CN116724072A

CN116724072A - Method for producing superabsorbent polymers

Info

Publication number: CN116724072A
Application number: CN202280011494.6A
Authority: CN
Inventors: 金基勋; 李惠民; 韩昌薰; 孙精敏
Original assignee: LG Chem Ltd
Current assignee: LG Chem Ltd
Priority date: 2021-12-24
Filing date: 2022-10-12
Publication date: 2023-09-08

Abstract

The present application relates to a method for producing a superabsorbent polymer capable of minimizing bubble loss during foaming polymerization by a radical polymerization method. According to the production method of the present application, a superabsorbent polymer having a high surface area can be produced by forming many small and uniform holes. Because it exhibits excellent absorption rate, it can be used in various products requiring high absorption properties.

Description

Method for producing superabsorbent polymers

Technical Field

Cross Reference to Related Applications

The present application claims the benefit of korean patent application 10-2021-0187268 filed by the korean intellectual property office at 24-12-2021 and korean patent application 10-2022-0129233 filed at 11-10-2022, the disclosures of which are incorporated herein by reference in their entireties.

The present application relates to a process for preparing superabsorbent polymers.

Background

Superabsorbent polymers (SAP) are a class of synthetic polymeric materials capable of absorbing 500 to 1000 times their own weight of moisture. The super absorbent polymer is practically applied to sanitary articles at first, and is now widely used not only for sanitary articles such as child diapers, but also for water-retaining soil products for gardening, water-stopping materials for civil engineering and construction, sheets for raising seedlings, antistaling agents or dressing materials in the field of food circulation, and the like.

In order to improve the absorption rate of superabsorbent polymers, a method of forming a porous structure by adding a foaming agent during polymerization is known. When an acrylic monomer as a main raw material of the super absorbent polymer encounters a foaming agent, the generated carbon dioxide gas forms a plurality of holes on the surface of the super absorbent polymer, thereby increasing the surface area of the super absorbent polymer and improving the absorption rate thereof.

However, in the case of the existing radical polymerization method using a photoinitiator, a large amount of carbon dioxide gas is lost before the monomer composition in a liquid phase is converted into a solid phase during the polymerization, so that a desired level of porosity cannot be achieved.

Thus, for a process for preparing superabsorbent polymers using a blowing agent, a process is needed that can compensate for the limitations described above during free radical polymerization.

Disclosure of Invention

Technical problem

In order to solve the above problems, a method of preparing a superabsorbent polymer capable of minimizing bubble loss during foaming polymerization by a radical polymerization method is provided.

Technical proposal

In the present application, there is provided a method for preparing a superabsorbent polymer, the method comprising the steps of:

preparing a monomer composition by mixing an acrylic monomer having an at least partially neutralized acidic group, an internal crosslinking agent, a photoinitiator, a thermal initiator, a foaming agent, a reducing agent and an oxidizing agent forming a redox pair with each other,

the hydrogel polymer is prepared by cross-linking polymerization of the monomer composition,

forming a base resin in powder form by drying and pulverizing the hydrogel polymer, and

forming a surface crosslinked layer by further crosslinking the surface of the base resin in the presence of a surface crosslinking agent,

wherein the content of each of the reducing agent and the oxidizing agent is 1 mol or more and 13 mol or less based on 1 mol of the photoinitiator.

Advantageous effects

According to the production method of the present application, the loss of bubbles can be minimized during the foaming polymerization using the radical polymerization method, and thus a superabsorbent polymer having many small and uniform holes can be obtained. Due to the high surface area of the superabsorbent polymer, it exhibits significantly improved absorption rate and excellent absorption properties, such as water retention capacity and absorbency under load, enabling its use in a variety of products requiring high absorbency.

Detailed Description

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. Singular is also intended to include plural unless the context clearly indicates otherwise. It will be further understood that the terms "comprises," "comprising," "includes" and "including," when used in this specification, specify the presence of stated features, steps, components, or groups thereof, but do not preclude the presence or addition of one or more other features, steps, components, or groups thereof.

While the application is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example and will herein be described in detail. It should be understood, however, that there is no intention to limit the application to the specific forms disclosed, and it is to be understood that the application includes all modifications, equivalents, and alternatives falling within the spirit and scope of the application.

As used herein, the "base resin" or "base resin powder" is a polymer obtained by polymerizing a water-soluble ethylenically unsaturated monomer, dried and pulverized to prepare a particle or powder, and means a polymer that has not been subjected to surface modification or surface crosslinking.

Hereinafter, a method of preparing the superabsorbent polymer will be described in more detail.

In the industrial production of superabsorbent polymers, the polymerization of acrylic monomers as main raw materials is generally carried out in a reactor equipped with a conveyor belt by a radical polymerization method using a photoinitiator. The advantage of this polymerization process is the ability to polymerize continuously. However, in the case of foam polymerization using a foaming agent, there is a problem in that the loss amount of carbon dioxide gas, which is generated when the foaming agent encounters a monomer and is lost to the air before the monomer composition is polymerized to form a solid phase, is large. When carbon dioxide bubbles are lost as described above, the foaming degree of the super absorbent polymer is reduced as compared with the addition amount of the foaming agent, and thus the effect of improving the absorption rate cannot be sufficiently obtained.

