阅读说明：本技术 超吸收性聚合物及其制备方法 (Superabsorbent polymer and method of making the same ) 是由 李俊宜 李昌勋 辛光寅 韩昌薰 李相琪 于 2018-12-06 设计创作，主要内容包括：本发明涉及具有高含水量的超吸收性聚合物及其制备方法。本发明的超吸收性聚合物与常规树脂相比具有相对高的含水量,但在与吸收相关的物理性能方面并没有劣化,因此可用于各种领域。(The present invention relates to superabsorbent polymers having a high water content and a method of making the same. The superabsorbent polymer of the present invention has a relatively high water content compared to conventional resins, but is not deteriorated in physical properties related to absorption, and thus can be used in various fields.)

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

A) a step of subjecting a water-soluble ethylenically unsaturated monomer having at least a part of the neutralized acid group to crosslinking polymerization in the presence of a polymerization initiator and an internal crosslinking agent to form a hydrogel polymer;

B) a step of drying, pulverizing and size-sorting the hydrogel polymer to form a base polymer powder;

C) a step of surface-crosslinking the base polymer powder by heat treatment in the presence of a surface-crosslinking agent to form superabsorbent polymer particles; and

D) a wetting step of exposing the superabsorbent polymer particles to conditions of relative humidity of 50 RH% or more and less than 80 RH% and temperature of 40 ℃ or more and less than 80 ℃.

2. The preparation method according to claim 1, wherein the ethylenically unsaturated monomer is a compound represented by the following chemical formula 1:

[ chemical formula 1]

R₁-COOM¹

In the chemical formula 1, the first and second,

R₁is an alkyl group having 2 to 5 carbon atoms containing an unsaturated bond, and

M¹is a hydrogen atom, a monovalent or divalent metal, an ammonium group or an organic amine salt.

3. The method according to claim 1, wherein the internal crosslinking agent comprises a crosslinking agent selected from the group consisting of N, N' -methylenebisacrylamide, trimethylolpropane tri (meth) acrylate, ethylene glycol di (meth) acrylate, polyethylene glycol di (meth) acrylate, propylene glycol di (meth) acrylate, polypropylene glycol di (meth) acrylate, butanediol 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 di (meth) acrylate, butylene glycol di (, One or more of the group consisting of propylene glycol, glycerol and ethylene carbonate.

4. The production method according to claim 1, wherein the internal crosslinking agent is used in an amount of 1,000ppmw to 10,000ppmw relative to the weight of the monomer.

5. The preparation method of claim 1, wherein the internal crosslinking agent comprises one or more selected from the group consisting of: alkylene carbonate having 3 to 10 carbon atoms, polyhydric alcohol having 2 to 10 carbon atoms, aminoalcohol having 1 to 10 carbon atoms, oxetane compound having 2 to 10 carbon atoms, epoxy compound having 2 to 10 carbon atoms, polyamine compound having 2 to 10 carbon atoms, calcium hydroxide, magnesium hydroxide, aluminum hydroxide, iron hydroxide, calcium chloride, magnesium chloride, aluminum chloride and iron chloride.

6. The method of claim 1, wherein step C) is performed at a temperature of 180 ℃ to 250 ℃.

7. The method of claim 1, wherein the step D) is performed for 1 to 15 minutes.

8. The production method according to claim 1, wherein the step D) is performed under conditions of a relative humidity of 50 RH% or more and 75 RH% or less and a temperature of 40 ℃ or more and 80 ℃ or less.

9. A superabsorbent polymer, comprising:

a surface-crosslinked resin in which a water-soluble ethylenically unsaturated monomer having at least a part of neutralized acid groups is polymerized and crosslinked by an internal crosslinking agent, and which has a surface-crosslinked layer modified by a surface-crosslinking agent,

wherein the superabsorbent polymer has a water content of from 0.1 wt% to 10 wt%.

10. The superabsorbent polymer of claim 9 comprising a surface crosslinked resin having a particle size of 150 μm or more and less than 850 μm, wherein the content of aggregates having a particle size of 850 μm or more is 1% by weight or less.

11. Superabsorbent polymer according to claim 9, having a centrifuge retention capacity of 20g/g or more, measured according to EDANA suggested test method WSP 241.3.

12. The superabsorbent polymer of claim 9, wherein the superabsorbent polymer has an Absorbency Under Pressure (AUP) at 0.7psi of 15g/g or more.

Technical Field

Cross Reference to Related Applications

This application claims the benefit of korean patent application No. 10-2018 and 0007384, filed on 19.1.2018 with the korean intellectual property office, the entire disclosure of which is incorporated herein by reference.

The present disclosure relates to superabsorbent polymers having a high water content and methods of making the same.

