system of the inorganic powder; for example, for aluminum oxide powder, α-type, β-type, and γ-type can be used equally effectively, and for titanium oxide powder, TiO,
It may be either Ti ₂ O ₃ or TiO ₂ . Furthermore, there is no particular limitation on the water content of these hydrated powders; for example, aluminum hydroxide powder has a
Al ₂ O ₃・H ₂ O powder, Al ₂ O ₃・2H ₂ O powder, Al ₂ O ₃・
3H ₂ O powder, TiO ₂ .H ₂ O powder, TiO ₂ .2H ₂ O powder, etc. are similarly used as titanium oxide powder.
In addition, the powder containing hydrated or anhydrous inorganic substances as a main component includes, for example, hydrated silicon dioxide such as colloidal silica, white carbon, ultrafine particulate silica, and/or anhydrous silicon dioxide (hereinafter referred to as fine particulate silica). Those whose main component is hydrated and anhydrous aluminum oxide, such as plate-shaped hydrated alumina and fibrous hydrated alumina, and those whose main component is rutile-type or anatase-type hydrated and anhydrous titanium oxide. For example, Among these inert inorganic powders, fine particulate silica, titanium dioxide powder, alumina powder, and the like are preferred. The average particle size of the inorganic powder is preferably 0.001 to 10 μm, preferably 0.005 to 1 μm.
More preferably, those having properties that improve the mutual dispersibility of water-absorbing resin particles in a water-absorbing and swollen state and improve fluidity are preferred. The crosslinking agent used in the present invention is a crosslinking agent having two or more functional groups that can react with groups such as carboxylate, hydroxyl group, sulfone group, and amino group present in the water absorbent resin improved by the present invention. Yes, such a material can be used without particular limitation. Examples of the crosslinking agent include diglycidyl ether compounds, polyvalent metal salts, haloepoxy compounds, aldehyde compounds, and isocyanate compounds. As the diglycidyl ether compound,
For example, diglycidyl ether compounds such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether are suitable, and among these, ethylene glycol diglycidyl ether is most suitable. . Examples of the polyvalent metal salts include compounds that can form crosslinks with carboxylates of water-absorbing resins through ionic reactions, and specific examples include divalent metals such as magnesium, calcium, barium, and zinc, or aluminum, iron, etc. trivalent metal halides, sulfates, nitrates, etc. More specifically, magnesium sulfate, aluminum sulfate, ferric chloride, calcium chloride, magnesium chloride,
Examples include aluminum chloride, polyaluminum chloride, iron nitrate, calcium nitrate, and aluminum nitrate. Specific examples of the haloepoxy compounds include:
Epichlorohydrin, epibromohydrin, α-
Aldehyde compounds such as methyl epichlorohydrin, glutaraldehyde, glyoxal, etc., and isocyanate compounds such as 2,
Examples include 4-tolylene diisocyanate and hexamethylene diisocyanate. The above-mentioned crosslinking agents may be used alone, and 2
Although more than one type may be used in combination, it is preferable to select and use an appropriate one depending on the type of water-absorbing resin to be improved by the present invention. The purpose of this is to add a crosslinked structure to the water-absorbing resin to be improved again, and to improve the gel strength of the water-absorbing resin after water absorption while maintaining the water-absorbing ability and water absorption rate. Of the crosslinking agents mentioned above, diglycidyl ether compounds, polyvalent metal salts, and haloepoxy compounds are suitable. In the present invention, 1 to 30 parts of inert inorganic powder, preferably 5 to 20 parts of inert inorganic powder, per 100 parts (by weight or less, the same) of the water absorbent resin improved by the present invention.
70 to 200 parts of water and 0.005 to 5.0 parts, preferably 0.01 to 1 part of crosslinking agent are used to produce a water absorbent resin with improved gel strength after water absorption. The role of water in the crosslinking reaction is to swell the water absorbent resin improved by the present invention and allow the crosslinking agent to penetrate into the water absorbent resin, thereby causing the crosslinking reaction to occur inside the water absorbent resin. Therefore, the more water you use, the more cross-linking reaction will occur internally, making it easier for the water-absorbing resin to maintain its network structure even after absorbing water and swelling, resulting in stable gel strength over time. A water-absorbing resin having the following properties is obtained. Furthermore, if the amount of water added to the water-absorbing resin improved by the present invention is increased, the swollen water-absorbing resin will aggregate and form a lump, making it difficult to maintain a uniform state of the water-absorbing resin and water. Using an active inorganic powder as a dispersant, uniformly stirring the water-absorbing resin improved by the present invention and the inorganic powder, and then adding a crosslinking agent and water by spraying, it is possible to obtain a uniformly dispersed state. By carrying out the crosslinking reaction in such a state and distilling off the water, a water-absorbing resin with improved gel strength after water absorption can be obtained. When the amount of the inert inorganic powder used is less than 1 part per 100 parts of the water absorbent resin improved by the present invention, water and a crosslinking agent are sprayed onto the mixture of the water absorbent resin and the inert inorganic powder. When the resin particles are brought into a semi-swollen state and stirred, agglomeration occurs between the resin particles in the water-swollen state, resulting in a lumpy state, making it impossible to carry out the crosslinking reaction homogeneously.