Accordingly, the present inventors studied a method capable of preventing the loss of bubbles even during the continuous radical polymerization using a conveyor belt. As a result, it was confirmed that when an oxidizing agent and a reducing agent forming an oxidation and reduction pair (i.e., redox pair) with each other are added to a monomer composition and the content in the monomer composition is more than 1 mol to 13 mol or less based on 1 mol of a photoinitiator, the loss of bubbles generated from a foaming agent can be minimized and a large number of smaller bubbles are generated to obtain a super absorbent polymer having a higher foaming degree, and thus the present application has been completed.

Thus, one embodiment of the present application is a method of making a superabsorbent polymer characterized by comprising the steps of:

forming surface crosslinks by further crosslinking the surface of the base resin in the presence of a surface crosslinking agent,

wherein the reducing agent and the oxidizing agent are each contained in an amount of more than 1 mole to 13 moles or less based on 1 mole of the photoinitiator.

The acrylic monomer is a compound represented by the following chemical formula 1:

[ chemical formula 1]

R ¹ -COOM ¹

In the chemical formula 1, the chemical formula is shown in the drawing,

R ¹ is a C2 to C5 hydrocarbon group having an unsaturated bond, and

M ¹ is a hydrogen atom, a monovalent or divalent metal, an ammonium group or an organic amine group.

Preferably, the acrylic monomer may include at least one selected from acrylic acid, methacrylic acid, and monovalent metal salts, divalent metal salts, ammonium salts, and organic amine salts thereof.

Here, the acrylic monomer may have an at least partially neutralized acidic group. Preferably, an acrylic monomer partially neutralized with a basic substance such as sodium hydroxide, potassium hydroxide, or ammonium hydroxide may be used. The degree of neutralization of the acrylic monomer may be 40 to 95 mole%, 40 to 80 weight%, or 45 to 75 mole%. The extent of neutralization can be adjusted according to the final properties. Too high a degree of neutralization results in precipitation of the neutralized monomer and thus polymerization is less likely to occur. Conversely, too low a degree of neutralization not only reduces the absorbency of the polymer, but also imparts difficult handling properties to the polymer, such as the properties of elastomeric rubber.

Further, the concentration of the acrylic monomer may be about 20 to 60% by weight, preferably about 40 to 50% by weight, based on the monomer composition comprising the raw material of the superabsorbent polymer and the solvent, and may be appropriately adjusted in consideration of the polymerization time and the reaction conditions. If the concentration of the monomer is too low, the yield of the super absorbent polymer is low and there may be a problem in terms of economic efficiency. In contrast, if the concentration is too high, there may be caused a problem that a part of the monomer is extracted during the process, or the crushing efficiency of the polymerized hydrogel polymer during the crushing process may be lowered, and thus, the physical properties of the superabsorbent polymer may be deteriorated.

The photoinitiator (photopolymerization initiator) used during the polymerization of the method for producing a superabsorbent polymer of the present application is not particularly limited if it is a photoinitiator generally used for producing a superabsorbent polymer.

Meanwhile, even when the photopolymerization method is applied, a certain amount of heat is generated by UV radiation or the like, and heat is generated as the polymerization reaction (exothermic reaction) proceeds. Therefore, in the present application, a thermal initiator (thermal polymerization initiator) may be used together with the photoinitiator.

Any compound capable of forming a radical by light, such as ultraviolet rays, may be used as the photoinitiator without limitation.

For example, the photoinitiator may be one or more compounds selected from the group consisting of benzoin ether, dialkyl acetophenone, hydroxy alkyl ketone, phenyl glyoxylate, benzyl dimethyl ketal, acyl phosphine, and alpha-amino ketone. Further, as specific examples of the acylphosphine, commercially available Lucirin TPO, i.e., diphenyl (2, 4, 6-trimethylbenzoyl) phosphine oxide, or Irgacure 819, i.e., bis (2, 4, 6-trimethylbenzoyl) phenylphosphine oxide, may be used. More various photoinitiators are fully disclosed on page 115 of "UV Coatings: basic, recent Developments and New Application (Elsevier, 2007)" by Reinhold Schwalm, and the application is not limited thereto.

The photoinitiator may be used in an amount of 0.001 parts by weight or more, 0.005 parts by weight or more, or 0.007 parts by weight or more, and may be 0.1 parts by weight or less, 0.05 parts by weight or less, or 0.01 parts by weight or less, based on 100 parts by weight of the acrylic monomer. If the content of the photoinitiator is too low, the polymerization rate may be slow, and if the concentration of the photoinitiator is too high, the molecular weight of the super absorbent polymer may become low and its properties may be uneven.