Background

Superabsorbent polymers (SAPs) are synthetic polymeric materials capable of absorbing about 500 to 1000 times their own weight of moisture, each manufacturer denominating them differently, such as SAM (superabsorbent material) or AGM (absorbent gel material), etc. Such super absorbent polymers are originally put into practical use in sanitary products, and are now widely used for producing sanitary products such as diapers for children and sanitary napkins, water-retaining soil products for gardening, water-stopping materials for civil engineering and construction, sheets for raising seedlings, freshness-retaining agents for food distribution grounds or materials for cataplasm.

As a method for producing such a superabsorbent polymer, reverse phase suspension polymerization and solution polymerization are known.

The preparation of the superabsorbent polymer by solution polymerization also includes a thermal polymerization method in which the hydrogel polymer is polymerized while being crushed and cooled in a kneader equipped with a plurality of shafts, and a photopolymerization method in which a high-concentration aqueous solution is irradiated with ultraviolet rays on a belt to thereby simultaneously perform polymerization and drying.

In addition, various post-treatment processes for improving physical properties related to absorption (e.g., absorption capacity and absorption rate of superabsorbent polymers), such as surface crosslinking and foaming, are known. In each process, a method of adding various additives for each purpose is known.

Superabsorbent polymers are typically prepared as dry powders having a certain particle size by drying, pulverizing, and size sorting processes.

Incidentally, such dry powders are electrostatically charged, which may cause process problems, and clogging of the bag filter or the like may occur during the process of preparing the super absorbent polymer, or during the subsequent processes such as diaper production or processing. During pneumatic conveyance for subsequent processes, crushing occurs, which may cause a problem of deterioration of physical properties.

In particular, in recent years, in connection with the occurrence of respiratory diseases caused by fine particles, the problem of health of human resources put into the process for producing superabsorbent polymers has become a big problem.

Therefore, there is a need to develop a method of preparing superabsorbent polymers that can solve the above-mentioned problems.

Disclosure of Invention

Technical problem

It is an object of the present invention to provide a superabsorbent polymer having a relatively high water content compared to conventional polymers, and a method of preparing the same.

Technical scheme

Provided herein is a method of preparing a superabsorbent polymer, the method comprising:

A) a step of subjecting a water-soluble ethylenically unsaturated monomer having at least a part of the neutralized acid group to crosslinking polymerization in the presence of a polymerization initiator and an internal crosslinking agent to form a hydrogel polymer;

B) a step of drying, pulverizing and size-sorting the hydrogel polymer to form a base polymer powder;

C) a step of surface-crosslinking the base polymer powder by heat treatment in the presence of a surface-crosslinking agent to form superabsorbent polymer particles; and

D) a wetting step of exposing the superabsorbent polymer particles to conditions of relative humidity of 50 RH% or more and less than 80 RH% and temperature of 40 ℃ or more and less than 80 ℃.

Further, provided herein is a superabsorbent polymer comprising a surface cross-linked resin, wherein a water-soluble ethylenically unsaturated monomer having at least a portion of neutralized acidic groups is polymerized and cross-linked by an internal cross-linking agent, and the surface cross-linked resin has a surface cross-linked layer modified by a surface cross-linking agent, wherein the superabsorbent polymer has a water content of 0.1% to 10% by weight.

Advantageous effects

According to the present disclosure, a superabsorbent polymer having a relatively high water content compared to conventional polymers without deterioration of physical properties related to absorption can be prepared.

Detailed Description

A method of preparing a superabsorbent polymer according to one aspect of the present disclosure includes:

A) a step of subjecting a water-soluble ethylenically unsaturated monomer having at least a part of the neutralized acid group to crosslinking polymerization in the presence of a polymerization initiator and an internal crosslinking agent to form a hydrogel polymer;

B) a step of drying, pulverizing and size-sorting the hydrogel polymer to form a base polymer powder;

C) a step of surface-crosslinking the base polymer powder by heat treatment in the presence of a surface-crosslinking agent to form superabsorbent polymer particles; and

D) a wetting step of exposing the superabsorbent polymer particles to conditions of relative humidity of 50 RH% or more and less than 80 RH% and temperature of 40 ℃ or more and less than 80 ℃.

Further, the superabsorbent polymer is prepared by the above-mentioned preparation method.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. Singular references include plural references unless expressly stated otherwise. It will be understood that terms such as "comprising" or "having," as used herein, are intended to specify the presence of stated features, integers, steps, components, or combinations thereof, but do not preclude the presence or addition of one or more other features, integers, steps, components, or groups thereof.

In addition, in the present disclosure, if it is mentioned that one layer or one element is formed "on" or "over" another layer or another element, it means that the layer or the element is directly formed on the another layer or the another element, or that another layer or another element is additionally formed between the layers or on the body or the substrate.

