KR20230013001A - Preparation method of super absorbent polymer - Google Patents

Abstract

The present invention relates to a manufacturing method of a superabsorbent polymer, and more specifically, to a manufacturing method of a superabsorbent polymer, in which, by performing a multi-step foaming process using a cell stabilizer, a hierarchical cell distribution is formed in the manufactured superabsorbent polymer, and accordingly, the super absorbent polymer having excellent absorption rate and absorption performance can be manufactured.

Description

Manufacturing method of super absorbent polymer {PREPARATION METHOD OF SUPER ABSORBENT POLYMER}

The present invention relates to a method for preparing a superabsorbent polymer. More specifically, by performing a multi-step foaming process using a cell stabilizer, a hierarchical cell distribution is formed in the superabsorbent polymer to be produced, thereby producing a superabsorbent polymer having excellent absorption rate and performance. It relates to a method for producing resin.

Super Absorbent Polymer (SAP) is a synthetic high-molecular substance that has the ability to absorb moisture 500 to 1,000 times its own weight. Material), etc., are named by different names. The superabsorbent polymer as described above has begun to be put into practical use as a sanitary tool, and is currently widely used as a material for gardening soil remediation agents, civil engineering and construction waterstop materials, seedling sheets, freshness retainers in the field of food distribution, and steaming. .

These superabsorbent polymers are widely used in sanitary materials such as diapers and sanitary napkins. In the sanitary material, it is common that the superabsorbent polymer is included in a spread state in the pulp. However, in recent years, efforts have been made to provide sanitary materials such as diapers with a thinner thickness, and as part of this, the content of pulp is reduced or, furthermore, so-called pulpless diapers in which pulp is not used at all. Development is actively progressing.

In sanitary materials such as pulpless diapers where the pulp content is reduced or pulp is not used, the superabsorbent polymer not only serves as an absorbent for absorbing liquids such as urine, but also serves as a pulp, resulting in high absorption. In addition to performance, it is required to exhibit a fast absorption rate.

In order to manufacture such a super absorbent polymer having an improved absorption rate, a method of improving a specific surface area by introducing a pore structure into the super absorbent polymer is mainly used. Specifically, in order to improve the specific surface area of the superabsorbent polymer, a foaming agent may be used to generate bubbles or a gas such as carbon dioxide gas, air, or nitrogen gas may be injected in the polymerization step.

However, since the bubbles formed by the foaming agent are unstable in the neutralization solution, the bubbles easily escape out of the neutralization solution, and it is difficult to control the size of the bubbles, making it impossible to prepare a superabsorbent polymer having a desired pore structure.

Accordingly, there is a continuous demand for the development of a super absorbent polymer exhibiting a fast absorption rate while maintaining the basic absorption performance of the super absorbent polymer.

Accordingly, the present invention relates to a method for preparing a super absorbent polymer, wherein a hierarchical pore distribution is formed in the super absorbent polymer to be produced by performing a multi-step foaming process using a cell stabilizer, and thus, improved absorption properties and absorption rate are obtained. .

In order to solve the above problems, the present invention,

preparing a monomer composition comprising a water-soluble acrylic acid-based monomer having an acidic group and at least partially neutralizing the acidic group, a polymerization initiator, and a first crosslinking agent;

firstly foaming a mixture obtained by mixing the monomer composition with a first foaming agent and a foam stabilizer;

secondary foaming of a mixture obtained by mixing a second foaming agent with the primary foamed mixture;

cross-linking the secondary foam mixture to form a water-containing gel polymer;

drying, pulverizing, and classifying the water-containing gel polymer to form a base resin powder; and

In the presence of surface crosslinking, crosslinking at least a portion of the surface of the base resin powder,

A method for preparing a superabsorbent polymer is provided.

According to the manufacturing method of the superabsorbent polymer of the present invention, by performing a multi-step foaming process using a foaming agent and a foam stabilizer, bubbles of different sizes can be easily formed in each step, and accordingly, processes such as drying and crushing are performed. By forming a hierarchical pore structure in the super absorbent polymer that is finally manufactured through the above process, the specific surface area can be increased to significantly improve the absorption properties and absorption rate of the super absorbent polymer.

1 schematically illustrates a multi-step foaming process in a method for manufacturing a superabsorbent polymer according to an embodiment of the present invention.
2 relates to OM images of bubbles generated under the conditions of the primary foaming process and the secondary foaming process of Example 1.
3 is a graph showing the change in the size of bubbles generated and the size of pores after curing according to the concentration change of the cell stabilizer in the multi-step foaming process of Example 1.
4 is a graph showing particle diameter distribution of pores in the superabsorbent polymer prepared according to Example 1 and Comparative Example 1.
5 schematically illustrates cross-sectional images of superabsorbent polymers prepared according to Example 1, Comparative Example 1, and Comparative Example 4.
6 relates to cross-sectional SEM images of superabsorbent polymers prepared according to Example 1, Comparative Example 1, and Comparative Example 4.

Terms used in this specification are only used to describe exemplary embodiments, and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly dictates otherwise. In this specification, terms such as "comprise", "comprise" or "having" are intended to indicate that there is an embodied feature, step, component, or combination thereof, but one or more other features or steps; It should be understood that the presence or addition of components, or combinations thereof, is not previously excluded.

The term "polymer" or "polymer" used in the specification of the present invention means a state in which acrylic acid-based monomers, which are water-soluble ethylenically unsaturated monomers, are polymerized, and may cover all moisture content ranges or particle size ranges. Among the polymers, polymers in a state after polymerization and before drying and having a moisture content (moisture content) of about 40% by weight or more may be referred to as hydrogel polymers, and particles obtained by pulverizing and drying such hydrogel polymers may be referred to as crosslinked polymers. there is.

Also, the term “superabsorbent polymer particle” refers to a particulate material including a crosslinked polymer containing an acidic group and an acrylic acid-based monomer monomer in which at least a part of the acidic group is neutralized is polymerized and crosslinked by an internal crosslinking agent.

In addition, the term “superabsorbent polymer” refers to a crosslinked polymer obtained by polymerizing an acrylic acid-based monomer containing an acidic group and at least a portion of the acidic group neutralized, or a powder composed of superabsorbent polymer particles in which the crosslinked polymer is pulverized, depending on the context. ) form, or covers all of the crosslinked polymers or base resins in a state suitable for commercialization through additional processes such as surface crosslinking, fine powder reassembly, drying, pulverization, classification, etc. used to do Accordingly, the term “super absorbent polymer” may be interpreted as including a plurality of super absorbent polymer particles.

In addition, the term “bubble” refers to a spherical space formed by the foaming gas formed in the mixture during the foaming process, and “pore” refers to a water-containing gel polymer, base resin, or superabsorbent material formed by curing the bubbles. It means a spherical space formed in the resin.

In addition, the term "average diameter" of pores or bubbles means the median value of the longest diameter value of each of the pores or bubbles to be measured. This is to make it less affected by outliers than the simple average.

Since the present invention can have various changes and various forms, specific embodiments will be exemplified and described in detail below. However, it should be understood that this is not intended to limit the present invention to the specific disclosed form, and includes all modifications, equivalents, and substitutes included in the spirit and scope of the present invention.

In addition, technical terms used herein are only for referring to specific embodiments and are not intended to limit the present invention. And, as used herein, the singular forms also include the plural forms unless the phrases clearly indicate the opposite.

In order to prepare a super absorbent polymer having a fast absorption rate, the specific surface area of the super absorbent polymer particles must be increased. Accordingly, a method of forming a plurality of pores in the super absorbent polymer by inducing a foaming process during the manufacturing process or a method of mechanically deforming the super absorbent polymer has been used to realize a super absorbent polymer having a high specific surface area. In particular, depending on how the foaming mechanism is controlled, the size of the foam formed varies, which causes the shape of the final particle after pulverization to change, and in order to increase the specific surface area, it is preferable to include a large number of fine pores even in the pulverized particles. However, since the bubbles formed in the normal foaming process are unstable in the neutralization solution, the bubbles easily escape out of the neutralization solution, and it is difficult to control the size of the bubbles, making it impossible to prepare a superabsorbent polymer having a desired pore structure. .

Therefore, the inventors of the present invention can stably form bubbles of different sizes in each step by performing a multi-step foaming process using a foaming agent and a foam stabilizer during the production of superabsorbent polymer, and accordingly, processes such as drying and crushing After confirming that a hierarchical pore structure can be formed in the superabsorbent polymer finally manufactured through the above process, the present invention was completed.