Further, as the thermal initiator, at least one selected from the group consisting of persulfate-type initiators, azo-type initiators, hydrogen peroxide, and ascorbic acid may be used. Specifically, as an example of the persulfate initiator, sodium persulfate (Na ₂ S ₂ O ₈ ) Potassium persulfate (K) ₂ S ₂ O ₈ ) And ammonium persulfate ((NH) ₄ ) ₂ S ₂ O ₈ ) Etc.; also, as examples of azo-based initiators, 2-azobis (2-amidinopropane) dihydrochloride, 2-azobis (N, N- "dimethylene") isobutyl amidine dihydrochloride, 2- (carbamoylazo) isobutyronitrile, 2-azobis [2- (2-imidazolin-2-yl) propane can be used]Dihydrochloride, 4-azobis (4-cyanovaleric acid), and the like. Further thermal initiators are well disclosed in "Principle of Polymerization (Wiley, 1981)" by Odian, page 203, and the application is not limited thereto.

The content of the thermal initiator may be 0.1 parts by weight or more, 0.15 parts by weight or more, or 0.2 parts by weight or more, and may be 1 parts by weight or less, 0.8 parts by weight or less, 0.6 parts by weight or less, or 0.4 parts by weight or less, based on 100 parts by weight of the acrylic monomer. If the thermal initiator is used in an amount of less than 0.1 parts by weight based on 100 parts by weight of the acrylic monomer, the content of unreacted residual monomer may increase, whereas if it exceeds 1 part by weight, the resin may be discolored.

The monomer composition of one embodiment of the present application includes an internal crosslinking agent. The internal crosslinking agent is used for crosslinking inside a polymer in which an acrylic monomer is polymerized, and is distinguished from a surface crosslinking agent used for crosslinking the surface of the polymer.

For example, the internal crosslinking agent may use at least one selected from the group consisting of N, N' -methylenebisacrylamide, trimethylolpropane tri (meth) acrylate, ethylene glycol di (meth) acrylate, propylene glycol di (meth) acrylate, polyethylene glycol di (meth) acrylate, polypropylene glycol (meth) acrylate, butanediol di (meth) acrylate, butylene glycol di (meth) acrylate, diethylene glycol di (meth) acrylate, hexanediol di (meth) acrylate, triethylene glycol di (meth) acrylate, tripropylene glycol di (meth) acrylate, tetraethylene glycol di (meth) acrylate, dipentaerythritol pentaacrylate, glycerol tri (meth) acrylate, pentaerythritol tetraacrylate, triarylamine, ethylene glycol diglycidyl ether, propylene glycol, glycerol, and ethylene carbonate.

The content of the thermal initiator may be 0.01 parts by weight or more, 0.05 parts by weight or more, 0.1 parts by weight or more, or 0.5 parts by weight or more, and may be 2 parts by weight or less, 1.5 parts by weight or less, or 1 part by weight or less, based on 100 parts by weight of acrylic acid.

As the foaming agent, an organic foaming agent and/or an inorganic foaming agent which are commonly used for producing superabsorbent polymers can be used without limitation, and for example, a carbonate-based foaming agent, a capsule-type foaming agent, or the like can be used. Among these blowing agents, carbonate-based blowing agents which are relatively inexpensive and which readily generate carbon dioxide gas under acidic conditions can be preferably used.

For example, as the carbonate foaming agent, at least one selected from sodium bicarbonate, sodium carbonate, potassium bicarbonate, potassium carbonate, calcium bicarbonate, calcium carbonate, magnesium bicarbonate, and magnesium carbonate may be used, but the present application is not limited thereto.

The foaming agent may be used in an amount of 0.05 parts by weight or more, 0.1 parts by weight or more, or 0.2 parts by weight or more, and may be 5 parts by weight or less, 3 parts by weight or less, 1 part by weight or less, or 0.5 parts by weight or less, based on 100 parts by weight of the acrylic monomer. If the content of the foaming agent is too low, the produced super absorbent polymer may not have sufficient porosity due to insufficient bubbles during polymerization. If the content of the foaming agent is too high, the porosity of the super absorbent polymer may be too high, and thus there is a problem in that the mechanical strength is lowered.

As reducing agent and oxidizing agent, organic and/or inorganic compounds known to form redox pairs with each other can be used. For example, the reducing agent may be sodium metabisulfite and the oxidizing agent may be sodium, potassium or ammonium persulfate. Alternatively, the reducing agent may be ascorbic acid and the oxidizing agent may be hydrogen peroxide.

For example, sodium metabisulfite and sodium persulfate, or ascorbic acid and hydrogen peroxide, may be contained as redox couples.

The reducing agent and the oxidizing agent, i.e., the redox couple, cause mutual redox reactions in the monomer composition, and the free radicals formed by the redox reaction of the redox couple initiate polymerization before polymerization begins. Therefore, when the redox couple is contained in the monomer composition, the polymerization rate increases as compared to the case without the redox couple, and bubble loss is minimized because polymerization can be performed before a large loss of carbon dioxide.