Since various modifications may be made to the present disclosure, and the present disclosure may take various forms, specific examples are illustrated in the following and described in detail. It should be understood, however, that there is no intention to limit the disclosure to the specific forms disclosed, and the invention is to cover all modifications, equivalents, or alternatives falling within the spirit and scope of the disclosure.

Hereinafter, a superabsorbent polymer and a method of preparing the same according to embodiments of the present disclosure will be described in more detail.

In the superabsorbent polymer, centrifuge retention capacity, pressure absorption rate and absorption rate were evaluated as important physical properties. Therefore, conventionally, there have been known a method of forming a large number of pores inside a superabsorbent polymer so as to rapidly absorb water, a method of reducing the particle size of the superabsorbent polymer, and the like.

However, since the superabsorbent polymer is prepared in the form of a fine dry powder, problems may arise in the process itself during the preparation process, or several problems related to the health of the allocated human resources as described above.

To solve these problems, it is necessary to increase the water content of the superabsorbent polymer particles produced in the form of a fine dry powder and to adjust it to a specific range. In the process of increasing the water content, agglomeration occurs between fine particles, resulting in aggregate particles having a relatively large particle size, so that there is a problem of deterioration of various physical properties associated with adsorption.

The present inventors have found that hydrogel polymers formed through polymerization and internal crosslinking reactions are dried, pulverized, classified and surface-crosslinked, and then a specific step is added to the superabsorbent polymer having a surface-crosslinked layer formed thereon, so that agglomeration does not occur between fine particles even at a relatively high water content, and it does not significantly contain aggregate particles having a large particle size, as compared to existing superabsorbent polymers. The present disclosure has been accomplished based on these findings.

Hereinafter, the superabsorbent polymer of the present disclosure and the method of preparing the same will be described in detail.

For reference, as used herein, "polymer" refers to the polymerization state of ethylenically unsaturated monomers, and may include all water content ranges or particle size ranges. Among these polymers, a polymer having a water content of about 40% by weight or more after polymerization and before drying may be referred to as a hydrogel polymer.

Also, "base polymer" or "base polymer powder" means a polymer in the form of powder prepared by drying and pulverization.

A method of preparing a superabsorbent polymer according to one aspect of the present disclosure includes:

A) a step of subjecting a water-soluble ethylenically unsaturated monomer having at least a part of the neutralized acid group to crosslinking polymerization in the presence of a polymerization initiator and an internal crosslinking agent to form a hydrogel polymer;

B) a step of drying, pulverizing and size-sorting the hydrogel polymer to form a base polymer powder;

C) a step of surface-crosslinking the base polymer powder by heat treatment in the presence of a surface-crosslinking agent to form superabsorbent polymer particles; and

D) a wetting step of exposing the superabsorbent polymer particles to conditions of relative humidity of 50 RH% or more and less than 80 RH% and temperature of 40 ℃ or more and less than 80 ℃.

Hereinafter, the present disclosure will be described in detail according to the respective steps.

In a method of preparing a superabsorbent polymer according to one embodiment of the present disclosure, first, a monomer composition including an ethylenically unsaturated monomer having an acid group at least a portion of which is neutralized, a polymerization initiator, and an internal crosslinking agent is polymerized to form a hydrogel polymer.

The ethylenically unsaturated monomer can have acidic groups, wherein at least a portion of the acidic groups are neutralized. Preferably, those monomers in which the monomers are partially neutralized with an alkaline substance (e.g., sodium hydroxide, potassium hydroxide, or ammonium hydroxide, etc.) may be used. In this case, the degree of neutralization of the ethylenically unsaturated monomer may be from about 40 mol% to about 95 mol%, or from about 40 mol% to about 80 mol%, or from about 45 mol% to about 75 mol%. The extent of neutralization may vary depending on the final physical properties. However, too high a degree of neutralization causes precipitation of the neutralized monomer and thus is not easily polymerized, while too low a degree of neutralization not only greatly reduces the absorption capacity of the polymer but also imparts to the polymer properties that are difficult to handle, like an elastic rubber.

Preferably, the ethylenically unsaturated monomer is a compound represented by the following chemical formula 1:

[ chemical formula 1]

R₁-COOM¹

In the chemical formula 1, the first and second,

R₁is an alkyl group having 2 to 5 carbon atoms containing an unsaturated bond, and

M¹is a hydrogen atom, a monovalent or divalent metal, an ammonium group or an organic amine salt.

Preferably, the ethylenically unsaturated monomer includes one or more selected from the group consisting of acrylic acid, methacrylic acid, and monovalent metal salts, divalent metal salts, ammonium salts, and organic amine salts of these acids.

