JP7642729B2 - Manufacturing method of water-absorbent resin - Google Patents

Description

The present invention relates to a method for producing a water-absorbent resin, and in particular, to a method for producing a water-absorbent resin having excellent liquid flow conductivity.

Water-absorbent resins are used in a wide range of applications, such as water retention agents in agriculture or horticulture, anti-condensation agents in building materials, materials for removing water from petroleum, waterproof outer coatings for cables, and personal hygiene products (e.g., disposable diapers, feminine hygiene products, and disposable towels), with disposable diapers being the main application.

The absorption performance of a paper diaper is determined by the absorption speed, absorption amount, and smoothness. In recent years, in order to make paper diapers thinner, the amount of pulp (hydrophilic fiber) used has been reduced and the amount of water-absorbing resin used has been increased, thereby increasing the proportion of water-absorbing resin in the absorbent structure of the paper diaper. However, if the amount of hydrophilic fiber used is reduced, the water storage space becomes smaller in a short time, the liquid penetration speed becomes slow, and the liquid leaks before being absorbed. In addition to having an excellent absorption speed, the water-absorbing resin must have high permeability to liquid. If the liquid permeability is poor, the water-absorbing resin will block the gaps between particles when absorbing liquid, causing a colloid blocking phenomenon, causing the liquid to flow out of the absorbent body of the paper diaper, which will reduce the absorption performance of the paper diaper.

Therefore, improving the absorption rate and permeability of water-absorbent resins is currently one of the main focuses of research in this field. In the conventional method, in order to improve the liquid flow conductivity of water-absorbent resins, surface modification with polyvalent metal cations or aluminum dihydroxyacetate can be used. In order to improve the liquid permeability of water-absorbent resins, thermoplastic polymers such as polyethylene and polypropylene can be used in the heat treatment process, or an amino group-containing azo compound can be added as a foaming agent to the acid group-containing monomer aqueous solution. In addition, by adding an encapsulated blowing agent or a water-soluble alkoxysilane compound to the acid group-containing monomer aqueous solution, the manufactured water-absorbent resin can have an excellent absorption rate and high colloidal stability. Furthermore, it is known that the diffusion and liquid permeability of the water-absorbent resin after absorbing liquid can be improved by mixing a water-soluble polyvalent metal powder, an adhesive, and a water-absorbent resin.

In addition, conventional water-absorbent resins are subjected to a surface cross-linking treatment to form further cross-links on the surface of the water-absorbent resin in order to achieve effects such as improved absorption speed, improved colloid strength, improved anti-caking properties, and improved liquid permeability. For example, conventional methods include dispersing a water-absorbent resin and a cross-linking agent in an organic solvent and performing a surface cross-linking treatment, mixing an inorganic powder into a water-absorbent resin and performing a cross-linking treatment, steaming after adding a cross-linking agent, performing a surface treatment using an organic solvent, water, and a polyhydric alcohol, and performing a surface treatment using an organic solvent, water, and an ether-based compound. However, although conventional surface treatment methods can improve the absorption speed and water absorption capacity under pressure, there is a risk that the retention force of the water-absorbent resin will be significantly reduced, which will reduce the practical effect.

In view of this, it is necessary to provide a water-absorbent resin that can improve both the liquid flow conductivity and the water absorption capacity under pressure, while maintaining a high retention force, and a method for producing the same.

One aspect of the present invention provides a method for producing a water-absorbent resin in which a reactive polymer is involved in a surface cross-linking reaction in order to improve the liquid flow conductivity of the resulting water-absorbent resin.

Another aspect of the present invention provides a water-absorbent resin produced by the above aspect.

According to one aspect of the present invention, a method for producing a water-absorbent resin is provided, which includes the steps of: subjecting a water-absorbent resin composition containing an aqueous unsaturated monomer solution, a polymerization initiator, and a free radical polymerization crosslinking agent to a free radical polymerization reaction to obtain a colloidal gel; pulverizing and sieving the colloidal gel to obtain a plurality of water-absorbent resin particles; and subjecting the water-absorbent resin particles to a surface crosslinking reaction with a reactive polymer containing a surface crosslinking agent, a polyethylene segment, and a polyacrylic acid segment to obtain a water-absorbent resin.