In particular, in the multi-stage foaming process, it was confirmed that it was easy to control the size of the cells formed in each foaming step by adjusting the concentration of the cell stabilizer used together with the foaming agent. Accordingly, in order to form a more stable pore structure, the components of the cell stabilizer used in the foaming process and the optimal concentration of the cell stabilizer in each foaming step were derived.

Manufacturing method of superabsorbent polymer

Hereinafter, a method for manufacturing superabsorbent polymer particles according to an embodiment will be described step by step.

(Preparation step of monomer composition)

First, according to an embodiment of the present invention, a step of preparing a monomer composition including a water-soluble acrylic acid-based monomer having an acidic group and neutralizing at least a portion of the acidic group, a polymerization initiator, and a first crosslinking agent is included.

The acrylic acid-based monomer is a compound represented by Formula 1 below:

[Formula 1]

R ¹ -COOM ¹

In Formula 1,

R ¹ is an alkyl group having 2 to 5 carbon atoms including an unsaturated bond;

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

Preferably, the acrylic acid-based monomer includes at least one selected from the group consisting of acrylic acid, methacrylic acid, and monovalent metal salts, divalent metal salts, ammonium salts, and organic amine salts thereof.

Here, the acrylic acid-based monomer may have an acidic group and at least a portion of the acidic group may be neutralized. Preferably, those obtained by partially neutralizing the monomers with alkali substances such as sodium hydroxide, potassium hydroxide, and ammonium hydroxide may be used. At this time, the degree of neutralization of the acrylic acid-based monomer may be 40 to 95 mol%, preferably, 40 mol% or more, 60 mol% or more, 70 mol% or more, 95 mol% or less, 90 mol% or less, 85 mol% or less mol% or less, or may be 40 to 90 mol%, 60 to 85 mol%, or 70 to 85 mol%. However, if the degree of neutralization is too high, neutralized monomers may precipitate out, making it difficult for the polymerization to proceed smoothly. Conversely, if the degree of neutralization is too low, the absorbency of the polymer is greatly reduced, and it may exhibit properties such as elastic rubber that are difficult to handle. there is.

The concentration of the acrylic acid-based monomer may be 20 to 60% by weight, preferably 20% by weight or more, 40% by weight or more, and 60% by weight based on the monomer mixture including the raw material of the superabsorbent polymer and the solvent. % or less, 50% by weight or less, or may be included in 40 to 50% by weight, which may be selected at an appropriate concentration in consideration of polymerization time and reaction conditions. However, if the concentration of the monomer is too low, the yield of the superabsorbent polymer is low and economic problems may arise. Conversely, if the concentration is too high, a part of the monomer is precipitated or the pulverization efficiency of the polymerized water-containing gel polymer is low. Process problems may occur, and physical properties of the superabsorbent polymer may be deteriorated.

In addition, the term 'first crosslinking agent' used herein is a term used to distinguish it from a second crosslinking agent for mainly crosslinking the surface of the superabsorbent polymer particles, which will be described later. It plays a role in crosslinking and polymerization. Crosslinking in the above step proceeds regardless of surface or internal, but when the surface crosslinking process of the superabsorbent polymer particles described later proceeds, the surface of the finally prepared superabsorbent polymer particle has a structure crosslinked by the second crosslinking agent, , The inside is composed of a structure crosslinked by the first crosslinking agent.

As the first crosslinking agent, any compound may be used as long as it enables introduction of crosslinking bonds during polymerization of the acrylic acid-based unsaturated monomer. Specifically, the first crosslinking agent includes a crosslinking agent having at least one ethylenically unsaturated group while having at least one functional group capable of reacting with the water-soluble substituent of the acrylic acid-based unsaturated monomer; Alternatively, a crosslinking agent having two or more functional groups capable of reacting with the water-soluble substituent of the monomer and/or the water-soluble substituent formed by hydrolysis of the monomer may be used.

As a non-limiting example, the first crosslinking agent is N,N'-methylenebisacrylamide, trimethylpropane tri(meth)acrylate, ethylene glycol di(meth)acrylate, polyethylene glycol (meth)acrylate, polyethylene glycol di( Meth)acrylate, propylene 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, glycerin tri Multifunctional crosslinking agents such as (meth)acrylate, pentaerythol tetraacrylate, triarylamine, ethylene glycol diglycidyl ether, propylene glycol, glycerin, or ethylene carbonate may be used alone or in combination of two or more. It is not limited.

The first crosslinking agent may be used in an amount of 0.01 to 5 parts by weight based on 100 parts by weight of the water-soluble ethylenically unsaturated monomer. For example, the first crosslinking agent is 0.01 parts by weight or more, 0.05 parts by weight or more, 0.1 parts by weight or more, 0.45 parts by weight or more, 5 parts by weight or less, 3 parts by weight or less, 2 parts by weight or less, based on 100 parts by weight of the water-soluble ethylenically unsaturated monomer. 0.1 to 3 parts by weight or 0.45 to 1 part by weight may be used. If the content of the upper first crosslinking agent is too low, crosslinking does not occur sufficiently, making it difficult to realize a strength higher than an appropriate level. If the content of the upper first crosslinking agent is too high, the internal crosslinking density increases, making it difficult to realize the desired water retention capacity.

The monomer composition may further include a polymerization initiator for initiating a polymerization reaction of the monomers. The polymerization initiator is not particularly limited as long as it is generally used in the production of super absorbent polymers.

Specifically, the polymerization initiator may be a thermal polymerization initiator or a photopolymerization initiator according to UV irradiation according to a polymerization method. However, even with the photopolymerization method, since a certain amount of heat is generated by irradiation such as ultraviolet light irradiation and a certain amount of heat is generated according to the progress of the polymerization reaction, which is an exothermic reaction, a thermal polymerization initiator may be additionally included.

As the photopolymerization initiator, any compound capable of forming radicals by light such as ultraviolet light may be used without limitation in its composition.

Examples of the photopolymerization initiator include benzoin ether, dialkyl acetophenone, hydroxyl alkylketone, phenyl glyoxylate, and benzyl dimethyl ketal. Ketal), acyl phosphine, and alpha-aminoketone (α-aminoketone) may be used at least one selected from the group consisting of. On the other hand, as a specific example of acylphosphine, commercially available lucirin TPO, that is, diphenyl (2,4,6-trimethylbenzoyl) -phosphine oxide can be used. can For more photoinitiators, see Reinhold Schwalm's "UV Coatings: Basics, Recent Developments and New Applications" (Elsevier 2007) p. 115, and is not limited to the examples described above.

The photopolymerization initiator may be included in a concentration of about 0.01 to about 1.0% by weight based on the monomer composition. If the concentration of the photopolymerization initiator is too low, the polymerization rate may be slowed down, and if the concentration of the photopolymerization initiator is too high, the molecular weight of the superabsorbent polymer may be small and physical properties may be non-uniform.

In addition, as the thermal polymerization initiator, at least one selected from the group consisting of a persulfate-based initiator, an azo-based initiator, hydrogen peroxide, and ascorbic acid may be used. Specifically, examples of the persulfate-based initiator include sodium persulfate (Na2S2O8), potassium persulfate (K2S2O8), ammonium persulfate ((NH4)2S2O8), and the like. Examples of initiators include 2,2-azobis(2-amidinopropane) dihydrochloride, 2,2-azobis-(N,N-dimethylene)isobu 2,2-azobis-(N, N-dimethylene)isobutyramidine dihydrochloride, 2-(carbamoylazo)isobutylonitril, 2,2-azobis [2-(2-imidazolin-2-yl)propane] dihydrochloride (2,2-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride), 4,4-azobis-( 4-cyanovaleric acid) (4,4-azobis-(4-cyanovaleric acid)) and the like. For more diverse thermal polymerization initiators, see Odian's 'Principle of Polymerization (Wiley, 1981)', p. 203, and is not limited to the examples described above.

The thermal polymerization initiator may be included in a concentration of about 0.001 to about 0.5% by weight based on the monomer composition. If the concentration of the thermal polymerization initiator is too low, additional thermal polymerization hardly occurs, and the effect of adding the thermal polymerization initiator may be insignificant. there is.

The total amount of the polymerization initiator may be 2 parts by weight or less based on 100 parts by weight of the water-soluble ethylenically unsaturated monomer. That is, when the concentration of the polymerization initiator is too low, the polymerization rate may be slowed and a large amount of residual monomer may be extracted into the final product, which is not preferable. Conversely, when the concentration of the polymerization initiator is higher than the above range, the polymer chain constituting the network is shortened, which is not preferable because the physical properties of the resin may be deteriorated, such as an increase in the content of water-soluble components and a decrease in absorbency under pressure.