In order to secure the above effect, the content of each of the reducing agent and the oxidizing agent is more than 1 mole and 13 moles or less based on 1 mole of the photoinitiator. Preferably, the respective content is 1.3 mol or more, or 2 mol or more and 12 mol or less, 10 mol or less, or 7 mol or less based on 1 mol of the photoinitiator. When the content of the redox couple is 1 mole or less based on 1 mole of the photoinitiator, the effect of reducing the bubble loss cannot be ensured. If it exceeds 13 moles, the effect of reducing the bubble loss is not increased, and the absorbability of the super absorbent polymer may be lowered.

Meanwhile, the content of each of the reducing agent and the oxidizing agent is more than 0.0015 mol%, 0.002 mol% or more or 0.003 mol% or more, and less than 0.02 mol%, 0.015 mol% or less or 0.01 mol% or less, based on 100 mol% of the acrylic monomer.

Thus, the molar ratio of reducing agent to oxidizing agent may be 1:1 to 1:2, or 1:1 to 1:1.5.

If necessary, the monomer composition may further contain additives such as a thickener, a plasticizer, a storage stabilizer, and an antioxidant.

The monomer composition may be prepared by mixing raw materials, that is, an acrylic monomer having an at least partially neutralized acidic group, an internal crosslinking agent, a photoinitiator, a thermal initiator, a foaming agent, and a reducing agent and an oxidizing agent forming a redox pair with each other in a solvent.

Here, any solvent capable of dissolving the above-mentioned compounds may be used without limitation. For example, the solvent may be a combination of at least one selected from the group consisting of water, ethanol, ethylene glycol, diethylene glycol, triethylene glycol, 1, 4-butanediol, propylene glycol, ethylene glycol monobutyl ether, propylene glycol monomethyl ether acetate, methyl ethyl ketone, acetone, methyl amyl ketone, cyclohexanone, cyclopentanone, diethylene glycol monomethyl ether, diethylene glycol diethyl ether, toluene, xylene, butyrolactone, carbitol, methyl cellosolve acetate, and N, N-dimethylacetamide.

Meanwhile, in one embodiment of the present application, the monomer composition may be prepared by: i) Adding a basic material to a mixture comprising acrylic monomers, an internal crosslinking agent, a photoinitiator, thereby neutralizing at least a part of the acidic groups of the acrylic monomers, and ii) adding a thermal initiator, a foaming agent, and a reducing agent and an oxidizing agent forming a redox pair with each other to the mixture of added basic materials obtained in step i). When the monomer composition is prepared by the above method, the redox couple may be introduced as late as possible while providing a sufficient mixing time for the monomer solution, thereby ensuring the stability of the process.

The polymerization of the monomer composition is carried out by radical polymerization caused by light irradiation, and may be carried out, for example, in a reactor equipped with a movable conveyor belt. When polymerization is carried out in the above-mentioned reactor, a hydrogel polymer in the form of a sheet having a conveyor belt width can be obtained. The thickness of the hydrogel polymer may vary depending on the concentration of the monomer composition to be injected and the injection rate, and it is preferable to supply the monomer composition such that the thickness of the polymer in the form of a sheet is about 0.5cm to about 5cm. If the monomer composition is supplied to such an extent that the thickness of the polymer sheet is too thin, the production efficiency may be low. If the thickness of the polymer sheet exceeds 5cm, the polymerization reaction may not occur uniformly throughout the thickness due to the excessive thickness.

The polymerization temperature of the monomer composition is not particularly limited, but may be, for example, 80 to 120 ℃, preferably 90 to 110 ℃.

Generally, the moisture content of the hydrogel polymer may be about 40% to about 80% by weight. Here, "moisture content" in the present application refers to the moisture content in the total weight of the polymer, and refers to a value obtained by subtracting the weight of the dry polymer from the weight of the polymer. Specifically, the moisture content is defined as a value calculated by measuring the weight loss due to evaporation of moisture from the polymer during the temperature increase of drying the polymer by heating with infrared rays. Here, the drying conditions for measuring the moisture content are as follows: the temperature was raised to about 180 ℃ and maintained at 180 ℃ for a total drying time of 20 minutes including a 5 minute heating step.

Next, the obtained hydrogel polymer is subjected to a drying step.

If necessary, a coarse pulverizing step may be further performed before drying to improve the efficiency of the drying step.

Here, the pulverizer used is not limited. Specifically, at least one selected from the group consisting of a vertical crusher, a turbine cutter, a turbine grinder, a rotary cutter grinder, a cutting grinder, a disc grinder, a crushing crusher, a chopper, and a disc cutter may be used, but the present application is not limited thereto.

In the pulverization step, the hydrogel polymer may be pulverized so that the particle diameter thereof is about 2mm to 10mm.