According to one embodiment of the present invention, the reaction polymer is an ethylene acrylic polymer, which comprises a polyethylene segment represented by the following formula (1) and a polyacrylic acid segment represented by the following formula (2):

M is a hydrogen atom, a Group IA element or a Group IIA element. When the ethylene acrylic polymer is 100 wt %, the polyethylene segment is 80 wt % to 99 wt %, and the polyacrylic acid segment is 1 wt % to 20 wt %.

According to one embodiment of the present invention, the weight of the polyacrylic acid segment in the reaction polymer is 8 wt% to 20 wt%, assuming that the weight of the polyethylene segment is 100 wt%.

According to one embodiment of the present invention, the weight of the water-absorbent resin particles is 100 wt %, and the amount of the reactive polymer added is 0.01 wt % to 10 wt %.

According to another aspect of the present invention, there is provided a water-absorbent resin produced by the above method and having a T20 value of 180 seconds or less.

According to one embodiment of the present invention, the liquid flow conductivity of the water absorbent resin is 30×10 ⁻⁷ cm ³ -s/g or more.

According to one embodiment of the present invention, the water-absorbent resin has a retention capacity of greater than 20 g/g.

The water-absorbent resin and the method for producing the same of the present invention have the effect of improving the liquid flow conductivity and water absorption capacity under pressure of the water-absorbent resin by involving a reactive polymer in the surface cross-linking reaction, and can also maintain the retention power of the water-absorbent resin at a high level, making them practical.

As used herein, "around," "about," "approximately," or "substantially" generally refers to within 20%, or within 10%, or within 5% of a numerical value or range.

As described above, the present invention provides a water-absorbent resin and a method for producing the same that are practical and have the effect of improving the liquid flow conductivity and water absorption capacity under pressure of the water-absorbent resin by involving a reactive polymer in the surface cross-linking reaction, while at the same time maintaining the retention power of the water-absorbent resin at a high level.

The method for producing a water-absorbent resin provided by the present invention includes a step of subjecting a water-absorbent resin composition to a free radical polymerization reaction to obtain a colloidal gel. In some embodiments, the water-absorbent resin composition includes an aqueous solution of an unsaturated monomer, a polymerization initiator, and a free radical polymerization crosslinker.

In some embodiments, the unsaturated monomer aqueous solution in the water absorbent resin composition includes an acid group monomer having an unsaturated double bond, such as acrylic acid. In some embodiments, the unsaturated monomer aqueous solution may be methacrylic acid, 2-acrylamido-2-methylpropanesulfonic acid, malic acid (cis-butenyl acid), cis-butenyl acid anhydride, fumaric acid (trans-butenyl acid), and trans-butenyl acid anhydride. The unsaturated monomer aqueous solution may include one type of monomer, but is not limited thereto, and two or more of the above monomer aqueous solutions may be selected.

In some embodiments, the concentration of the unsaturated monomer aqueous solution may be, but is not limited to, 20 wt% to 55 wt%, preferably 30 wt% to 45 wt%. In general, when the concentration of the acid-group monomer aqueous solution is 20 wt% to 55 wt%, the viscosity of the product after polymerization is appropriate, machining is easy, and the reaction heat during the free radical polymerization reaction is easy to control.

In other embodiments, other hydrophilic monomers having unsaturated double bonds, such as acrylamide, methacrylamide, 2-carboxyethyl acrylate, 2-carboxyethyl methacrylate, methyl acrylate, ethyl acrylate, dimethylamine acrylamide, and trimethylamine chloride, may be selectively added. However, the amount of the hydrophilic monomers added should not impair the physical properties (e.g., retention and absorption speed) of the water absorbent resin.

In some embodiments, the acid-group monomer aqueous solution may be directly polymerized, or may be partially neutralized with a neutralizing agent to make the acid-group monomer aqueous solution neutral or weakly acidic, and then polymerized. In these embodiments, the neutralizing agent includes hydroxide or carbonate compounds of alkali metals or alkaline earth metals (e.g., sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, potassium bicarbonate), amine compounds, and combinations thereof. In some embodiments, the neutralization concentration of the acid-group monomer aqueous solution is 45 mol% to 85 mol%. When the neutralization concentration of the acid-group monomer aqueous solution is in the aforementioned range, the acid-group monomer aqueous solution has a pH suitable for free radical polymerization reactions and can reduce damage caused by accidental contact with the human body. As a supplementary explanation, the neutralization concentration in this specification is defined as the ratio of the moles of the basic solution to the moles of the acid-group monomer aqueous solution, and may be the percentage of the acid groups in the acid-group monomer aqueous solution that are neutralized.