The monomer composition may further include additives such as a thickener, a plasticizer, a storage stabilizer, and an antioxidant, if necessary.

And, the monomer composition including the monomer may be, for example, in a solution state dissolved in a solvent such as water, and the solid content in the monomer composition in such a solution state, that is, the concentration of the monomer, the first crosslinking agent, and the polymerization initiator It may be appropriately adjusted in consideration of polymerization time and reaction conditions. For example, the solid content in the monomer mixture may be 10 to 80% by weight, preferably 10% by weight or more, 15% by weight or more, 30% by weight or more, 80% by weight or less, 60% by weight or less, 50 wt% or less, 15 to 60 wt%, or 30 to 50 wt%.

When the monomer composition has a solid content in the above range, the gel effect phenomenon that occurs in the polymerization reaction of a high-concentration aqueous solution is used to eliminate the need to remove unreacted monomers after polymerization, while increasing the pulverization efficiency when pulverizing the polymer, which will be described later. It can be advantageous to adjust.

The solvent that can be used at this time can be used without limitation in composition as long as it can dissolve the above-mentioned components. For example, water, ethanol, ethylene glycol, diethylene glycol, triethylene glycol, 1,4-butanediol, propylene Glycol, ethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monomethyl ether acetate, methyl ethyl ketone, acetone, methyl amyl ketone, cyclohexanone, cyclopentanone, diethylene glycol monomethyl ether, diethylene glycol ethyl At least one selected from ether, toluene, xylene, butyrolactone, carbitol, methylcellosolveacetate, and N,N-dimethylacetamide may be used in combination.

On the other hand, the content of each component in the above-described monomer composition means the total mixing content of the monomer composition of the first foaming step described later and the monomer composition additionally added in the second foaming step.

(1st foaming step)

Next, the manufacturing method of the superabsorbent polymer according to one embodiment of the present invention includes the step of firstly foaming a mixture obtained by mixing the monomer composition with a first foaming agent and a cell stabilizer.

In the primary foaming step, bubbles are generated by the first foaming agent included in the mixture of the monomer composition, the first foaming agent, and the foam stabilizer, and the foam formed by the foam stabilizer used together can be stably trapped in the mixture. there will be The bubbles formed in the primary foaming step form a plurality of pores in the water-containing gel polymer in the crosslinking polymerization step described later, and form a porous structure in the superabsorbent polymer finally manufactured through processes such as drying and grinding to reduce the specific surface area. increase

Meanwhile, as will be described later, the foaming step is performed at a low temperature of about 50° C. or less. Accordingly, bubbles are formed before the polymerization step, and the bubbles stably trapped in the mixture by the bubble stabilizer are the water-containing gel polymer formed in the polymerization step. to form pores within it. The first foaming agent may be a low-temperature foaming agent that foams at 50° C. or lower, and thus, the foaming process may be performed first before the polymerization step.

Specific examples of the first blowing agent include sodium carbonate, sodium bicarbonate, potassium bicarbonate, azodicarbonamide, p,p'-oxybisbenzenesulfonylhydrazide, dinitrosopentamethylenetetramine, and p-toluenesulfonyl. At least one selected from the group consisting of hydrazide and benzenesulfonylhydrazide may be used, and preferably sodium carbonate, potassium bicarbonate, and the like may be used.

In the primary foaming step, the first blowing agent may be used in an amount of 200 ppmw to 8,000 ppmw based on the water-soluble ethylenically unsaturated monomer in the mixture, preferably 200 ppmw or more, 500 ppmw or more, 500 ppmw or more, or 8,000 ppmw or less , 6,000 ppmw or less, 4,000 ppmw or less, and may be used at 500 to 6,000 ppmw or 800 ppmw to 40,000 ppmw. When included within the above content range, generated fine bubbles may be stably formed. When the amount of the first foaming agent is insignificant, the foaming effect is insignificant, and when it is included in an excessive amount, the gel strength of the superabsorbent polymer finally produced may decrease.

The cell stabilizer is a component that allows the bubbles formed from the foaming agent to be stably captured in the mixture without escaping out of the mixture, and is dispersed between the bubbles formed by the foaming agent and the interface between the mixture to capture and stabilize the bubbles. . In addition, the concentration of the foam stabilizer is different in the first foaming step and the second foaming step to be described later, and thus the size of the generated bubbles can be controlled.

Specifically, as the concentration of the cell stabilizer in the mixture increases, it is easier to stably form fine cells. In the present invention, after the cell stabilizer is added at a relatively high concentration in the first foaming step, the foaming agent, monomer, and solvent are additionally added in the second foaming step to lower the concentration of the cell stabilizer. Accordingly, in the first foaming step, fine cells having a relatively small average diameter are formed, and in the second foaming step, cells having a relatively large average diameter are formed.

The cell stabilizer may be a hydrophilic surface-modifying compound, and is thus located at the interface between the foam formed by the foaming agent and the mixture, enabling the cell to be stably captured.

Specifically, the cell stabilizer is a particulate material capable of being dispersed in a mixture having an average diameter of about 0.05 μm to 5 μm after being modified with a hydrophilic surface.

The cell stabilizer is a compound in which a hydrophobic compound is hydrophilic surface modified with a specific surface modifier, specifically, polyethylene glycol, polyvinyl alcohol, polyvinylpyrrolidine, polyvinyl acetate, polyvinylpyrrolidine polyacrylic acid, polyacrylic acid. A compound obtained by modifying the hydrophobic compound into a hydrophilic surface using at least one polymer selected from the group consisting of amide, polydopamine, poly(4-styrenesulfonate) polyethylene glycol and polypropylene glycol, or a copolymer thereof as a surface modifier. can be

The polymer used as the surface modifier may be a homopolymer or a block copolymer.

As the hydrophobic compound, a hydrophobic synthetic high molecular compound having a melting point of 100° C. or higher, for example, poly(L-lactide), poly(D,L-lactic-co-glycolic acid), polylactic acid, polystyrene, polymethylmethacrylate, polystyrene-polybutadiene copolymer, polystyrene-polyimide copolymer, nylon, polyester; hydrophobic natural high molecular compounds having a melting point of 100° C. or higher, such as lignin, polylactic acid-lignin blend, cellulose acetate (eg, cellulose triacetate, cellulose acetate propionate, cellulose acetate butylate, cellulose acetate phthalate); solid hydrocarbons having a melting point of 100° C. or higher, such as fatty acids (eg, stearic acid, palmitic acid), fatty acid metal salts, glyceride esters, and sugar esters; In addition, among hydrophobic compounds used as defoamers and lubricants, hydrophilic surface-modifiable particles (eg, talc, etc.) may be exemplified, but are not particularly limited thereto.

On the other hand, the hydrophilic surface-modified cell stabilizer is surface-modified through a method of dispersing hydrophobic particles and a surface modifier in a mixed solution of water and a co-solvent (acetone, ethanol, etc.) and stirring when the surface modification target is an insoluble hydrophobic compound. It can be. In addition, when the surface modification target is dissolved in an organic solvent, the organic solvent in which the hydrophobic compound is dissolved is dropped into water to precipitate (Dropping method), or an emulsion is formed and then the volatile organic solvent is evaporated (emulsion evaporation method) The surface can be modified through the method.

The hydrophilic surface-modified cell stabilizer may have an average diameter of about 0.05 μm to about 5 μm when dispersed in the mixture.

In the primary foaming step, the concentration of the cell stabilizer with respect to the total weight of the mixture is greater than the concentration of the cell stabilizer with respect to the total weight of the mixture in the secondary foaming step. This is because, in the secondary foaming step, the second foaming agent and the like are included in addition to the previous primary foaming mixture, so the relative concentration (content) of the cell stabilizer becomes small.

Specifically, in the primary foaming step, the cell stabilizer may be included in an amount of 0.02% to 1.0% by weight based on the total weight of the mixture, and at this time, in the secondary foaming step, the concentration of the cell stabilizer is the primary foaming step is smaller. In the primary foaming step, the content of the cell stabilizer is preferably 0.02% by weight or more, 0.05% by weight or more, 0.1% by weight or more, 0.3% by weight or more, 1.0% by weight or less, 0.8% by weight or less, 0.5% by weight or less. % or less, and may be included in 0.05% by weight to 0.8% by weight or 0.3% by weight to 0.5% by weight. When included within the above content range, the generated bubbles can be stably formed, and in particular, the diameter of the generated bubbles can be controlled to 100 nm or less. Therefore, in the primary foaming step, fine cells having a diameter of 100 nm or less can be formed.

The first foaming agent and the foam stabilizer may be used as a first foaming agent composition in which they are dissolved or dispersed.