Since the hydrogel polymer has a high moisture content, it is technically difficult to crush to a particle diameter of less than 2mm, and agglomeration between crushed particles may occur. On the other hand, if the particle diameter is larger than 10mm, the effect of improving the efficiency of the subsequent drying step may not be significant.

The crushed hydrogel polymer as described above is dried, or dried immediately after polymerization without the crushing step. The drying temperature in the drying step may be from about 150 ℃ to about 250 ℃. If the drying temperature is lower than 150 ℃, the drying time may become excessively long and the physical properties of the finally formed super absorbent polymer may be lowered. If the drying temperature exceeds 250 ℃, only the surface of the polymer is excessively dried, fine powder may be generated during the subsequent pulverization, and the physical properties of the final superabsorbent polymer may be degraded. Thus, drying may preferably be performed at a temperature of about 150 ℃ to about 200 ℃, more preferably at a temperature of about 160 ℃ to about 180 ℃.

Meanwhile, the drying time may be about 20 minutes to about 90 minutes in consideration of process efficiency, but is not limited thereto.

The drying method in the drying step is not particularly limited as long as it is a drying method commonly used in the drying process of hydrogel polymers. Specifically, the drying step may be performed by supplying hot air, infrared radiation, microwave radiation, ultraviolet radiation, and the like. After the drying step, the moisture content of the polymer may be from about 0.1 wt% to about 10 wt%.

Subsequently, a step of pulverizing the dried polymer obtained by the drying step is performed.

The particle diameter of the polymer powder obtained after the pulverizing step may be about 150 μm to about 850 μm. As the pulverizer for pulverizing to the above particle size, a pin mill, a hammer mill, a screw mill, a roller mill, a disc mill, a hand mill, or the like can be used, but the present application is not limited thereto.

Further, the polymer powder obtained after the pulverizing step may be subjected to a separate classification step according to the particle diameter, and the polymer powder may be classified according to the particle size at a predetermined weight ratio.

Subsequently, the surface of the dried and pulverized polymer, i.e., the base resin, is further crosslinked to form a surface crosslinked layer.

Specifically, a surface crosslinking agent is mixed with a base resin, and then the mixture is heated to perform a surface crosslinking reaction on the pulverized polymer.

The surface crosslinking step is a step of initiating a crosslinking reaction at the surface of the base resin in the presence of a surface crosslinking agent, thereby forming a superabsorbent polymer having improved physical properties. A surface cross-linked layer (surface modified layer) is formed on the surface of the base resin by surface cross-linking.

The surface cross-linking agent is applied to the surface of the superabsorbent polymer particles. Thus, surface cross-linking reactions occur on the surface of the superabsorbent polymer particles, which improve the cross-linking properties of the particle surface without substantially affecting the interior of the particles. Thus, the surface-crosslinked superabsorbent polymer particles have a higher degree of crosslinking on the surface than the interior.

Meanwhile, as the surface crosslinking agent, a compound capable of reacting with the functional group of the polymer is used. For example, polyhydric alcohol compounds, polyepoxide compounds, polyamine compounds, halogenated epoxy compounds, condensation products of halogenated epoxy compounds, oxazoline compounds or alkylene carbonate compounds can be used.

Specifically, as the polyhydric alcohol compound, at least one selected from the group consisting of di-, tri-, tetra-, and polyethylene glycols, 1, 3-propanediol, dipropylene glycol, 2,3, 4-trimethyl-1, 3-pentanediol, polypropylene glycol, glycerin, polyglycerol, 2-butene-1, 4-diol, 1, 4-butanediol, 1, 3-butanediol, 1, 5-pentanediol, 1, 6-hexanediol, and 1, 2-cyclohexanedimethanol can be used.

Further, as the polyepoxide compound, ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, or glycidol can be used. As the polyamine compound, at least one selected from ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, polyethyleneimine and polyamide polyamine can be used.

Further, as the halogenated epoxy compound, epichlorohydrin, epibromohydrin or α -methyl epichlorohydrin may be used. Meanwhile, as the mono-, di-, or polyoxazolidone compound, 2-oxazolidone may be used.

Further, as the alkylene carbonate compound, ethylene carbonate and the like can be used. Which may be used alone or in combination with each other.

The addition amount of the surface cross-linking agent may be appropriately selected depending on the type of the surface cross-linking agent added or the reaction conditions, and may be about 0.001 to about 5 parts by weight, preferably about 0.01 to about 3 parts by weight, more preferably about 0.05 to about 2 parts by weight, based on 100 parts by weight of the base resin.

If the content of the surface cross-linking agent is too low, the surface cross-linking reaction hardly occurs. If the content of the surface cross-linking agent exceeds 5 parts by weight based on 100 parts by weight of the polymer, absorption properties such as water retention capacity may be deteriorated due to excessive surface cross-linking reaction.

Further, the method of adding the surface cross-linking agent to the base resin powder is not particularly limited. For example, a method of adding the surface cross-linking agent and the base resin powder to the reactor for mixing, a method of spraying the surface cross-linking agent onto the base resin powder, or a method of mixing the base resin powder with the surface cross-linking agent while continuously supplying it to a continuously operating mixer may be used.