In addition, the concentration of the ethylenically unsaturated monomer in the monomer composition may be appropriately controlled in consideration of the polymerization time and the reaction conditions, and the concentration thereof may preferably be about 20% by weight to about 90% by weight, or about 40% by weight to about 70% by weight. These concentration ranges are advantageous for adjusting the pulverization efficiency in the pulverization of the polymer as a subsequent process without removing unreacted monomers after polymerization by utilizing the gel effect phenomenon occurring in the polymerization reaction of the high-concentration aqueous solution. However, when the concentration of the monomer is too low, the yield of the superabsorbent polymer may be reduced. On the contrary, if the monomer concentration is too high, there may occur process problems such that a part of the monomer may precipitate during pulverization of the polymerized hydrogel polymer, or the pulverization efficiency may be reduced, etc., and the physical properties of the superabsorbent polymer may be deteriorated.

Meanwhile, in the monomer composition, a polymerization initiator, which is generally used to prepare the superabsorbent polymer, may be included. As a non-limiting example, the polymerization initiator may be a thermal polymerization initiator or a photopolymerization initiator depending on the polymerization method. However, even in the method of performing photopolymerization, a certain amount of heat is generated by ultraviolet irradiation or the like. In addition, as the exothermic polymerization reaction proceeds, a certain amount of heat is generated. Therefore, a thermal polymerization initiator may be further included.

Here, as the photopolymerization initiator, for example, one or more compounds selected from the group consisting of benzoin ethers, dialkyl acetophenones, hydroxyalkyl ketones, benzoyl formate, benzyl dimethyl ketal, acyl phosphines, and α -amino ketones may be used. In particular, as a specific example of acylphosphine, commercially available Lucirin TPO, i.e., 2,4, 6-trimethyl-benzoyl-trimethylphosphine oxide, may be used. Further various photopolymerization initiators are well disclosed in "UVcoatings: bases, Recent Developments and New Applications" (Elsevier, 2007), page 115, by Reinhold Schwalm, which is incorporated herein by reference.

Further, as the thermal polymerization initiator, one or more compounds selected from the group consisting of a persulfate-based initiator, an azo-based initiator, hydrogen peroxide, and ascorbic acid may be used. Specific examples of persulfate initiators may include sodium persulfate (Na)₂S₂O₈) Potassium persulfate (K)₂S₂O₈) Ammonium persulfate ((NH)₄)₂S₂O₈) And the like. Further, examples of the azo-based initiator may include 2, 2-azobis- (2-amidinopropane) dihydrochloride, 2-azobis- (N, N-dimethylene) isobutyramidine dihydrochloride, 2- (carbamoylazo) isobutyronitrile, 2-azobis [2- (2-imidazolin-2-yl) propane]Dihydrochloride, and 4, 4-azobis- (4-cyanovaleric acid), and the like. More various thermal polymerization initiatorsHair agents are fully disclosed in Odian, written "More varied thermal polymerization in Principle of polymerization" (Wiley, 1981), page 203, which is incorporated herein by reference.

Such a polymerization initiator may be added at a concentration of about 0.001 to 1% by weight with respect to the monomer composition. That is, if the concentration of the polymerization initiator is too low, the polymerization rate becomes low, and a large amount of residual monomer may be extracted in the final product, which is not desirable. On the contrary, if the concentration of the polymerization initiator is too high, polymer chains constituting the network may become short, and thus, physical properties of the polymer may be deteriorated, such as an increase in the content of water-soluble components and a decrease in pressure absorption, which is undesirable.

Meanwhile, the monomer composition contains an internal crosslinking agent to improve the physical properties of the polymer through polymerization of the water-soluble ethylenically unsaturated monomer. The crosslinking agent is used for internal crosslinking of the hydrogel polymer and is used separately from the surface crosslinking agent ("crosslinker") to crosslink the surface of the hydrogel polymer.

As the internal crosslinking agent, any compound can be used without particular limitation so long as it can introduce a crosslinking bond during polymerization of the water-soluble ethylenically unsaturated monomer. As non-limiting examples, the internal crosslinking agent may include multifunctional crosslinking agents such as N, N' -methylenebisacrylamide, trimethylolpropane tri (meth) acrylate, ethylene glycol di (meth) acrylate, polyethylene glycol di (meth) acrylate, propylene glycol di (meth) acrylate, polypropylene glycol 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 di (meth) acrylate, propylene glycol di (meth, Glycerin and ethylene carbonate, which may be used alone or in combination of two or more, but are not limited thereto.

Such internal crosslinking agents may be used in a range of from about 1,000ppmw to about 10,000ppmw, based on the monomer composition, or added at a concentration of from about 0.001 wt% to about 1 wt%, based on the monomer composition.