The first foaming agent composition may further include an additional solvent to dissolve or disperse the first foaming agent and the foam stabilizer. Examples of the solvent include water, ethanol, ethylene glycol, diethylene glycol, triethylene glycol, 1,4-butanediol, propylene glycol, ethylene glycol monobutyl ether, propylene glycol monomethyl ether, and propylene glycol monomethyl ether. Acetate, methyl ethyl ketone, acetone, methyl amyl ketone, cyclohexanone, cyclopentanone, diethylene glycol monomethyl ether, diethylene glycol ethyl ether, toluene, xylene, butyrolactone, carbitol, methyl cellosolve acetate , N,N-dimethylacetamide, or a mixture thereof may be used, and water may be preferably used.

The primary foaming step may be performed at 35° C. to 50° C., preferably at 35° C. or higher, 40° C. or higher, and 50° C. or lower, in which the first foaming agent is foamed to form the mixture. Microbubbles can be easily trapped inside.

In the primary foaming step, fine cells having a diameter of 100 nm or less are formed. The microbubbles may have a diameter of preferably 20 nm or more, 100 nm or less, or 20 to 100 nm. The microbubbles are stably captured in the mixture due to the high concentration of the cell stabilizer, and have a diameter of 2 μm to 180 μm in the superabsorbent polymer finally prepared through an additional foaming process, a crosslinking polymerization process, a surface crosslinking process, and a grinding process. It contributes to the formation of micropores.

(Second foaming step)

Next, the manufacturing method of the superabsorbent polymer according to one embodiment of the present invention includes the step of secondary foaming a mixture obtained by mixing the primary foaming mixture with a second foaming agent.

Here, the primary foaming mixture means a mixture that is primarily expanded and includes fine cells.

After the primary foaming, additional bubbles are generated through the secondary foaming step, and bubbles are generated by the foaming agent (the remaining first foaming agent and the added second foaming agent) in the mixture. The bubbles generated in the secondary foaming step can be stably captured in the mixture by the bubble stabilizer present in the primary foam mixture.

However, in the secondary foaming step, no additional cell stabilizer is added, and the concentration of the cell stabilizer in the mixture is relatively low in the secondary foaming step. Accordingly, the cells formed in the second foaming step have a larger average diameter than the cells formed in the first foaming step. In addition, a hierarchical porous structure can be implemented in the final superabsorbent polymer according to the size of the particle diameters of the cells formed in the first foaming step and the cells formed in the second foaming step.

The second foaming agent may be the same components as the first foaming agent described above.

In addition, the content of the second foaming agent is used on the same basis as the first foaming agent described above, and as will be described later, when a monomer is added in the secondary foaming step, it may be additionally added according to the amount of the monomer. Specifically, the content of the second blowing agent may be used in the range of 200 ppmw to 8,000 ppmw based on the water-soluble ethylenically unsaturated monomer in the mixture to be foamed, preferably 200 ppmw or more, 500 ppmw or more, 500 ppmw or more, or 8,000 ppmw or less, 6,000 ppmw or less, 4,000 ppmw or less, and may be used in 500 to 6,000 ppmw or 800 ppmw to 40,000 ppmw. When included within the above content range, generated bubbles may be stably formed.

The second blowing agent may be used as a second blowing agent composition comprising an additional solvent. Here, the same solvent as the solvent used in the first foaming agent composition may be used as the solvent.

On the other hand, in the secondary foaming step, the concentration of the cell stabilizer included in the mixture is smaller than that in the first foaming step, and the concentration is reduced by a maximum of 0.1 to 0.5 times that of the first foaming step (this means that the mixture to be foamed is 2 2-fold to 10-fold dilution).

Specifically, the concentration of the cell stabilizer is relatively reduced according to the content of the second foaming agent composition added, and the concentration of the cell stabilizer can be reduced by additionally adding the monomer composition in the secondary foaming step.

In the secondary foaming step, the additionally added monomer composition may include a water-soluble acrylic acid-based monomer having an acidic group and at least a portion of the acidic group neutralized, a polymerization initiator, and a first crosslinking agent. Components and contents of the water-soluble acrylic acid-based monomer, polymerization initiator, and first crosslinking agent are as described above.

The secondary foaming step may be performed at 35° C. to 50° C., and the foaming agent is foamed in the temperature range so that bubbles can be easily trapped in the mixture.

In the secondary foaming step, cells having a particle size 2 to 10 times larger than the cells formed in the primary foaming process may be formed.

Specifically, when fine cells having a diameter of 100 nm or less are formed in the first foaming step, bubbles having a diameter of 100 nm to 1,000 nm may be formed in the second foaming step. Preferably, the diameter of the bubble is 100 nm or more, 150 nm or more, 1,000 nm or less, 500 nm or less, 400 nm or less, 250 nm or less, 100 nm to 500 nm, 100 nm to 400 nm or 150 nm to 250 nm. nm. The bubbles contribute to forming pores having an average diameter of 180 μm or more in the superabsorbent polymer finally manufactured through a crosslinking polymerization process, a surface crosslinking process, a pulverization process, and the like.

Meanwhile, if necessary, the aforementioned foaming step may be additionally performed a plurality of times, and cells having different average diameters may be formed by adjusting the concentration of the cell stabilizer in the additional foaming step.

(crosslink polymerization step)

Next, the manufacturing method of the superabsorbent polymer according to one embodiment of the present invention includes forming a water-containing gel polymer by cross-linking the secondary foam mixture.

Here, the secondary foaming mixture includes hierarchical cells formed in the first and second foaming steps. Cells included in the secondary foaming mixture are stably captured in the foaming mixture by a cell stabilizer, and the cells form pores in the water-containing gel polymer formed in the crosslinking polymerization step. Specifically, a hierarchical porous pore structure having different average diameters is formed in the water-containing gel polymer.

The cross-linking polymerization step may be performed at 50° C. to 100° C., which is higher than the temperature of the aforementioned foaming step. When polymerization is performed within the above temperature range, an appropriate pore structure may be formed in the water-containing gel polymer by bubbles formed in the foaming step.

The cross-linking polymerization method is largely divided into thermal polymerization and photopolymerization according to the polymerization energy source. In case of general thermal polymerization, it may be carried out in a reactor having an agitation shaft such as a kneader, and in case of photopolymerization, a movable conveyor may be performed. Although it may be carried out in a reactor equipped with a belt, the above polymerization method is an example, and the present invention is not limited to the above polymerization method.

For example, as described above, the water-containing gel polymer obtained by thermal polymerization by supplying hot air to a reactor such as a kneader equipped with an agitation shaft or heating the reactor is directed to the outlet of the reactor according to the shape of the agitation shaft provided in the reactor. The discharged water-containing gel polymer may be in the form of several centimeters to several millimeters. Specifically, the size of the obtained water-containing gel polymer may vary depending on the concentration and injection speed of the monomer composition to be injected, and a water-containing gel polymer having a weight average diameter of 2 to 50 mm may be obtained.

In addition, as described above, when photopolymerization is performed in a reactor equipped with a movable conveyor belt, the form of a water-containing gel polymer that is usually obtained may be a sheet-like water-containing gel polymer having the width of a belt. At this time, the thickness of the polymer sheet varies depending on the concentration and injection rate of the monomer composition to be injected, but it is preferable to supply the monomer composition so that a polymer sheet having a thickness of about 0.5 to about 5 cm can be obtained. When the monomer composition is supplied to such an extent that the thickness of the polymer on the sheet is too thin, production efficiency is low, which is undesirable. When the thickness of the polymer on the sheet exceeds 5 cm, the polymerization reaction does not occur evenly over the entire thickness due to the excessively thick thickness. may not be

In this case, the water-containing gel polymer obtained in this way may have a normal moisture content of about 40 to about 80% by weight. On the other hand, "moisture content" throughout the present specification is the content of water occupied with respect to the total weight of the polymer, and means a value obtained by subtracting the weight of the polymer in a dry state from the weight of the polymer. Specifically, it is defined as a value calculated by measuring the weight loss due to moisture evaporation in the polymer during drying by raising the temperature of the polymer through infrared heating. At this time, the drying condition is a method of raising the temperature from room temperature to about 180 ° C and then maintaining it at 180 ° C. The total drying time is set to 20 minutes including 5 minutes of the temperature raising step, and the moisture content is measured.

(Drying, Grinding and Classifying Steps)

Next, according to one embodiment of the present invention, the step of forming a base resin powder by drying, pulverizing, and classifying the water-containing gel polymer is included.

Specifically, a step of drying the obtained water-containing gel polymer is performed. If necessary, a step of coarsely pulverizing the water-containing gel polymer may be further performed before drying to increase the efficiency of the drying step.