In some embodiments, the pH value of the acid group monomer aqueous solution is 5.5 or more, preferably 5.5 to 7.0, and more preferably 5.5 to 6.5. When the pH of the acid group monomer aqueous solution is 5.5 to 7.0, a large amount of unreacted monomer is unlikely to remain in the aqueous solution after polymerization, and the water absorbent resin produced subsequently has good physical properties and a large absorption capacity.

In some embodiments, a water-soluble polymer may be selectively added to the water-absorbent resin composition to reduce manufacturing costs. In some embodiments, the water-soluble polymer includes partially or completely saponified polyvinyl alcohol, polyethylene glycol, polyacrylic acid, polyacrylamide, starch or starch derivatives (e.g., methyl cellulose, methyl cellulose acrylate, ethyl cellulose, etc.), and preferably, starch and partially or completely saponified polyvinyl alcohol are used alone or in combination. In the above embodiments, the molecular weight of the water-soluble polymer is not limited, and the amount of the water-soluble polymer added is generally 0 wt% to 20 wt%, preferably 0 wt% to 10 wt%, and more preferably 0 wt% to 5 wt%, assuming that the amount of the unsaturated monomer aqueous solution used is 100 wt%, as a rule, so as not to reduce the physical properties of the water-absorbent resin.

The prepolymerization reaction starts with the generation of radicals due to the decomposition of the polymerization initiator. In some embodiments, the amount of the polymerization initiator used is 0.001 wt% to 10 wt%, preferably 0.1 wt% to 5 wt%, assuming that the amount of the unsaturated monomer aqueous solution used is 100 wt%. When the amount of the polymerization initiator used is within the above range, the speed of the free radical polymerization reaction is appropriate, it is economically beneficial, the reaction heat is easily controlled, and the formation of colloidal solids due to excessive polymerization can be avoided.

In some embodiments, the polymerization initiator includes a thermal decomposition initiator, a redox initiator, and a combination thereof. In some embodiments, the thermal decomposition initiator includes a peroxide [hydrogen peroxide, di-tert-butyl peroxide, peroxide amide or persulfate (including ammonium salts and alkali metal salts)], and an azo compound [e.g., 2,2-azobis(2-amidinopropane) dihydrochloride, 2,2-azobis(N,N-dimethyleneisobutylamidine) dihydrochloride]. In some embodiments, the redox initiator includes an acid sulfite, ascorbic acid, or a ferrous salt. The polymerization initiator is preferably a combination of a thermal decomposition initiator and a redox initiator, which reacts with the redox initiator to generate free radicals, which transfer to the monomer to initiate the polymerization reaction, and the large amount of heat released by the polymerization reaction increases the temperature of the reaction system. When the reaction system reaches a certain temperature, the thermal decomposition initiator can be further decomposed to make the polymerization reaction more complete, thereby avoiding excess unreacted monomer remaining.

The free radical polymerization reaction crosslinking agent in the water absorbent resin composition can provide the water absorbent resin composition with an appropriate degree of crosslinking, and can improve the processability of the water absorbent resin composition after the polymerization reaction. In some embodiments, the free radical polymerization reaction crosslinking agent may be a compound containing two or more unsaturated double bonds, such as N,N-bis(2-propenyl)amine, N,N-methylenebisacryloylamine, N,N-methylenebismethacryloylamine, propylene acrylate, ethylene glycol diacrylate, polyethylene glycol diacrylate, ethylene glycol dimethacrylate, polyethylene glycol dimethacrylate, glycerin trimethacrylate, triacrylate or trimethacrylate obtained by adding ethylene oxide to glycerin, trimethylolpropane trimethacrylate, trimethylolpropane triacrylate, N,N,N-tris(2-propenyl)amine, ethylene glycol diacrylate, polyoxyethylene triacrylate, diethyl polyoxyethylene triacrylate, dipropylene triglycol, etc. In some embodiments, the free radical polymerization reactive crosslinker may be selected from compounds containing two or more epoxy groups, such as sorbitol polyglycidyl ether, polyglycerol polyglycidyl ether, ethylene glycol diglycidyl ether, diethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, diglycerol polyglycidyl ether, etc. The free radical polymerization reactive crosslinker may be used alone or in combination of two or more.