In the case of adding the surface crosslinking agent, it may be additionally mixed with water and added in the form of a surface crosslinking agent solution. When water is added thereto, there is an advantage in that the surface cross-linking agent can be uniformly dispersed in the polymer. At this time, the addition amount of water may be appropriately controlled in order to cause uniform dispersion of the surface cross-linking agent, prevent agglomeration of the polymer powder, and optimize the surface penetration depth of the surface cross-linking agent. For example, the amount of water added may be preferably about 1 to 10 parts by weight based on 100 parts by weight of the base resin.

At the same time, the surface modification is performed by heating the mixture of the base resin and the surface cross-linking agent solution to increase the temperature thereof.

Depending on the type of surface cross-linking agent, the surface modification may be performed under known conditions, for example, may be performed at a temperature of 100 to 200 ℃ for 20 to 60 minutes. In more specific embodiments, when the surface cross-linking agent is a multivalent polyepoxide, it can be heated at about 120 ℃ to 180 ℃ or about 120 ℃ to 150 ℃ for about 10 minutes to about 50 minutes or about 20 to about 40 minutes. If the temperature of the surface modification is lower than 100 ℃, or the reaction time is too short, the surface crosslinking reaction cannot normally occur and the transmittance may be lowered. If the temperature exceeds 200℃or the reaction time is too long, a problem of reduced water retention capacity may occur.

The heating means for the surface crosslinking reaction is not particularly limited. The heat medium may be provided thereto or the heat source may be provided directly thereto. At this time, usable heat medium may be a heated fluid such as vapor, hot air, hot oil, etc., but the present application is not limited thereto. Further, the temperature of the heat medium to be supplied thereto may be appropriately selected in consideration of the manner of the heat medium, the heating rate, and the target temperature of heating. Meanwhile, as a directly supplied heat source, an electric heater or a gas heater may be used, but the present application is not limited thereto.

After surface modification, the obtained superabsorbent polymer powder is additionally subjected to a classification process according to particle size.

The superabsorbent polymer prepared according to the preparation method has a porous structure in which a large number of small and uniform pores are formed, thereby exhibiting excellent absorption properties.

For example, the superabsorbent polymer may have a vortex time (absorption rate) of 35 seconds or less, 33 seconds or less, 30 seconds or less, 29 seconds or less, or 25 seconds or less. The lower limit is theoretically 0 seconds, but may be 5 seconds or more, 10 seconds or more, or 12 seconds or more, since the shorter the swirling time is estimated to be better.

Furthermore, the superabsorbent polymer may have a Centrifuge Retention Capacity (CRC) of 25g/g or more, 29g/g or more, or 32g/g or more and 40g/g or less, 38g/g or less, or 35g/g or less, as measured according to the EDANA WSP 241.3 method.

Furthermore, the superabsorbent polymer may have an absorbency under load of 0.9psi (AUL) of 20g/g or more, or 22g/g or more, and of 35g/g or less, 33g/g or less, or 30g/g or less, as measured according to the EDANA WSP 242.3 method.

Hereinafter, the present application will be described in more detail with the following preferred examples, which are provided for illustrative purposes only. It will be apparent to those skilled in the art that various changes and modifications can be made within the scope and spirit of the application. It is therefore evident that the particular embodiments disclosed, as such variations and modifications are intended to be within the scope of the present application.

Examples (example)

Example 1

By mixing 0.5 part by weight of polyethylene glycol diacrylate (weight average molecular weight: 500 g/mol) as an internal crosslinking agent and 0.009 part by weight of a photoinitiator819 (bis (2, 4, 6-trimethylbenzoyl) phenylphosphine oxide, manufactured by Ciba) was mixed with 100 parts by weight of acrylic acid to prepare a monomer solution. Subsequently, 140 parts by weight of a 31 wt% aqueous sodium hydroxide solution was continuously mixed on line to prepare an aqueous monomer solution while continuously supplying the aqueous monomer solution to a metering pump. At this time, after confirming that the temperature of the aqueous monomer solution was raised to about 72 ℃ or higher by the neutralization heat, the solution was allowed to stand until the temperature was cooled to 40 ℃. When cooled to a temperature of 40 ℃, 0.225 parts by weight of solid sodium bicarbonate as a foaming agent and 5.6 parts by weight of sodium persulfate as a thermal initiator were added to the aqueous monomer solution. Then, sodium Metabisulfite (SMBS) and Sodium Persulfate (SPS) were added in amounts of 0.018 mol% each, respectively, as a reducing agent and an oxidizing agent forming a redox pair, based on 100 mol% of acrylic acid. The solution was poured into a barrel tray (15 cm wide by 15cm long) mounted in a square polymerizer provided with a light irradiation device on top and preheated to 80 ℃, and then subjected to light irradiation to initiate polymerization. After light irradiation for 60 seconds, the reaction was further carried out for 120 seconds to obtain a sheet-like hydrogel polymer.