When the concentration of the internal crosslinking agent is too low, the absorption rate of the polymer decreases and the gel strength may be weakened, which is undesirable. In contrast, when the concentration of the internal crosslinking agent is too high, the absorption capacity of the polymer is reduced, which may be undesirable as an absorbent.

Meanwhile, the crosslinking reaction of the monomer composition may be performed in the presence of a blowing agent. During the polymerization and crosslinking reactions, the blowing agent may decompose to form pores. As non-limiting examples, the foaming agent may include one or more compounds selected from the group consisting of sodium bicarbonate, sodium carbonate, potassium bicarbonate, potassium carbonate, calcium bicarbonate, calcium carbonate, magnesium bicarbonate, magnesium carbonate, azodicarbonamide (ADCA), Dinitrosopentamethylenetetramine (DPT), p' -oxybis (benzenesulfonylhydrazide) (OBSH), p-Toluenesulfonylhydrazide (TSH), sucrose stearate, sucrose palmitate, and sucrose laurate.

The blowing agent is preferably present in the monomer composition in the range of from about 1,000ppmw to 3,000 ppmw. Specifically, the blowing agent is present in the monomer composition at greater than about 1,000ppmw, or greater than 1,100ppmw, or greater than 1,200ppmw, and less than 3,000ppmw, or less than 2,500ppmw, or less than 2,000 ppmw.

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

Such a monomer composition may be prepared in the form of a solution in which raw materials such as the above-mentioned monomer, polymerization initiator, internal crosslinking agent, and the like are dissolved in a solvent. In this case, any usable solvent may be used without limitation in composition as long as it can dissolve the above raw materials. Examples of the solvent may include 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 pentanone, cyclohexanone, cyclopentanone, diethylene glycol monomethyl ether, diethylene glycol diethyl ether, toluene, xylene, butyrolactone, carbitol, methyl cellosolve acetate, N-dimethylacetamide, or a mixture thereof.

In addition, the formation of the hydrogel polymer by polymerization of the monomer composition may be carried out by a general polymerization method, and the method is not particularly limited. As a non-limiting example, polymerization methods are largely classified into thermal polymerization, which may be performed in a reactor equipped with a stirring shaft such as a kneader, and photo polymerization, which may be performed in a reactor equipped with a movable conveyor belt, according to the type of polymerization energy source.

As an example, the monomer composition is injected into a reactor equipped with a stirring shaft, such as a kneader, and thermal polymerization is carried out by supplying hot air to the reactor or heating the reactor, thereby obtaining a hydrogel polymer. In this case, the hydrogel polymer discharged from the outlet of the reactor may be obtained as particles having a size of the order of centimeters or millimeters, depending on the type of stirring shaft provided in the reactor. Specifically, various forms of hydrogel polymers can be obtained depending on the concentration of the monomer composition injected therein, the injection speed, or the like, and hydrogel polymers having a (weight average) particle diameter of about 2nm to about 50nm can be generally obtained.

By way of example, sheet-like hydrogel polymers can be obtained when photopolymerization of the monomer composition is carried out in a reactor equipped with a movable conveyor belt. In this case, the thickness of the sheet varies depending on the concentration of the monomer composition injected therein and the injection speed, and the polymer sheet is preferably controlled to have a typical thickness of about 0.5cm to about 5cm, thereby enabling uniform polymerization of the entire sheet while ensuring the production speed and the like.

The hydrogel polymer obtained by the above method may have a water content of about 40% to about 80% by weight. As used herein, "water content" means the weight of water relative to the total weight of the hydrogel polymer, which may be the value obtained by subtracting the dry polymer weight from the weight of the hydrogel polymer. Specifically, the water content may be defined as a value calculated by measuring weight loss due to evaporation of water by raising the temperature of the polymer through infrared heating during drying. At this time, the drying conditions were determined as follows: the drying temperature is raised from room temperature to about 180 c, and then the temperature may be maintained at about 180 c, and the total drying time may be set to about 20 minutes, including a raising step of about 5 minutes.

Meanwhile, the method for preparing the superabsorbent polymer includes the step of drying the hydrogel polymer formed through the above-mentioned steps.

In this case, in order to improve the efficiency of the drying step, a step of coarsely pulverizing the hydrogel polymer may be further performed before the drying, if necessary.

By way of non-limiting example, crushers that may be employed may include, for example, vertical crushers, turbine cutters, vortex mills, rotary shredders, disc mills, crushers, and disk cutters, among others.

In this case, coarse pulverization may be carried out so that the hydrogel polymer has a particle diameter of about 2mm to about 10 mm. That is, in order to improve the drying efficiency, the hydrogel polymer is preferably pulverized to a particle size of 10mm or less. However, excessive pulverization may cause agglomeration between particles, and therefore, the hydrogel polymer is preferably pulverized to a particle size of about 2mm or more.