The crosslinking reaction itself may become difficult to proceed. Furthermore, even if the amount exceeds 30 parts, not only will the effect of the amount exceeding 30 parts not be obtained, but the water absorption capacity per weight of the water absorbent resin with improved gel strength after water absorption will decrease. There is a tendency to When the amount of water used is less than 70 parts per 100 parts of the water absorbent resin improved by the present invention, the gel strength and sticky feeling of the gel after water absorption of the crosslinked and improved water absorbent resin are sufficiently good. can't be, and can't be
If more than 200 parts are used, even if an inert inorganic powder is used, the semi-swollen water-absorbing resin will aggregate between the resin particles and become lumpy, making it impossible to carry out the crosslinking reaction homogeneously. It may disappear or the reaction itself may become difficult to proceed. When the amount of water used is 70 to 200 parts, an improved water-absorbing resin that maintains water absorption capacity and water absorption rate, has high gel strength after water absorption, and does not feel sticky even after water absorption is used. It can be grown. Moreover, even without using a hydrophilic organic solvent as in conventional methods, the presence of only inert inorganic powder prevents resin particles in a water-absorbing and swollen state from agglomerating and forming into lumps. The mixture becomes homogeneous and can be easily crosslinked under sufficient stirring conditions. Furthermore, as mentioned above, since no organic solvent is used in the present invention, the volumetric efficiency (yield per unit volume) of the resulting improved water-absorbing resin can be greatly improved, and moreover, the volumetric efficiency (yield per unit volume) of the resulting improved water-absorbent resin can be greatly improved. Processes such as recovery and regeneration are no longer necessary, and this can also contribute to lowering the cost of the improved water-absorbing resin. The amount of crosslinking agent used in the present invention depends on the type of crosslinking agent, the type of water absorbent resin to be improved, the amount of water used, the type and amount of inert inorganic powder, the purpose of use of the improved water absorbent resin, etc. Varies depending on the situation, but typically 0.005 for the water-absorbing resin being improved.
About 5.0% is preferable, and 0.01 to 1.0% is more preferable. In general, if the amount of crosslinking agent used is less than 0.005%, there will be little effect of improving the strength of the gel after water absorption, and if it is more than 5%, the degree of crosslinking will become too high and the water absorption capacity will tend to decrease. arise. The water-absorbing resin with improved gel strength after water absorption according to the present invention can be obtained, for example, by mixing an inert inorganic powder with the water-absorbing resin improved according to the present invention, and then spraying and adding an aqueous solution of a crosslinking agent while stirring. The crosslinking agent and water are added separately by spraying, then the reaction system is heated to a predetermined temperature, and while or after the crosslinking reaction is carried out, the added water is heated to normal pressure to
By distilling it out of the system under reduced pressure, a water-absorbing resin with the desired improved gel strength after water absorption can be obtained. Another method for obtaining the improved water-absorbent resin is to mix an inert inorganic powder with the water-absorbent resin improved by the present invention, raise the temperature to a predetermined temperature in advance, and then mix the crosslinking agent with stirring. There is a method in which the aqueous solution is added by spraying, or the crosslinking agent and water are added by spraying separately, and then the crosslinking reaction and drying are carried out by holding at a predetermined temperature. As for the method of adding the crosslinking agent and water in the above manufacturing method, it is preferable to add the predetermined amount of these to the water-absorbing resin by spraying using a shearing method or a spray method, which can be added substantially uniformly and is industrially preferable. There are no particular restrictions on the stirring method used when adding the crosslinking agent and water to the water-absorbent resin improved by the present invention by the method described above, or after adding the crosslinking agent and water.