At this time, the grinder used is not limited in configuration, but specifically, a vertical pulverizer, a turbo cutter, a turbo grinder, a rotary cutter mill, a cutting Includes any one selected from the group of crushing devices consisting of a cutter mill, a disc mill, a shred crusher, a crusher, a chopper, and a disc cutter However, it is not limited to the above example.

At this time, in the coarsely pulverizing step, the water-containing gel polymer may be pulverized to have a particle diameter of 2 to 10 mm. Grinding to a particle diameter of less than 2 mm is not technically easy due to the high water content of the hydrogel polymer, and also may cause agglomeration of the pulverized particles. On the other hand, when the particle diameter is greater than 10 mm, the effect of increasing the efficiency of the subsequent drying step may be insignificant.

Drying is performed on the water-containing gel polymer immediately after polymerization that has been coarsely pulverized or has not undergone the coarse pulverization step. At this time, the drying step is performed at a temperature of 150 ° C. or more for 30 minutes or more. Preferably, it may be 150 °C to 250 °C or 170 °C to 200 °C. When the drying temperature is less than 150 ° C, the drying time is too long and there is a concern that the physical properties of the finally formed superabsorbent polymer may deteriorate, and when the drying temperature exceeds 250 ° C, only the polymer surface is excessively dried, resulting in a subsequent pulverization process There is a concern that fine powder may be generated in the water, and physical properties of the finally formed superabsorbent polymer may be deteriorated.

Meanwhile, the drying step is performed for 30 minutes or more. If it is carried out for less than 30 minutes, sufficient drying is not achieved. Preferably, it may proceed for 30 to 90 minutes, but is not limited thereto.

As long as the drying method of the drying step is also commonly used as a drying process for hydrogel polymers, the composition may be selected and used without limitation. Specifically, the drying step may be performed by a method such as hot air supply, infrared ray irradiation, microwave irradiation, or ultraviolet ray irradiation. The water content of the polymer after the drying step may be about 0.1 to about 10% by weight.

Next, a step of pulverizing the dried polymer obtained through such a drying step is performed.

The polymer powder obtained after the grinding step may have a particle size of 300 μm to 800 μm. The grinder used for grinding to such a particle size is specifically, a pin mill, a hammer mill, a screw mill, a roll mill, a disc mill, or a jog mill. A jog mill or the like may be used, but is not limited to the above example.

In addition, in order to manage the physical properties of the superabsorbent polymer powder produced as a final product after the pulverization step, a separate process of classifying the polymer powder obtained after pulverization according to the particle size may be performed. Preferably, polymers having a particle diameter of 300 μm to 800 μm may be classified, and only polymer powder having such a particle diameter may be commercialized through a surface crosslinking reaction step. More specifically, the classified base resin powder has a particle size of 300 μm to 800 μm, and may include 50% by weight or more of particles having a particle size of 300 to 600 μm.

(Surface Crosslinking Process)

Meanwhile, according to one embodiment of the present invention, after preparing the base resin powder through the above-described classification process, crosslinking at least a part of the surface of the base resin powder in the presence of surface crosslinking is included.

In the presence of the second crosslinking agent, the base resin powder is heat treated to thermally crosslink at least a portion of the surface of the base resin powder to form superabsorbent polymer particles. The surface cross-linking step is to induce a cross-linking reaction on the surface of the base resin powder in the presence of a second cross-linking agent, and a surface cross-linking layer may be formed on at least a portion of the surface of the base resin powder through such surface cross-linking. This is to increase the surface cross-linking density of the super-absorbent polymer. When the super-absorbent polymer further includes a surface cross-linking layer as described above, it has a structure in which the external cross-linking density is higher than the internal cross-linking density.

The surface crosslinking step may be performed at a temperature of about 180° C. or more for about 30 minutes or more.

As the second crosslinking agent, any second crosslinking agent previously used in the manufacture of superabsorbent polymers may be used without particular limitation. For example, the second crosslinking agent is ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, 1,2-hexanediol, 1,3-hexanediol, 2 - 1 selected from the group consisting of methyl-1,3-propanediol, 2,5-hexanediol, 2-methyl-1,3-pentanediol, 2-methyl-2,4-pentanediol, tripropylene glycol and glycerol more than one polyol; At least one carbonate-based compound selected from the group consisting of ethylene carbonate and propylene carbonate; epoxy compounds such as ethylene glycol diglycidyl ether; oxazoline compounds such as oxazolidinone; polyamine compounds; oxazoline compounds; mono-, di- or polyoxazolidinone compounds; or cyclic urea compounds; etc. may be included. Preferably, the same as the above-mentioned first crosslinking agent may be used, and for example, a diglycidyl ether-based compound of alkylene glycol such as ethylene glycol diglycidyl ether may be used.

The second crosslinking agent may be used in an amount of 0.001 to 2 parts by weight based on 100 parts by weight of the base resin powder. Preferably, it is 0.005 parts by weight or more, 0.01 parts by weight or more, or 0.02 parts by weight or more, and may be used in an amount of 0.5 parts by weight or less and 0.3 parts by weight or less. By adjusting the content range of the second crosslinking agent to the above range, a superabsorbent polymer exhibiting various physical properties such as excellent absorption performance and liquid permeability can be prepared.

Meanwhile, in order to further improve liquid permeability, etc., the superabsorbent polymer according to one embodiment may further use aluminum salts such as aluminum sulfate salts and other various polyvalent metal salts during surface crosslinking. Such a polyvalent metal salt may be included on the surface crosslinking layer of the finally prepared superabsorbent polymer.

Meanwhile, the superabsorbent polymer according to one embodiment of the present invention may have a particle diameter of 300 to 800 μm. More specifically, at least 95% by weight of the base resin powder and the super absorbent polymer including the same may have a particle size of 300 to 800 μm, and 50% by weight or more of particles having a particle size of 300 to 600 μm may be included.

(super absorbent polymer)

The super absorbent polymer prepared according to the method for manufacturing a super absorbent polymer according to an embodiment of the present invention includes a plurality of pores, and the average diameter of the plurality of pores through the multi-step foaming process of the present application described above is 20 μm to 150 μm. μm.

In addition, 70% or more of the pores having a diameter of 2 μm to 180 μm are included in the pores, and thus, the superabsorbent polymer has a hierarchical pore structure. The ratio of the micropores is preferably 70% or more, 73% or more, 78% or more, 81% or more, 84% or more, 99% or less, 95% or less, 92% or less, or 70 to 99%, 73 to 92%.

Even if a normal foaming process is performed, the formed microbubbles easily disappear through processes such as polymerization and pulverization. A large number of micropores having a diameter of 2 μm to 180 μm exist in the resin, and thus the absorption rate can be remarkably improved.

On the other hand, when the multi-step foaming process is not performed during the preparation of the superabsorbent polymer, or the cell stabilizer of the present application is not used even after the multi-step process is performed, it is difficult to introduce the hierarchical pore structure desired by the present invention.

At this time, the average diameter of the pores in the super absorbent polymer is determined by observing the internal structure of the super absorbent polymer particles to be measured with an optical microscope (OM; magnification: X50 or X500) and an electron microscope (SEM; magnification: X200 or X5000). You can check. More specifically, after measuring the longest diameter of each of the pores included in the superabsorbent polymer particle, the median of the measured longest diameter may be obtained as an average diameter. At this time, it is preferable to obtain an average diameter after measuring the diameter of 800 or more pores in one superabsorbent polymer sample.

In addition, the content of micropores with a diameter of 2 μm to 180 μm was measured by measuring the longest diameter of each of 800 or more pores in one superabsorbent polymer sample, and among them, micropores with a corresponding diameter of 2 μm to 180 μm It means the ratio of the number of pores.

Meanwhile, the superabsorbent polymer may include particles having a particle diameter of about 300 μm to about 800 μm in an amount of 90% by weight or more based on the total weight, and the particle diameter of the superabsorbent polymer particles is determined by European Disposables and Nonwovens Association, EDANA) standard EDANA WSP 220.3 method.

In addition, the superabsorbent polymer has a centrifugal retention capacity (CRC) of 28 g/g or more, preferably 28.8 g/g or more, or 29.5 g/g or more, as measured by the EDANA method WSP 241.3. g/g or less, 33 g/g or less, 31.7 or less, 28 to 35 g/g, or 28.8 to 31.7 g/g. The method for measuring the centrifugal water retention capacity (CRC) will be described in more detail in the experimental examples section to be described later.