After uniformly spraying 150g of water for lubrication onto the hydrogel polymer, pulverization was performed using a chopper having a 10mm orifice plate. The crushed hydrogel polymer is then dried in a dryer capable of changing the direction of the wind up and down. The hydrogel polymer was uniformly dried by flowing hot air of 180℃from the bottom to the top for 15 minutes and then from the top to the bottom for 15 minutes, so that the moisture content of the dried powder was less than about 2%. The dried polymer was pulverized using a pulverizer, and then classified to obtain a base resin powder having a size of 150 μm to 850 μm.

10g of a polyethylene glycol diglycidyl ether containing 0.12 parts by weightEJ1030, manufactured by JSI) was sprayed onto 100 parts by weight of the base resin powder, and stirred at room temperature to uniformly mix the surface cross-linking agent aqueous solution on the base resin powder. Then, the base resin powder mixed with the surface crosslinking solution was put into a surface crosslinking reactor, and surface crosslinking reaction was performed at 140℃for 40 minutes. Thereafter, the powder obtained after the surface crosslinking reaction was classified using an ASTM standard sieve to obtain a super absorbent polymer having a particle diameter of 150 μm to 850 μm.

Example 2

Superabsorbent polymers were prepared in the same manner as in example 1 except that the amount of SMBS and SPS added to the aqueous monomer solution was 0.01 mole% each, based on 100 mole% of acrylic acid.

Example 3

Superabsorbent polymers were prepared in the same manner as in example 1 except that the amount of SMBS and SPS added to the aqueous monomer solution was 0.005 mol% each, based on 100 mol% of acrylic acid.

Example 4

Superabsorbent polymers were prepared in the same manner as in example 1 except that the amount of SMBS and SPS added to the aqueous monomer solution was 0.003 mole% each, based on 100 mole% of acrylic acid.

Example 5

Superabsorbent polymers were prepared in the same manner as in example 1 except that ascorbic acid and hydrogen peroxide were added to the aqueous monomer solution as redox pairs in amounts of 0.018 mole% each, instead of SMBS and SPS, based on 100 mole% acrylic acid.

Example 6

Superabsorbent polymers were prepared in the same manner as in example 1 except that ascorbic acid and hydrogen peroxide were added to the aqueous monomer solution in an amount of 0.01 mol% each as a redox pair in place of the SMBS and SPS, based on 100 mol% of acrylic acid.

Example 7

Superabsorbent polymers were prepared in the same manner as in example 1 except that ascorbic acid and hydrogen peroxide were added to the aqueous monomer solution in an amount of 0.005 mol% each as a redox pair in place of the SMBS and SPS, based on 100 mol% of acrylic acid.

Example 8

Superabsorbent polymers were prepared in the same manner as in example 1 except that ascorbic acid and hydrogen peroxide were added to the aqueous monomer solution as redox pairs in amounts of 0.003 mol% each, instead of SMBS and SPS, based on 100 mol% of acrylic acid.

Comparative example 1

Superabsorbent polymers were prepared in the same manner as in example 1 except that no redox couple was added to the aqueous monomer solution.

Comparative example 2

Superabsorbent polymers were prepared in the same manner as in example 1 except that the amount of SMBS and SPS added to the aqueous monomer solution was 0.02 mol% each, based on 100 mol% of acrylic acid.

Comparative example 3

Superabsorbent polymers were prepared in the same manner as in example 1 except that the amounts of SMBS and SPS added to the aqueous monomer solution were each 0.0015 mol% based on 100 mol% of acrylic acid.

Comparative example 4

Superabsorbent polymers were prepared in the same manner as in example 1 except that ascorbic acid and hydrogen peroxide were added to the aqueous monomer solution in an amount of 0.02 mol% each as a redox pair in place of the SMBS and SPS, based on 100 mol% of acrylic acid.

Comparative example 5

Superabsorbent polymers were prepared in the same manner as in example 1 except that ascorbic acid and hydrogen peroxide were added to the aqueous monomer solution as redox pairs in amounts of 0.0015 mol% each, instead of SMBS and SPS, based on 100 mol% of acrylic acid.

Test example 1

The physical properties of the superabsorbent polymers in examples and comparative examples were measured by the following methods, and the results thereof are summarized in Table 1.

(1) Centrifuge Retention Capacity (CRC)

The centrifuge retention capacity of each superabsorbent polymer was measured according to EDANA WSP 241.3.

A sample of 0.2g (W0) of superabsorbent polymer having a particle diameter of 300 μm to 600 μm was uniformly inserted into a nonwoven fabric envelope and sealed, and then immersed in saline (0.9% by weight aqueous sodium chloride solution) at room temperature. After 30 minutes, the envelope was centrifuged at 250G for 3 minutes to drain, and the weight W2 (G) of the envelope was weighed. In addition, after the same operation was performed without using the resin, the weight W1 (g) of the envelope was measured. Then, CRC (g/g) is calculated using the obtained weight value according to the following equation.