In some embodiments, the amount of the free radical polymerization crosslinking agent is 0.001 wt% to 5 wt%, preferably 0.01 wt% to 3 wt%, of the acid-group monomer aqueous solution, when considered as 100 wt%. When the amount of the free radical polymerization crosslinking agent added is within the above range, the viscosity of the polymer aqueous solution after the reaction is appropriate, it is easy to machine, and the absorbency of the water-absorbent resin produced subsequently is large, i.e., the performance of the water-absorbent resin is good.

In some embodiments, the free radical polymerization reaction may be carried out in a batch reactor or a conveyorized reactor.

In some embodiments, the colloidal gel may be selectively pulverized in a pulverizer to a particle size of 20 mm or less, preferably 10 mm or less, before the sieving step. In some embodiments, the sieving step is to select colloidal gel with a particle size of 2.0 mm or less, preferably 0.05 mm to 1.50 mm, and then return the colloidal gel with a particle size of more than 2.0 mm to the reactor for another grinding step. It is necessary to control the particle size within the aforementioned range in order to avoid a large amount of fines being generated in the subsequent process, to have good thermal conductivity, and to avoid a high amount of residual monomer in the finished product. In general, the narrower the particle size distribution of the colloidal gel, the better the physical properties and the more favorable the subsequent drying process.

In some embodiments, the colloidal gel may optionally be subjected to a drying process before subsequent operations. In some embodiments, the drying process is performed at a temperature of 100°C to 180°C. By performing the drying process in the aforementioned temperature range, the drying time can be effectively controlled, and the degree of crosslinking can be effectively controlled to avoid a large amount of residual unreacted monomer.

Next, the method for producing the water-absorbent resin includes a step of crushing and sieving the colloidal gel to obtain water-absorbent resin particles. In some embodiments, the particle size of the water-absorbent resin particles is selected to be 0.06 mm to 1.00 mm, preferably 0.10 mm to 0.85 mm. By controlling the particle size of the water-absorbent resin particles to the aforementioned range, the amount of fine powder in the finished product can be reduced, and the absorption performance of the water-absorbent resin can be improved. Similarly, the narrower the size distribution of the water-absorbent resin particles, the better the physical properties.

Then, the water-absorbent resin particles are subjected to a surface cross-linking reaction with a surface cross-linking agent and a reactive polymer to obtain a water-absorbent resin. Since the water-absorbent resin is a hydrophilic polymer that does not dissolve and has a uniform cross-linked structure inside the resin, it is necessary to further cross-link the surface of the resin in order to improve properties such as absorption speed, colloid strength, anti-caking property, and liquid permeability. The surface cross-linking reaction is carried out with a surface cross-linking agent having a functional group that can react with an acid group. In addition, in the present invention, a reactive polymer, for example an ionomer, is added to further increase the liquid flow conductivity of the produced water-absorbent resin, and a surface cross-linking reaction is carried out together.

In some embodiments, the surface crosslinking agent includes a polyol, a polyamine, a compound having two or more epoxy groups, and an alkylene carbonate, the polyol may be, for example, glycerol, ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, and propylene glycol, the polyamine may be, for example, ethylenediamine, diethylenediamine, and triethylenediamine, and the compound containing an epoxy group may be, for example, sorbitol polyglycidyl ether, polyglycerol polyglycidyl ether, ethylene glycol, propylene glycol, and propylene glycol. The alkylene carbonate may be, for example, ethylene glycol carbonate, 4-methyl-1,3-dioxolan-2-one, 4,5-dimethyl-1,3-dioxolan-2-one, 4,4-dimethyl-1,3-dioxolan-2-one, 4-ethyl-1,3-dioxolan-2-one, 1,3-dioxan-2-one, 4,6-dimethyl-1,3-dioxan-2-one, and 1,3-dioxepan-2-one. The reaction may be carried out using a single surface crosslinking agent or a combination of two or more types. Depending on the surface crosslinking agent selected, the surface crosslinking agent may be added directly, or may be added after preparing an aqueous solution or a hydrophilic organic solution. Hydrophilic organic solvents include, but are not limited to, methanol, ethanol, propanol, isobutanol, acetone, methyl ether, and ethyl ether.

In some embodiments, assuming that the amount of water-absorbent resin particles used is 100 wt%, the amount of surface cross-linking agent added is 0.001 wt% to 10 wt%, preferably 0.005 wt% to 5 wt%. When the amount of surface cross-linking agent added is within the above range, a cross-linked structure can be formed on the surface of the water-absorbent resin, thereby achieving good absorption performance.