In addition, the superabsorbent polymer has a vortex time of 40 seconds or less according to the vortex method. The absorption rate is excellent as the value is small, and the lower limit of the absorption rate is 0 seconds in theory, but, for example, 5 seconds or more, 10 seconds or more, 20 seconds or more, 40 seconds or less, 38 seconds or less, 37 seconds 35 seconds or less, 33 seconds or less, 32 seconds or less, 31 seconds or less, or 10 seconds to 40 seconds. The absorption rate according to the vortex method means the time (time, unit: seconds) at which the vortex of the liquid disappears due to rapid absorption when the superabsorbent polymer is added to physiological saline and stirred, and the time is short. It can be seen that the higher the absorbent polymer, the faster the initial absorption rate. Here, physiological saline means a 0.9% by weight sodium chloride (NaCl) aqueous solution. The method for measuring the absorption rate will be described in more detail in the Experimental Example section to be described later. On the other hand, the absorption rate according to the vortex method can be measured by the same method for the base resin powder before surface crosslinking.

The present invention is explained in more detail in the following examples. However, the following examples are only to illustrate the present invention, and the detailed description of the present invention is not limited by the following examples.

<Production Example>

Hereinafter, the first foaming agent composition including the foam stabilizer used in the Examples was prepared as follows.

Preparation Example 1-1: Preparation of the first foaming agent composition (A-1)

First, 500 g of water containing a PEO copolymer (Pluronic® F-127, Sigma-Aldrich) at a concentration of 0.25% w / v was put into a high shear mixer, and room temperature (24 ± 1 ° C) and atmospheric pressure (1 atmospheric pressure), while stirring at 20,000 rpm, dichloromethane containing 15% w/v of polylactic acid (Poly(L-lactide), average Mn 20,000, Sigma-Aldrich) was added at 20% v/v. After adding dichloromethane, additional stirring was performed at 20,000 rpm for 15 minutes to form an oil in water emulsion.

Thereafter, dichloromethane was evaporated while stirring slowly for 20 hours to form a PLA cell stabilizer surface-modified with a PEO copolymer. The bubble stabilizer was recovered by centrifugation and redispersed in water, and at this time, the average diameter of the bubble stabilizer in the aqueous dispersion was 0.3 μm.

9 g of the prepared foam stabilizer was added to 800 g of water (here, considering the water content in the aqueous dispersion containing the foam stabilizer, the total water content was 800 g.) Next, the first foaming agent was sodium bicarbonate. (SBC) 1.35g was added and stirred at room temperature (24±1° C.) to prepare a first foaming agent composition.

Preparation Example 1-2: Preparation of the first foaming agent composition (A-2)

In Preparation Example 1-1, PLA/lignin (added to the organic solvent at a weight ratio of 9:1) was used in the same amount instead of PLA when preparing the aqueous dispersion of the foam stabilizer. At this time, the foam stabilizer was used in the aqueous dispersion. The average diameter of was 0.3 μm.

In addition, the first foaming agent composition (A-2) was prepared in the same manner as in Preparation Example 1-1.

Preparation Example 1-3: Preparation of the first foaming agent composition (A-3)

In Preparation Example 1-1, PE/PP wax (LG chem) was used in the same amount instead of PLA, and at this time, the average diameter of the cell stabilizer in the aqueous dispersion was 0.2 μm .

In addition, the first foaming agent composition (A-3) was prepared in the same manner as in Preparation Example 1-1.

Preparation Example 1-4: Preparation of the first foaming agent composition (A-4)

In Preparation Example 1-1, cellulose ester (180955, Sigma-Aldrich) was used in the same amount instead of PLA, and at this time, the average diameter of the cell stabilizer in the aqueous dispersion was 0.6 μm.

In addition, a first foaming agent composition (A-4) was prepared in the same manner as in Preparation Example 1-1.

Preparation Example 1-5: Preparation of the first foaming agent composition (A-5)

In Preparation Example 1-1, Mg stearate (415057, Sigma-Aldrich) was used in the same amount instead of PLA, and at this time, the average diameter of the cell stabilizer in the aqueous dispersion was 2 μm.

In addition, a first foaming agent composition (A-5) was prepared in the same manner as in Preparation Example 1-1.

Preparation Example 1-6: Preparation of the first foaming agent composition (A-6)

In Preparation Example 1-1, Polyvinyl alcohol (360627, Mw 9,000-10,000, Sigma-Aldrich) was used in the same amount instead of the surface modifier PEO copolymer, and at this time, the average diameter of the bubble stabilizer in the aqueous dispersion was 0.3 μm.

In addition, a first foaming agent composition (A-6) was prepared in the same manner as in Preparation Example 1-1.

Preparation Example 1-7: Preparation of the first foaming agent composition (A-7)

In Preparation Example 1-1, a first foaming agent composition (A-7) was prepared in the same manner as in Preparation Example 1-1, except that the same amount of potassium bicarbonate (DUKSAN) was used instead of the foaming agent SBC.

<Examples and Comparative Examples>

Example 1

(Step 1) A mixture was prepared by slowly adding 5,000 g of acrylic acid, 1,750 g of NaOH, and 2,300 g of water dropwise to a glass container equipped with a stirrer and a thermometer. Here, a solution of 615 g of water, 4.5 g of IRGACURE 819 as a photoinitiator, 9 g of sodium persulfate as a thermal initiator, and 22.5 g of polyethylene glycol diacrylate (PEGDA, molecular weight 400 g/mol) as a first crosslinking agent was slowly added dropwise. A monomer composition was prepared. The degree of neutralization of acrylic acid in the obtained composition was 70 mol%.

(Step 2) The prepared monomer composition was introduced into the transfer pipe, and the first foaming agent composition of Preparation Example 1 was additionally introduced. Here, the content of SBC, the first foaming agent, was added so that the content of acrylic acid was 1,500 ppmw (the foam stabilizer of Preparation Example 1 was added by increasing the content by 5.5 times), and at this time, the foam stabilizer was 0.36 included in wt%.

The temperature of the mixture was raised to 50° C. to perform primary foaming. The flow rate was adjusted so that the Reynolds Number was 5,000 or more so that the monomer composition and the first blowing agent composition could be easily mixed inside the transfer pipe.

(Step 3) The prepared monomer composition is further added to the primary foaming mixture, and a second foaming agent composition in which SBC is dispersed in water is sequentially added so that the SBC is 1,500 ppmw based on the total monomers in the mixture. Secondary foaming was performed by raising the temperature of the foaming mixture to 50°C.

At this time, the content of the additional monomer composition was adjusted so that the concentration of the cell stabilizer relative to the total content of the mixture was 0.036 wt% (the cell stabilizer was adjusted to be diluted 10 times). However, since some of the cell stabilizer is absorbed and consumed at the interface after the first foaming process, the concentration of the cell stabilizer in the mixture is slightly reduced.

The flow rate was adjusted so that the Reynolds Number was 5,000 or more so that each component could be easily mixed inside the transfer pipe.

Meanwhile, FIG. 1 schematically shows a flow chart of the multi-step foaming process in Example 1.

(Step 4) The secondary foam mixture exiting the transfer pipe was supplied at a speed of 500 to 2000 mL/min on a conveyor belt rotating at a speed of 50 cm/min with a belt having a width of 10 cm and a length of 2 m. And, simultaneously with the supply of the secondary foaming mixture, UV light having an intensity of 10 mW/cm ² was irradiated to conduct a polymerization reaction for 60 seconds, thereby obtaining a sheet-type water-containing gel polymer having a water content of 55% by weight.

(Step 5) Next, the hydrogel polymer in the form of a sheet is cut into a size of about 5 cm × 5 cm, and then put into a meat chopper to pulverize the polymer to obtain hydrogel particle crumbs having a size of 1 mm to 10 mm got it Thereafter, the crumb was dried in an oven capable of transferring air volume upwards and downwards. Hot air of 180 ° C. or higher was flowed from bottom to top for 15 minutes and from top to bottom for 15 minutes to dry evenly, and after drying, the water content of the dried body was set to 2% or less. After drying, the base resin powder was prepared by pulverizing with a grinder and then classifying to select a size of 300 to 800 μm.

(Step 6) 6 parts by weight of the second crosslinking agent solution containing 3 parts by weight of ethylene carbonate was sprayed on 100 parts by weight of the prepared base resin powder, and stirred at room temperature to evenly distribute the surface crosslinking liquid on the base resin powder. Subsequently, the base resin powder mixed with the surface cross-linking liquid was put into a surface cross-linking reactor and a surface cross-linking reaction was performed.