Further, when the coarse pulverization is carried out in this manner before the hydrogel polymer is dried, the polymer is in a high water content state, and therefore, a phenomenon in which it adheres to the surface of the pulverizing apparatus may occur. In order to minimize this phenomenon, the following substances may be added as necessary during the coarse pulverization: steam, water, a surfactant, an anti-agglomerating agent for fine particles such as clay or silica, a thermal polymerization initiator such as a persulfate-based initiator, an azo-based initiator, hydrogen peroxide, and ascorbic acid; or a crosslinking agent such as an epoxy-based crosslinking agent, a diol-based crosslinking agent, a crosslinking agent including 2-functional or 3-functional or more acrylate ester, or a monofunctional compound containing a hydroxyl group.

Meanwhile, drying the hydrogel polymer immediately after the coarse pulverization or polymerization may be performed at a temperature of about 120 ℃ to about 250 ℃, about 150 ℃ to about 200 ℃, or about 160 ℃ to about 180 ℃ (in this case, the temperature may be defined as a temperature at which a heating medium for drying is provided thereto, or an internal temperature of a drying reactor including the heating medium and the polymer during the drying process). If the drying temperature is low and thus the drying time becomes long, the physical properties of the final polymer may be deteriorated. To prevent this problem, the drying temperature is preferably about 120 ℃ or higher. In addition, when the drying temperature is higher than necessary, only the surface of the hydrogel polymer is dried, and thus there is a concern that a large amount of fine particles are generated in the subsequent pulverization process, and the physical properties of the final polymer may be deteriorated. To avoid this problem, the drying temperature is therefore preferably about 250 ℃ or less.

In this case, the drying time in the drying step is not particularly limited, but may be controlled to be at the above drying temperature for about 20 to about 90 minutes in consideration of process efficiency.

The drying method for drying may also be applied without limitation as long as it is a method commonly used in the drying process of hydrogel polymers. Specifically, the drying step may be performed by a method such as hot air supply, infrared irradiation, microwave irradiation, or ultraviolet irradiation.

The polymer dried by the above process may exhibit a moisture content of about 0.1 wt% to about 10 wt%, preferably about 0.1 wt% to about 5 wt%, more preferably about 0.1 wt% to about 1 wt%. If the water content of the dried polymer is too low, production costs may be increased due to excessive drying and degradation of the crosslinked polymer, which is disadvantageous. Further, if the water content of the polymer is too high, processability may be reduced and defects may occur in a subsequent pulverization or surface crosslinking step, which is undesirable.

Subsequently, a step of pulverizing the dried polymer is performed. The pulverization is for optimizing the surface area of the dried polymer, and may be performed such that the particle size of the pulverized polymer is about 150 μm to about 850 μm, specifically about 150 μm or more and about 850 μm or less.

In this case, the pulverizing device may include a commonly used pin mill, hammer mill, screw mill, roll mill, disc mill, or point mill (jog mill), or the like. In order to manage the physical properties of the finally produced superabsorbent polymer, a step of selectively size-sorting polymer particles having a particle size of about 150 μm to about 850 μm among the polymer particles obtained through the pulverizing step may be further performed.

The surface crosslinking step of the polymer (for example, base polymer powder) pulverized by the above-mentioned steps is carried out using a surface crosslinking agent.

Surface crosslinking is a step of forming a superabsorbent polymer having further improved physical properties by inducing a crosslinking reaction of the surface of the pulverized polymer in the presence of a surface crosslinking agent. A surface cross-linked layer may be formed on the surface of the pulverized polymer particles by surface cross-linking.

The surface modification may be carried out by a general method of increasing the crosslinking density of the surface of the polymer particles, for example, a solution containing a surface crosslinking agent is mixed with the pulverized polymer to allow the crosslinking reaction.

Herein, the surface cross-linking agent is a compound that can react with the functional group of the polymer, and its constitution is not particularly limited.

However, as non-limiting examples, it preferably comprises one or more selected from the group consisting of: alkylene carbonate having 3 to 10 carbon atoms, polyhydric alcohol having 2 to 10 carbon atoms, aminoalcohol having 1 to 10 carbon atoms, oxetane compound having 2 to 10 carbon atoms, epoxy compound having 2 to 10 carbon atoms, polyamine compound having 2 to 10 carbon atoms, calcium hydroxide, magnesium hydroxide, aluminum hydroxide, iron hydroxide, calcium chloride, magnesium chloride, aluminum chloride and iron chloride.