The water-absorbing resin and the reactive polymer need to be mixed uniformly in order to effectively achieve the effects of the present invention, and therefore a mixing device with high mixing efficiency needs to be used. In some embodiments, the mixing device may be a V-type mixer, a column mixer, a high-speed stirring mixer, a spiral mixer, a flow mixer, a double-arm kneader, a double-arm cone mixer, a screw mixer, an internal mixer, a grinding kneader, a rotary mixer, or a screw extruder.

The ionomer is a polymer that contains repeating units of ionic units within a polymer of electrically neutral repeating units, and the ionic groups are actually part of the polymer backbone. In some examples, the ionic groups may be, for example, carboxylic acid groups. In these examples, the ionomer may be an ethylene-acrylic acid copolymer (EAA), which is a compound that includes a polyethylene segment of formula (1) below and a polypropylene segment of formula (2) below.

M is a hydrogen atom, a Group IA element (e.g., lithium, sodium, or potassium), or a Group IIA element (e.g., calcium or magnesium). In this embodiment, the polyethylene segment is 80 wt% to 99 wt%, and the polyacrylic acid segment is 1 wt% to 20 wt%, assuming that the ethylene acrylic polymer is 100 wt%.

In another embodiment, the ionomer may be an ethylene acrylic compound represented by formula (3):

M is a hydrogen atom, a Group IA element (e.g., lithium, sodium, or potassium), or a Group IIA element (e.g., calcium or magnesium). In some embodiments, M is preferably a Group IA element.

In some embodiments, in the ethylene acrylic compound, if the weight of the polyethylene segment (i.e., formula (1)) is 100 wt%, the weight of the polyacrylic acid segment (i.e., formula (2)) is 8 wt% to 20 wt%. In the ethylene acrylic compound, the polyethylene segment is a hydrophobic group and the polyacrylic acid segment is a hydrophilic group, so it is easy to absorb water. Therefore, if there are too many polyethylene segments (for example, if the weight of the polyacrylic acid segment is 8 wt% or less), the absorption capacity of the water-absorbent resin decreases, and conversely, if there are too many polyacrylic acid segments (for example, if the weight of the polyacrylic acid segment exceeds 20 wt%), the ionomer itself absorbs moisture in the air and forms aggregates, so the mixing effect with the water-absorbent resin particles is reduced, and the intended effect of the present invention cannot be achieved.

In some embodiments, the amount of reactive polymer added is 0.01 wt % to 10 wt % when the weight of the water absorbent resin particles is 100 wt %. Only when the amount of reactive polymer added is within the above range, the effect of improving the absorption capacity (e.g., water absorption capacity under pressure) of the water absorbent resin can be achieved.

In some embodiments where the ionomer is an ethylene acrylic compound, the surface cross-linking reaction includes a heat treatment at a temperature of 150°C to 210°C. By performing the heat treatment at the aforementioned temperature, the ethylene acrylic compound can be completely melted, and a high-quality water-absorbent resin, i.e., a water-absorbent resin with high water-absorption capacity, can be obtained.

As described above, the present invention can produce a water-absorbent resin having a saline flow conductivity (SFC), and the saline flow conductivity can be used to evaluate the permeability of a liquid. If the liquid permeability of the water-absorbent resin is high, the problems of rewetting of the absorbent body, reduction in smoothness, and leakage can be reduced. The liquid flow conductivity indicates that the water-absorbent resin that has absorbed a liquid has liquid permeability at high pressure, so that when the liquid enters the absorbent body again, the liquid is likely to diffuse through the water-absorbent resin that has already absorbed the liquid to other water-absorbent resins that have not absorbed the liquid. The liquid flow conductivity of the water-absorbent resin of the present invention is 30×10 ⁻⁷ cm ³ -s/g or more, preferably 40×10 ⁻⁷ cm ³ -s/g or more.

Furthermore, the absorbent resin must have a good centrifuge retention capacity (CRC) and absorption against pressure (AAP) so that the absorbent resin is not damaged or its liquid absorption capacity is not affected by pressure applied from the outside after absorbing liquid. In some embodiments, the centrifuge retention capacity of the absorbent resin of the present invention is 20 g/g or more, preferably 25 g/g or more. In some examples, the absorption against pressure capacity of the absorbent resin of the present invention is greater than 15 g/g, preferably greater than 20 g/g, more preferably greater than 23 g/g.