In this surface crosslinking reactor, it was confirmed that the temperature of the base resin powder gradually increased from an initial temperature of around 80° C., and was operated to reach the maximum reaction temperature of 190° C. after 30 minutes. After reaching the maximum reaction temperature, the reaction was further conducted for 15 minutes, and a sample of the superabsorbent polymer was finally prepared. After the surface crosslinking process, the superabsorbent polymer of Example 1 having a particle diameter of 300 μm to 800 μm was prepared by classifying with a standard ASTM mesh sieve.

Examples 2 to 7

In Example 1, a superabsorbent polymer was prepared in the same manner as in Example 1, except that the first foaming agent compositions (A-2 to A-7) prepared in Preparation Examples 2 to 7 were used.

Comparative Example 1

In Example 1, the same cell stabilizer and foaming agent were used, but the first high concentration (cell stabilizer: 0.36wt%) foaming process was not performed, and the same conditions as the second low concentration (cell stabilizer: 0.036wt%) were performed. A superabsorbent polymer was prepared in the same manner as in Example 1, except that the foaming process was performed.

Comparative Example 2

In Example 4, the same cell stabilizer and foaming agent were used, but the first high concentration (cell stabilizer: 0.36wt%) foaming process was not performed, and the same conditions as the second low concentration (cell stabilizer: 0.036wt%) were performed. A superabsorbent polymer was prepared in the same manner as in Example 4, except that the foaming process was performed.

Comparative Example 3

In Example 1, when preparing the first foaming agent composition of Preparation Example 1, Polystyrene (LG chem, latex beads 0.3㎛) was used in the same content as the foam stabilizer, but the first high concentration (cell stabilizer: 0.36wt%) foaming A superabsorbent polymer was prepared in the same manner as in Example 1, except that the foaming process was performed under the same conditions as the secondary low concentration (bubble stabilizer: 0.036 wt%) without performing the process.

Comparative Example 4

In Example 1, a superabsorbent polymer was prepared in the same manner as in Example 1, except that the cell stabilizer was not used.

Comparative Example 5

A solution (solution A) of a mixture of 8.6 g of IRGACURE 819 initiator diluted at 0.5% by weight in acrylic acid (80 ppmw for monomers) and 12.3 g of polyethylene glycol diacrylate (PEGDA, Mw = 400) diluted at 20% by weight in acrylic acid. ) was prepared. 540 g of acrylic acid and the above solution A were injected into a 2L glass reactor surrounded by a jacket in which a heat medium pre-cooled to 25° C. was circulated. Then, 832 g (C solution) of 25% by weight caustic soda solution was slowly added dropwise to the glass reactor and mixed. After confirming that the temperature of the mixed solution rose to about 72° C. or higher due to the heat of neutralization, the mixed solution was waited for cooling. The degree of neutralization of acrylic acid in the mixed solution thus obtained was about 70 mol%. Separately, water was introduced into a micro bubbler (O2 Bubble, OB-750S) circulating at a flow rate of 500 kg/h per hour to prepare D solution in which bubbles were generated. Silica was added here and put into an ultrasonic device (O2 Bubble, OB-750S) to prepare F solution. And, when the temperature of the neutralized mixed solution is cooled to about 45 ° C., the previously prepared solution F was injected into the mixed solution and mixed. At this time, the amount of silica was 0.05 parts by weight based on 100 parts by weight of the mixed solution.

Subsequently, the mixed solution prepared above was poured into a Vat-shaped tray (15 cm Х 15 cm in width) installed in a square polymerization reactor equipped with a light irradiation device and preheated to 80 ° C inside. Thereafter, light was irradiated to the mixed solution. It was confirmed that a gel was formed from the surface after about 20 seconds from the time of light irradiation, and it was confirmed that the polymerization reaction occurred simultaneously with foaming after about 30 seconds from the time of light irradiation. Subsequently, the polymerization reaction proceeded for an additional 2 minutes, and the polymerized sheet was taken out and cut into a size of 3 cm Х 3 cm. Then, the cut sheet was prepared into crump using a meat chopper and through a chopping process. The average particle size of the prepared crump was 1.5 mm. Subsequently, the crump was dried in an oven capable of controlling air volume up and down. So that the water content of the dried powder is about 2% by weight or less, hot air at 180 ° C. flows from bottom to top for 15 minutes, and from top to bottom for another 15 minutes. Crump dried evenly. The dried powder was pulverized with a grinder and then classified to obtain a base resin having a size of 150 to 850 μm.

Thereafter, to 100 g of the prepared base resin, 4.5 g of water, 1 g of ethylene carbonate, 0.05 g of Aerosil 200 (Evonik), 0.25 g of 20% by weight aqueous silica (Snowtex, ST-O) solution After mixing the mixed crosslinking agent solution, a surface crosslinking reaction was performed at 190° C. for 30 minutes. Then, the obtained product was pulverized and a surface-crosslinked superabsorbent polymer having a particle size of 150 to 850 μm was obtained using a sieve. 0.1 g of Aerosil 200 was additionally mixed with the obtained superabsorbent polymer in a dry manner to prepare a superabsorbent polymer.

<Experimental example>

Experimental Example 1: Cell size measurement in multi-step foaming process

(1-1) In order to confirm the structure of the bubbles generated in the multi-step foaming process of the manufacturing process of Example 1, using an optical microscope (product name: Ti-U, manufacturer: Nikon), the foamed mixture in each foaming step was It was photographed at 500 magnification. Specifically, OM images of cells in the primary foaming mixture and the secondary foaming mixture are shown in FIG. 2 in the following manner.

Figure 2 (a) shows the bubbles generated after the first foaming process (foaming under high-concentration bubble stabilizer conditions), and FIG. to be. Referring to (a) of FIG. 2, it can be seen that in the first foaming process, a relatively high concentration of the cell stabilizer is included, so that fine bubbles of a small particle size are trapped in the foaming mixture. Referring to (b) of FIG. 2, It was confirmed that the second foaming process contained a relatively low concentration of the cell stabilizer compared to the first foaming process, and thus, bubbles having a relatively large particle size were trapped in the foamed mixture.

(1-2) The size of bubbles generated in the foaming process by varying the concentration of the cell stabilizer in the multi-step foaming process of the manufacturing process of Example 1 was measured through the OM image of (1-1) of Experimental Example 1, After curing, the pore size was measured through the SEM image of Experimental Example 2 described later, and the change is shown in the graph of FIG. 3.

Referring to FIG. 3 , it was confirmed that the size of the bubbles generated is significantly smaller as the concentration of the bubble stabilizer increases during the foaming process.

Experimental Example 2: Measurement of pore size of superabsorbent polymer

In order to confirm the pore structure of the super absorbent polymer prepared in the above Examples and Comparative Examples, an internal image of the super absorbent polymer particle was taken from 200 to 200 using a SEM (Scanning Electron Microscope, product name: JCM-6000, manufacturer: JEOL) device. It was photographed at 5,000 magnification. Specifically, the level of pores of the superabsorbent polymer by the following method?? Diameter was measured. Accordingly, the particle size distribution of pores in Example 1 and Comparative Example 1 is shown in FIG. 4 . In addition, cross-sectional images of Example 1, Comparative Example 1, and Comparative Example 4 are shown in FIGS. 5 and 6, respectively.

1) First, 2 g of sample was prepared by separating the examples and superabsorbent polymers into individual particles having a particle diameter of 200 μm to 500 μm by classifying the examples and superabsorbent polymers using a Retsch particle classifier at 1.0 amplitude for 1 minute without damaging the particles. .

2) After that, the prepared sample particles were randomly arranged and placed on the SEM stage.

3) Next, the sample particles randomly arranged in the SEM stage were fixed with carbon tape, and the pore size formed on the surface of the superabsorbent particles was measured at a magnification of 200 to 5,000. At this time, an average of 1,000 or more superabsorbent polymer particles were targeted, and the pore size of 800 or more particles in which pores were clearly visible was measured. Here, assuming that the air bubbles are formed in a spherical shape, the criterion for 'clearly visible pores' is determined to be most clearly visible when the air bubbles take the shape of a hemisphere on the surface of the superabsorbent polymer particle through a pulverization process.

4) Next, the diameter of each particle was measured by obtaining the finally measured particle pore size of 800 or more, and the ratio of micropores having a pore diameter of 2 μm to 180 μm was calculated and shown in Table 1.

5) In addition, the average diameter was statistically obtained as a median value for the measured particle diameter, and the results are shown in Table 1. At this time, the interquartile range was used to confirm the pore size distribution, which is to exclude measurement errors due to measurers and outliers.