[ equation 1]

CRC(g/g)＝{[W2(g)-W1(g)]/W0(g)}–1

(2) Absorption under load of 0.9psi (0.9 AUL)

The absorption of each polymer at 0.9psi loading was measured according to EDANA WSP 242.3. In this measurement, the same superabsorbent polymer sample having a particle diameter of 300 μm to 600 μm as in the CRC measurement was used.

A400 mesh stainless steel screen was mounted on the bottom of a plastic cylinder with an inner diameter of 25 mm. The super absorbent polymer of W3 (g, 0.16 g) was uniformly scattered on the screen at room temperature and humidity of 50%. Thereafter, a piston capable of uniformly providing a load of 0.9psi was placed thereon. Here, the outer diameter of the piston is slightly smaller than 25mm, no gap exists between the piston and the inner wall of the cylinder, and the movement of the cylinder is not interrupted. At this time, the weight W4 (g) of the apparatus was measured.

Subsequently, a glass filter having a diameter of 90mm and a thickness of 5mm was placed in a petri dish having a diameter of 150mm, and brine (0.9 wt% sodium chloride aqueous solution) was poured into the dish. At this time, the physiological saline was injected until the level of the physiological saline was leveled with the upper surface of the glass filter. A piece of filter paper with a diameter of 90mm was placed thereon. After placing the measuring device on the filter paper, the liquid is absorbed under load for 1 hour. After 1 hour, the measuring device was lifted and the weight W5 (g) was measured.

Then, the absorption under load (g/g) was calculated according to the following equation by using the obtained weight value.

[ equation 2]

0.9AUL(g/g)＝[W5(g)–W4(g)]/W3(g)

(3) Vortex time (absorption rate, second)

The vortex time was measured according to the japanese standard method (JIS K7224).

Specifically, 50mL of brine (0.9 wt% aqueous sodium chloride solution) at 24℃and a magnetic bar (diameter 8mm, length 31.8 mm) were placed in a 100mL beaker and stirred at 600 rpm. 2.0g of a super absorbent polymer having a particle diameter of 300 μm to 600 μm was added to the brine being stirred, and then the time taken until the vortex disappeared was measured in seconds to calculate the vortex time.

TABLE 1

* Based on 100 mol% of acrylic acid

Referring to table 1, it was confirmed that the super absorbent polymers of examples 1 to 8 to which the reducing agent and the oxidizing agent forming the redox pair each other had equal or higher centrifuge retention capacity and absorption under load while having significantly improved absorption rate, compared to the super absorbent polymer of comparative example 1 to which the redox pair was not added. However, as can be seen from comparative examples 2 to 5, when the addition amount of the redox couple is less than 1 mole or exceeds 13 moles based on 1 mole of the photoinitiator, the effect of improving the absorption rate cannot be obtained.

Claims

1. A method of preparing a superabsorbent polymer, the method comprising the steps of:

preparing a hydrogel polymer by cross-linking polymerization of the monomer composition,

wherein the content of each of the reducing agent and the oxidizing agent is more than 1 mol to 13 mol or less based on 1 mol of the photoinitiator.

2. The method for producing a super absorbent polymer as set forth in claim 1, wherein the reducing agent and the oxidizing agent are each contained in an amount of 2 to 12 moles based on 1 mole of the photoinitiator.

3. The method for producing a superabsorbent polymer according to claim 1, wherein the reducing agent and the oxidizing agent are each contained in an amount of more than 0.0015 mol% to less than 0.02 mol%, based on 100 mol% of the acrylic monomer.

4. The method for producing a super absorbent polymer as set forth in claim 1, wherein the reducing agent is sodium metabisulfite and the oxidizing agent is sodium persulfate.

5. The method for producing a super absorbent polymer as set forth in claim 1, wherein the reducing agent is ascorbic acid and the oxidizing agent is hydrogen peroxide.

6. The method for producing a super absorbent polymer as set forth in claim 1, wherein the foaming agent is at least one selected from the group consisting of sodium hydrogencarbonate, sodium carbonate, potassium hydrogencarbonate, potassium carbonate, calcium hydrogencarbonate, calcium carbonate, magnesium hydrogencarbonate and magnesium carbonate.

7. The method for producing a superabsorbent polymer according to claim 1, wherein the foaming agent is contained in an amount of 0.05 to 5 parts by weight based on 100 parts by weight of the acrylic monomer.

8. A process for producing a superabsorbent polymer according to claim 1,

wherein the preparation of the monomer composition comprises the following steps:

i) Adding a basic material to a mixture comprising an acrylic monomer, an internal crosslinking agent, and a photoinitiator, thereby neutralizing at least a portion of the acidic groups of the acrylic monomer, and

ii) adding a thermal initiator, a blowing agent, and a reducing agent and an oxidizing agent forming a redox pair with each other to the mixture obtained in step i).