The ability of a dried absorbent resin to absorb liquid when it starts to come into contact with the liquid may also be expressed as a T20 value. A low T20 value of the absorbent resin indicates that the dried absorbent resin is more likely to absorb liquid. The T20 value of the absorbent resin of the present invention is 180 seconds or less, preferably 160 seconds or less. As a supplementary explanation, the T20 value is defined as the time required for 1 gram of the absorbent resin to absorb 20 grams of saline and 0.01 wt % aqueous solution of an alcohol ethoxy compound under a pressure of 0.3 psi, where the alcohol ethoxy compound has 12 to 14 carbon atoms. In some embodiments, the T20 value of the absorbent resin of the present invention is less than 180 seconds, preferably less than 160 seconds. The absorbent resin of the present invention has both a low T20 value and a high liquid flow conductivity, and can reduce the amount of rewetting of the absorbent body and improve the smoothness of the disposable diaper.

The absorbent body is formed into a sheet using the water-absorbent resin and hydrophilic fibers of the present invention. When actually used, the absorbent body may be placed on a polyethylene (PE) film that is not permeable to liquids, and a nonwoven fabric that is permeable to liquids may be used as a surface layer, or the water-absorbent resin may be fixed to a pulp fiber material (airlaid) and/or a nonwoven fabric. The pulp fiber may be crushed wood pulp, crosslinked cellulose fiber, cotton, wool, vinyl acetate fiber, etc. In general, assuming that the absorbent body is 100 wt%, the content of the water-absorbent resin (also called the core concentration) in the absorbent body is 20 wt% or more and 100 wt% or less, preferably 40 wt% or more and 100 wt% or less, and more preferably 50 wt% or more and 100 wt% or less. If a water-absorbent resin with such a high core concentration is used, the effect of the present invention can be further achieved. Generally, the basis weight (weight per unit area) of the absorbent body is 0.01 g/cm ² to 0.30 g/cm ² , and the thickness of the absorbent body is 30 mm or less.

The application of the present invention will be described below using several examples, which are not intended to limit the present invention, and those skilled in the art may make various changes and modifications without departing from the spirit and scope of the present invention.
Preparation of reactive polymer Production Example 1

Take 1180g of ethylene acrylic compound (CAS NO. 9010-86-0, purchased from Aldrich-Sigma, containing 18wt% polyacrylic acid segment), 118g of sodium hydroxide, and 5g of pure water, and mix them in a Banbury mixer at a temperature of 150°C for 30 minutes to obtain an ethylene acrylic compound (A) having neutral sodium ions.
Production Examples 2 to 3

Preparation Example 1 was repeated, with the difference being that the ethylene acrylic compound in Preparation Example 2 contained 15 wt % of polyacrylic acid segment (CAS No. 9010-77-9, purchased from Aldrich-Sigma, containing 15 wt % of polyacrylic acid segment), and in Preparation Example 3, 172 g of magnesium hydroxide was used instead of sodium hydroxide to prepare an ethylene acrylic compound (B) having a neutralizing sodium ion and an ethylene acrylic compound (C) having a neutralizing magnesium ion, respectively.
Production Examples 4 to 5

Preparation Example 1 was repeated, with the difference being that in Preparation Example 4, Escor ^™ 5080 containing 10 wt % polyacrylic acid segments, and in Preparation Example 5, Escor ^™ 5020 containing 7.5 wt % polyacrylic acid segments (purchased from Exxon) were used to prepare ethylene acrylic compounds (D) and (E) having neutral sodium ions, respectively.
Production Example 1 of Water Absorbent Resin

503.12 g of 48 wt% sodium hydroxide aqueous solution was gradually added to a 2000 cc Erlenmeyer flask containing 621.03 g of acrylic acid and 670.74 g of water, the sodium hydroxide/acrylic acid drop ratio was set to the range of 0.85-0.95, the drop time was set to 2 hours, and the temperature of the neutralization reaction system in the flask was maintained within the range of 15°C to 40°C, to obtain an unsaturated monomer aqueous solution with a monomer concentration of 42 wt%, in which 70 mol% of the acrylic acid was partially neutralized to sodium acrylate and the pH value was 5.69.

Next, 1.36 g of N,N'-methylenebisacrylamide (free radical polymerization reaction crosslinking agent) is added to the unsaturated monomer aqueous solution, and the temperature is maintained at about 20°C. After that, 0.35 g of hydrogen peroxide, 4.15 g of sodium hydrogen sulfite, and 4.15 g of ammonium persulfate are added as polymerization reaction initiators to carry out a free radical polymerization reaction.