Any other method can be used as long as these components are substantially uniform, and for example, agitators with stirring blades of various shapes, kneaders, pipeline mixers, etc. can be used as they are. The temperature conditions for smoothly carrying out the crosslinking reaction cannot be determined unconditionally because they vary depending on the type of crosslinking agent used, the type and amount of inert inorganic powder, the use of the water absorbent resin with improved gel strength, etc. However, it is usually preferable to carry out the reaction at a temperature in the range of 40 to 150°C. The water absorbent resin improved by the method of the present invention is
It maintains the water absorption capacity and water absorption rate, and provides a gel with high gel strength and a smooth feeling after water absorption. Furthermore, according to the method of the present invention, the improved water-absorbing resin described above can be easily and efficiently produced. Next, the method of the present invention will be explained based on Examples, but the present invention is not limited thereto. Example 1 100 g of powder of a cross-linked polyacrylic acid potassium salt (Arasorb manufactured by Arakawa Chemical Co., Ltd.) and Aerosil 200 (average particle size of about 0.012 μm, manufactured by Nippon Aerosil Co., Ltd.)
Place 8 g of fine particulate silicon dioxide) into a 300 ml three-necked separable flask, stir thoroughly with a stirrer, and while continuing to stir, add 0.10 g of ethylene glycol digidyl ether (EGDG) and 84 g of water.
A solution consisting of g was added by spraying to obtain a uniformly dispersed state. Thereafter, it was heated at about 80°C for 1 hour to effect crosslinking. Water was then distilled off at about 120°C, and finally remaining water was distilled off under reduced pressure (about 30 mmHg) for about 10 minutes to obtain 94 g of a water-absorbing resin with improved gel strength after water absorption. By the following method using the obtained water absorbent resin,
Water absorption capacity, water absorption rate, and gel strength after water absorption were measured. The results are shown in Table 1 along with the state of the water absorbent resin during the crosslinking reaction. (Water absorption capacity) Add 150 g of deionized water or physiological saline (0.9% saline) and 0.12 g of the water absorbent resin obtained according to the present invention to a 200 ml beaker, and leave it for 30 minutes.
Filter through a 200-mesh wire mesh, measure the amount of water flowing out, and calculate the water absorption capacity using the formula below. Water absorption capacity = Weight of water initially added - Weight of water flowing out / Weight of water absorbent resin (water absorption rate) Physiological saline (0.9%) was placed in a 100ml beaker in advance.
Add 50g of saline solution and a stirrer, stir at a speed of 600rpm with a magnetic stirrer, and add 2.0g of water-absorbing resin into the mixture.Gellation occurs due to water absorption and swelling, resulting in poor fluidity. The water flow vortex at the stirring center disappears. The time required for the vortex to disappear after the introduction of the water-absorbing resin is measured and determined as the water absorption rate. (Gel strength) A gel (hereinafter referred to as 30x gel) was prepared by mixing and stirring 60 g of physiological saline and 2.0 g of water absorbent resin.
The hardness of the gel was measured after 1 hour, 1 day, and 3 days using a Neocard meter manufactured by Iio Electric Co., Ltd. Here, the hardness of a gel refers to the elastic force required to break the gel. Examples 2 to 9 and Comparative Examples 2 to 3 Water absorbent resins with improved gel strength after water absorption were obtained in the same manner as in Example 1 using the reaction system compositions shown in Table 1, and their physical properties were evaluated. The results are shown in Table 1 along with the state of the water absorbent resin during the crosslinking reaction. Comparative Examples 1 and 5 An attempt was made to produce a water absorbent resin with improved gel strength after water absorption using the reaction composition shown in Table 1 in the same manner as in Example 1, but blocking occurred during the reaction and the desired product was not produced. I couldn't get it. Comparative Example 4 An attempt was made to produce a water absorbent resin with improved gel strength after water absorption using the reaction composition shown in Table 1 in the same manner as in Example 1 except that a crosslinking agent and water solution were added dropwise. However, blocking occurred during the reaction and the desired product could not be obtained.

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Claims (1)

[Claims] 1. While stirring a water-absorbing resin containing a monomer unit having a carboxylate as a component of the polymer and an inert inorganic powder, a crosslinking agent and water (based on the solid content of the water-absorbing resin) are added. A method for producing a water-absorbing resin with improved gel strength, which comprises spraying and adding 70 to 200% by weight of a water-absorbing resin, followed by heating to cause a crosslinking reaction, and then distilling off water. 2 The water-absorbing resin is a crosslinked product of a (meth)acrylic acid polymer, a crosslinked product of a polysaccharide-(meth)acrylic acid graft copolymer, or a crosslinked product of a (meth)acrylic acid-acrylamide-sulfonated acrylamide terpolymer. The method according to claim 1, which is at least one selected from the group consisting of crosslinked products and their alkali metal salts or alkaline earth metal salts. 3. The manufacturing method according to claim 1, wherein the inert inorganic powder is at least one selected from the group consisting of particulate silica, titanium dioxide powder, and alumina powder. 4. The method according to claim 1, wherein the crosslinking agent is at least one selected from the group consisting of diglycidyl ether compounds, polyvalent metal salts, and haloepoxy compounds.