Specific examples of the alkylene carbonate compound may include 1, 3-dioxolan-2-one, 4-methyl-1, 3-dioxolan-2-one, 4, 5-dimethyl-1, 3-dioxolan-2-one, 4-dimethyl-1, 3-dioxolan-2-one, 4-ethyl-1, 3-dioxolan-2-one, 4-hydroxymethyl-1, 3-dioxolan-2-one, 1, 3-dioxan-2-one, 4-methyl-1, 3-dioxan-2-one, 4, 6-dimethyl-1, 3-dioxan-2-one and 1, 3-dioxepin-2-one, and the like.

According to another embodiment of the present disclosure, the surface cross-linking agent may further include a polycarboxylic acid compound as a random copolymer derived from a hydrophilic monomer such as alkoxy polyalkylene glycol mono (meth) acrylate-based monomers including methoxy polyethylene glycol monomethacrylate (MPEGMAA) and the like; and (meth) acrylate-based monomers including acrylic acid and (meth) acrylic acid.

Specific examples of the polycarboxylic acid compound are disclosed in korean unexamined patent publication No. 2015-0143167 and the like.

In this case, the content of the surface cross-linking agent may be appropriately controlled according to the kind of the cross-linking agent or the reaction conditions, and is preferably about 0.001 parts by weight to about 5 parts by weight based on 100 parts by weight of the pulverized polymer. If the content of the surface cross-linking agent is too low, surface cross-linking may not be properly performed, which may result in deterioration of physical properties of the final polymer. On the contrary, if the surface cross-linking agent is excessively used, an excessive surface cross-linking reaction may occur, resulting in a decrease in the absorption capacity of the polymer, which is undesirable.

When the surface-crosslinking solution is added, water may be further added. The surface cross-linking agent and water are added together, so that the surface cross-linking agent is dispersed more uniformly, and the penetration depth of the surface cross-linking agent in the polymer particles is further optimized. In view of these objects and effects, the amount of water added together with the surface cross-linking agent may be controlled to be about 0.5 parts by weight to about 10 parts by weight based on 100 parts by weight of the pulverized polymer.

Meanwhile, the surface crosslinking may be performed at a temperature of about 180 ℃ to about 250 ℃. When the surface crosslinking is performed at the above temperature, the surface crosslinking density can be increased, and thus it is preferable. More preferably, surface crosslinking can be performed at a temperature above about 190 ℃ and below about 240 ℃, below about 230 ℃, below about 220 ℃, below about 210 ℃, or below about 200 ℃.

Further, the surface crosslinking reaction may be performed for about 50 minutes or more. That is, in order to induce the minimum surface crosslinking reaction and prevent the degradation of physical properties due to the deterioration of polymer particles during an excessive reaction, the surface crosslinking reaction may be performed under the above-mentioned conditions. The reaction may be carried out for about 120 minutes or less, about 100 minutes or less, or about 60 minutes or less.

And, after surface crosslinking, a wetting step of exposing the superabsorbent polymer particles to conditions of a relative humidity of 50 RH% or more and less than 80 RH% and a temperature of 40 ℃ or more and less than 80 ℃ is performed.

As mentioned above, since the superabsorbent polymer in the form of dry fine powder causes some problems, it is necessary to further increase the water content.

Conventionally, in order to increase the water content, a method of adding water or other additives to superabsorbent polymer particles having a surface cross-linked layer formed thereon, or a method of adjusting the water content while evaporating water again after spraying in a spray form and aging has been employed.

However, in these methods, it is difficult to precisely adjust the water content, and some particles are re-agglomerated to form aggregate particles of large particle size during the process of increasing the water content, resulting in a significant decrease in physical properties associated with adsorption.

Accordingly, in the method of preparing the superabsorbent polymer of the present disclosure, the water content is increased by exposing the superabsorbent polymer having the surface-crosslinked layer to conditions of relatively high humidity and temperature for a certain period of time.

As described above, when the surface-crosslinked superabsorbent polymer particles are exposed to a specific condition to increase the water content, there is an advantage in that the water content of the superabsorbent polymer particles can be precisely adjusted by adjusting the humidity or the exposure time. In addition, the superabsorbent polymer particles do not come into direct contact with water during wetting and thus do not form aggregate particles, which does not result in deterioration of physical properties.

The wetting step may be performed differently depending on the target moisture content range, which ranges from about 1 minute to about 15 minutes, preferably from about 1 minute to about 10 minutes. If the exposure time is too short, there is a problem that it is difficult to increase the water content. If the exposure time is too long, problems may occur with the surface cross-linked layer of superabsorbent polymer and the physical properties associated with absorption may deteriorate.