The colloidal gel obtained by the reaction is pulverized using a cutter mill, and colloidal gel with a particle size of 2 mm or less is selected. It is then dried at a temperature of 130°C for 2 hours. It is then further selected using a sieve with a uniform particle size of 0.1 mm to 0.85 mm to obtain water-absorbent resin particles.

Next, 2.5 g of a surface crosslinking agent, which is an aqueous solution obtained by mixing ethylene glycol, aluminum sulfate, and water at a volume ratio of 1:0.5:2, is added to 200 g of the obtained water absorbent resin particles, and 0.1 g of an ethylene acrylic compound (A) having a neutral sodium ion is added, and a heat treatment is performed at a temperature of 160° C. for 1 hour. After cooling, a water absorbent resin can be obtained.
Examples 2 to 6

The water absorbent resins of Examples 2 to 6 are manufactured by the same process as Example 1. The difference is that the amount of ethylene acrylic compound (A) used in Example 2 is 1.0 g, 0.1 g of ethylene acrylic compound (B) is used in Example 3, 0.1 g of ethylene acrylic compound (C) is used in Example 4, 0.1 g of ethylene acrylic compound (D) is used in Example 5, and 4.63 g of polyethylene glycol 600-diacrylate (UM82-080, manufactured by Nissho Chemical Industry Co., Ltd.) is used as a free radical polymerization reaction crosslinking agent instead of N,N'-methylenebispropyleneamine in Example 6.
Comparative Examples 1 to 6

The water absorbent resins of Comparative Examples 1 to 6 are manufactured by the same process as that of Example 1. The differences are that in Comparative Example 1, a non-neutralized ethylene acrylic compound (CAS NO. 9010-86-0, purchased from Aldrich-Sigma, containing 18 wt% polyacrylic acid segment) is used instead of the ethylene acrylic compound (A) having neutralized sodium ions, in Comparative Example 2, 0.1 g of ethylene acrylic compound (E) is used, in Comparative Example 3, no ethylene acrylic compound is added, in Comparative Example 4, 5 g of ethylene acrylic compound (E) is used, in Comparative Example 5, heat treatment is performed at a temperature of 140°C, and in Comparative Example 6, heat treatment is performed at a temperature of 250°C.

Evaluation method

In order to evaluate the properties of the water-absorbing resin of the present invention, the physical properties are analyzed by the following test methods, and unless otherwise specified, all of the following measurements are performed under the conditions of room temperature of 23±2° C. and relative humidity of 45±10%. The water-absorbing resin needs to be thoroughly mixed before it is analyzed.
Holding Power

The Centrifuge Retention Capacity (CRC) is tested according to the test method ERT441.3(10) specified by the European Disposables and Nonwovens Association (EDANA). The results of the retention capacity test of the water-absorbent resin are shown in Table 1.
Water absorption rate under pressure

The absorption capacity under pressure (AAP) is tested according to the test method ERT442.3(10) specified by EDANA, and the absorption capacity under pressure is tested against a 0.9% sodium chloride aqueous solution under a pressure of 4.9 kPa for 60 minutes. The test results of the absorption capacity under pressure of the water absorbent resin are shown in Table 1.
Liquid Flow Conductivity

Saline flow conductivity (SFC, unit: ^10-7 cm3- ^sec /g) was measured and calculated according to the method described in U.S. Patent No. 5,562,646, specifically, the water-absorbent resin was placed in Jayco synthetic urine for 60 minutes, and then the flow conductivity of a 0.118 M aqueous sodium chloride solution was measured under a pressure of 0.3 psi. The flow conductivity of the water-absorbent resin is shown in Table 1.
T20 value

The T20 value (unit: seconds) was measured and calculated according to the method described in U.S. Patent 9,285,302, and is the time it takes for 1 gram of water absorbent resin to absorb 20 grams of saline and 0.01 wt % aqueous solution of alcohol ethoxy compound under a pressure of 0.3 psi, where the alcohol ethoxy compound has 12-14 carbon atoms. The average results of three repeated tests are shown in Table 1.

Fabrication of absorber

Using an absorbent molding machine, 10.0 grams of water-absorbent resin and 10.0 grams of crushed wood pulp are mixed and molded, the molding mesh is a wire mesh of 400 mesh (38 μm), and the absorbent area is 160 square centimeters (8 cm x 20 cm). The molded absorbent is placed on a polyethylene film, and then a nonwoven fabric is placed on it. Next, the absorbent is pressed for 5 minutes with a pressure of 18.39 kPa (area 160 square centimeters, weight 30 kg), and the periphery is glued with white adhesive to obtain a test absorbent.
Examples 7 to 12 and Comparative Examples 7 to 12

In Examples 7 to 12, absorbents were prepared by the above method using the water-absorbent resins of Examples 1 to 6, respectively, and in Comparative Examples 7 to 12, absorbents were prepared by the above method using the water-absorbent resins of Comparative Examples 1 to 6, respectively. The basis weights and thicknesses of the absorbents are shown in Table 2.
Rewetting performance of absorbent body

The lower the rewet amount (i.e., smoothness) of the absorbent, the better the urine resistance of the water-absorbent resin. The test method is to place a weight of 4.8 kPa (area 160 square centimeters, weight 7.8 kg) on the absorbent prepared in Examples 7 to 12 and Comparative Examples 7 to 12 above, and drip 180 mL of synthetic urine liquid (Jayco synthetic urine liquid described in U.S. Patent Publication No. 20040106745) at the center point in three separate doses (30 minutes apart each time), and remove the weight from the absorbent 30 minutes after the dripping is completed. Then, 30 sheets of filter paper (8 cm x 20 cm) whose total weight W1 has been measured in advance are placed on the absorbent, and a 4.8 kPa weight is immediately placed on the absorbent for 5 minutes to allow the filter paper to absorb the rewet liquid. Then, the weight W2 of the 30 sheets of filter paper is measured. The rewet amount of the synthetic urine liquid on the absorbent is (W2 - W1). The test results are shown in Table 2.

According to the above test results, in Examples 1 to 6, compared to Comparative Examples 1 to 6, an ethylene acrylic compound having a specific amount of neutralized metal ions is involved in the surface crosslinking reaction, thereby obtaining a water-absorbent resin having both high liquid flow conductivity and a low T20 value. In other words, the water-absorbent resins of Examples 1 to 6 not only have excellent liquid flow conductivity, but also have good liquid diffusibility and conduction ability in a dry state. In addition, according to Examples 7 to 12, the absorbent bodies made of the water-absorbent resins of Examples 1 to 6 have a liquid rewet amount lower than 3.0 g, which is clearly lower than the liquid rewet amounts of Comparative Examples 7 to 12, which indicates that they are also excellent in smoothness.

Therefore, by applying the method for producing a water-absorbent resin according to the present invention and involving a reactive polymer in the surface cross-linking reaction of water-absorbent resin particles, the liquid flow conductivity and liquid diffusibility in a dry state of the produced water-absorbent resin can be effectively improved, while the retention power of the water-absorbent resin can be maintained at a high level, making it practical.

The present invention has been disclosed above based on the embodiments, but the embodiments are not intended to limit the present invention, and a person skilled in the art may make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention is based on the definition in the appended claims.

Claims (5)

A step of subjecting a water absorbent resin composition comprising an unsaturated monomer aqueous solution, a polymerization initiator, and a free radical polymerization reaction crosslinking agent to a free radical polymerization reaction to obtain a colloidal gel;
pulverizing and sieving the colloidal gel to obtain a plurality of water-absorbent resin particles;
A step of subjecting the water-absorbent resin particles to a surface cross- linking reaction with a surface cross-linking agent and a reactive polymer to obtain a water-absorbent resin;
Including,
The surface cross-linking reaction comprises a heat treatment at a temperature of 150° C. to 210° C.
The reaction polymer is an ethylene acrylic acid copolymer, and the ethylene acrylic acid copolymer contains a polyethylene segment represented by the following formula (1) and a polyacrylic acid segment represented by the following formula (2),
In formula (2), M is a Group IA element or a Group IIA element;
When the weight of the polyethylene segment in the reaction polymer is taken as 100 wt %, the weight of the polyacrylic acid segment is 8 wt % to 20 wt % . 2. The method for producing an absorbent resin according to claim 1 , wherein the amount of the reactive polymer added is 0.01 wt % to 10 wt % when the weight of the water-absorbent resin particles is 100 wt %. The method for producing a water-absorbent resin according to claim 1, wherein the T20 value of the water-absorbent resin is 180 seconds or less. 2. The method for producing a water-absorbing resin according to claim 1, wherein the water-absorbing resin has a liquid flow conductivity of 30×10 ⁻⁷ cm ³ -s/g or more. The method for producing a water-absorbent resin according to claim 1, wherein the retention force of the water-absorbent resin is greater than 20 g/g.