Also, the above-mentioned wetting step may be performed under the following conditions in terms of adjusting the water content and maintaining the physical properties of the surface cross-linked layer: the relative humidity is about 50 RH% or more and about 75 RH% or less, more preferably the relative humidity is about 60 RH% or more and about 75 RH% or less, and the temperature is about 40 ℃ or more and about 75 ℃ or less, more preferably the temperature condition is about 60 ℃ or more and about 75 ℃ or less.

The superabsorbent polymer prepared according to the above-mentioned preparation method may have excellent absorption-related physical properties and relatively high water content compared to existing superabsorbent polymers, and thus, problems caused by the superabsorbent polymer (fine dry particles) may be prevented in advance.

A superabsorbent polymer according to one aspect of the present disclosure comprises,

a surface-crosslinked resin in which a water-soluble ethylenically unsaturated monomer having at least a part of neutralized acid groups is polymerized and crosslinked by an internal crosslinking agent, and which has a surface-crosslinked layer modified by a surface-crosslinking agent,

wherein the superabsorbent polymer has a water content of from 0.1 wt% to 10 wt%.

The ethylenically unsaturated monomer may preferably be a compound represented by the following chemical formula 1, as described in the manufacturing method.

[ chemical formula 1]

R¹-COOM¹

In the chemical formula 1, the first and second,

R₁is an alkyl group having 2 to 5 carbon atoms containing an unsaturated bond, and

M¹is a hydrogen atom, a monovalent or divalent metal, an ammonium group or an organic amine salt.

Preferably, the ethylenically unsaturated monomer includes one or more selected from the group consisting of acrylic acid, methacrylic acid, and monovalent metal salts, divalent metal salts, ammonium salts, and organic amine salts of these acids.

Here, the ethylenically unsaturated monomer may have an acidic group, wherein at least a part of the acidic group is neutralized. Preferably, those in which the monomer is partially neutralized by an alkaline substance such as sodium hydroxide, potassium hydroxide, ammonium hydroxide, or the like can be used. In this case, the degree of neutralization of the ethylenically unsaturated monomer may be from about 40 mol% to about 95 mol%, or from about 40 mol% to about 80 mol%, or from about 45 mol% to about 75 mol%. The range of neutralization may vary depending on the final physical properties. However, too high a degree of neutralization causes precipitation of the neutralized monomer and thus is not easily polymerized, while too low a degree of neutralization not only greatly reduces the absorption capacity of the polymer but also gives the polymer properties that are difficult to handle, like an elastic rubber.

The water content may vary depending on the purpose of use, and may vary depending on the adjustment of the exposure conditions of the wetting step in the preparation process. The water content may be 0.1 wt% or more, preferably about 0.5 wt% or more, more preferably about 1 wt% or more, and 10 wt% or less, preferably about 7 wt% or less, more preferably about 5 wt% or less.

In addition, the superabsorbent polymer of one embodiment of the present disclosure comprises,

a surface-crosslinked polymer having a particle size of about 150 μm or more and less than about 850 μm, wherein the polymer has a content of aggregates having a particle size of about 850 μm or more of about 1% by weight or less, or about 0.5% by weight or less, or about 0.1% by weight, or may substantially not contain the above-mentioned aggregate particles, and therefore, it is possible to prevent deterioration of physical properties related to absorption due to formation of the aggregates.

The superabsorbent polymer according to one embodiment of the present disclosure has a centrifuge retention capacity of about 20g/g or more, or about 25g/g or more, preferably about 26g/g or more, as measured according to EDANA suggested test method WSP 241.3. The upper limit of the centrifuge retention capacity is not particularly limited, but may be about 50g/g or less, preferably about 40g/g or less, or about 35g/g or less, or about 30g/g or less.

Centrifuge Retention Capacity (CRC) was measured according to EDANA suggested test method WSP 241.3 and is represented by the following equation 1:

[ equation 1]

CRC(g/g)＝{[W2(g)-W1(g)]/W0(g)}–1

In the case of the equation 1, the,

w0(g) is the initial weight of the polymer (g),

w1(G) is the weight (G) of the apparatus excluding the polymer measured after dewatering at 250G for 3 minutes using a centrifuge, and

w2(G) is the weight of the apparatus including the polymer (G) measured after immersion in 0.9 wt% saline solution for 30 minutes at room temperature and then dehydration for 3 minutes at 250G using a centrifuge.

Further, according to another embodiment of the present disclosure, the superabsorbent polymer has an Absorbency Under Pressure (AUP) at 0.7psi of about 15g/g or more, preferably about 19g/g or more, or about 20g/g or more. The upper limit is also not particularly limited, but may be about 40g/g or less, preferably about 30g/g or less, or about 27g/g or less, or about 25g/g or less.

Hereinafter, the action and effect of the present disclosure will be described in more detail by specific examples. However, these examples are for illustrative purposes only, and the scope of the present disclosure is not determined thereby.