5(a) schematically shows a cross-sectional image of the super absorbent polymer prepared by the multi-step foaming process of Example 1, and there are micropores inside the super absorbent polymer particles finally manufactured through polymerization, drying, pulverization, and the like. It can be seen that there are many of them, and thus the absorption rate can be significantly improved. FIG. 5(b) schematically shows a cross-sectional image of the super absorbent polymer prepared by a single foaming process of Comparative Example 1, and shows the relative inside of the super absorbent polymer particles finally prepared through polymerization, drying, pulverization, and the like. It can be seen that the average diameter of the pores is large, and accordingly, the effect of improving the absorption rate may be lowered compared to the embodiment. FIG. 5(c) schematically shows a cross-sectional image of the super absorbent polymer prepared in a single foaming process of Comparative Example 4, and shows pores in the finally prepared super absorbent polymer particles through polymerization, drying, pulverization, and the like. It was confirmed that the average diameter was the largest and the number of pores was the smallest. Accordingly, it can be seen that the effect of improving the absorption rate may be lowered compared to the embodiment.

Figure 6 (a) shows a cross-sectional SEM image of the super absorbent polymer manufactured by the multi-step foaming process of Example 1, and Figure 6 (b) is a cross-sectional view of the super absorbent polymer manufactured by the single foaming process of Comparative Example 1. As for the SEM image, FIG. 6(c) shows a cross-sectional SEM image of the superabsorbent polymer prepared in the single foaming process of Comparative Example 4. Referring to FIG. 6 , it can be seen that in the case of the embodiment, a plurality of micropores are formed in the superabsorbent polymer, and accordingly, the absorption rate can be remarkably improved.

Experimental Example 3: Measurement of physical properties of superabsorbent polymer

The physical properties of the superabsorbent polymers prepared in Examples and Comparative Examples were evaluated in the following manner, and are shown in Table 1 below.

Unless otherwise indicated, all of the following physical property evaluations were conducted at room temperature (24 ± 1 ° C), and physiological saline or saline means a 0.9 wt% sodium chloride (NaCl) aqueous solution.

(1) Centrifuge Retention Capacity (CRC)

The water retention capacity by water absorption capacity under no load of each resin was measured according to EDANA WSP 241.3.

Specifically, the superabsorbent polymer W ₀ (g) (about 0.2 g) was uniformly put into a bag made of nonwoven fabric, sealed, and immersed in physiological saline (0.9% by weight) at room temperature. After 30 minutes, water was drained from the bag for 3 minutes under the condition of 250 G using a centrifuge, and the mass W ₂ (g) of the bag was measured. Moreover, after carrying out the same operation without using resin, the mass W1 (g) _at that time was measured. Using each obtained mass, CRC (g/g) was calculated according to the following equation.

[Equation 1]

CRC (g/g) = {[W ₂ (g) - W ₁ (g)]/W ₀ (g)} - 1

(2) Absorption rate (Vortex time)

The absorption rate (vortex time) of the superabsorbent polymers of Examples and Comparative Examples was measured in the following manner.

① First, 50 mL of 0.9% saline was added to a 100 mL beaker with a flat bottom using a 100 mL Mass Cylinder.

② Next, after placing the beaker in the center of the magnetic stirrer, a circular magnetic bar (30 mm in diameter) was placed in the beaker.

③ Then, the stirrer was operated so that the magnetic bar was stirred at 600 rpm, and the lowest part of the vortex generated by the stirring was brought into contact with the top of the magnetic bar.

④ After confirming that the temperature of the brine in the beaker has reached 24.0℃, insert 2±0.01 g of the superabsorbent polymer sample and simultaneously operate the stop watch. It was measured as a unit, and this was taken as the absorption rate.

division blowing agent foam stabilizer foaming process groundbreaking
Average diameter (μm) Ratio of micropores ^* (%) CRC
(%) Vortex
(sec) hydrophobic particles surface modifier Example 1 SBC PLA PEO copolymer Step 1 (high concentration)
Stage 2 (low concentration) 92 92 30.6 31 Example 2 SBC PLA/lignin PEO copolymer Step 1 (high concentration)
Stage 2 (low concentration) 98 87 31.7 32 Example 3 SBC PE/PP wax PEO copolymer Step 1 (high concentration)
Stage 2 (low concentration) 115 78 29.5 38 Example 4 SBC cellulose ester PEO copolymer Step 1 (high concentration)
Stage 2 (low concentration) 102 85 28.8 33 Example 5 SBC Mg-stearate PEO copolymer Step 1 (high concentration)
Stage 2 (low concentration) 121 73 30.1 40 Example 6 SBC PLA PVA polymer Step 1 (high concentration)
Stage 2 (low concentration) 109 81 31.2 37 Example 7 PBC PLA PEO copolymer Step 1 (high concentration)
Stage 2 (low concentration) 86 84 29.8 37 Comparative Example 1 SBC PLA PEO copolymer Stage 2 (low concentration) 158 65 29.3 43 Comparative Example 2 SBC cellulose ester PEO copolymer Stage 2 (low concentration) 162 64 29.9 44 Comparative Example 3 SBC polystyrene PEO copolymer Stage 2 (low concentration) 215 50 28.9 49 Comparative Example 4 SBC - - Step 1
Step 2 227 42 31.5 49 Comparative Example 5 SBC Silica - Step 1 (Micro Bubble Generator)
Step 2
(ultrasonic device) 187 58 30.8 47 *Micropores: Pores with a diameter of 2㎛ to 180㎛

Referring to the contents of Table 1, the examples show that a hierarchical pore distribution is formed in the superabsorbent polymer prepared by performing a multi-step foaming process using a cell stabilizer, thereby exhibiting improved absorption properties and absorption rate. I was able to confirm.

In the case of Comparative Examples 1 to 3 using a cell stabilizer but not undergoing a multi-step foaming process, it was difficult to form a hierarchical pore distribution, and accordingly, it was confirmed that the absorption rate was increased compared to the examples.

In the case of Comparative Example 4, which performed a multi-step foaming process but did not use a cell stabilizer, it was difficult to form fine bubbles, and accordingly, it was confirmed that the absorption rate was significantly increased compared to Examples.

In addition, even in the case of Comparative Example 5 in which a multi-step foaming process was performed but a physical foaming process was performed, it was difficult to form fine bubbles, and accordingly, it was confirmed that the absorption rate was significantly increased compared to the examples.

Claims (12)

preparing a monomer composition comprising a water-soluble acrylic acid-based monomer having an acidic group and at least partially neutralizing the acidic group, a polymerization initiator, and a first crosslinking agent;
firstly foaming a mixture obtained by mixing the monomer composition with a first foaming agent and a foam stabilizer;
secondary foaming of a mixture obtained by mixing a second foaming agent with the primary foamed mixture;
cross-linking the secondary foam mixture to form a water-containing gel polymer;
drying, pulverizing, and classifying the water-containing gel polymer to form a base resin powder; and
In the presence of surface crosslinking, crosslinking at least a portion of the surface of the base resin powder,
A method for producing a superabsorbent polymer.
According to claim 1,
In the first foaming step, the concentration of the cell stabilizer based on the total weight of the mixture is greater than the concentration of the cell stabilizer based on the total weight of the mixture in the second foaming step.
A method for producing a superabsorbent polymer.
According to claim 1,
In the first foaming step, the cell stabilizer is included in an amount of 0.02 to 1.0% by weight based on the total weight of the mixture,
In the second foaming step, the concentration of the cell stabilizer is smaller than that of the first foaming step,
A method for producing a superabsorbent polymer.
According to claim 1,
The cell stabilizer is a hydrophilic surface-modified compound,
A method for producing a superabsorbent polymer.
According to claim 1,
The cell stabilizer is a hydrophobic compound, polyethylene glycol, polyvinyl alcohol, polyvinylpyrrolidine, polyvinyl acetate, polyvinylpyrrolidine polyacrylic acid, polyacrylamide, polydopamine, poly(4-styrenesulfonate) polyethylene A hydrophilic surface-modified compound with one or more polymers or copolymers thereof selected from the group consisting of glycol and polypropylene glycol,
A method for producing a superabsorbent polymer.
According to claim 1,
The first foaming agent and the second foaming agent are each independently foamed at 50 ° C. or less,
A method for producing a superabsorbent polymer.
According to claim 1,
The first blowing agent and the second blowing agent are each independently sodium carbonate, sodium bicarbonate, potassium bicarbonate, azodicarbonamide, p,p'-oxybisbenzenesulfonylhydrazide, dinitrosopentamethylenetetramine, p - At least one selected from the group consisting of toluenesulfonylhydrazide and benzenesulfonylhydrazide,
A method for producing a superabsorbent polymer.
According to claim 1,
In the second foaming step,
Further mixing a monomer composition comprising a water-soluble acrylic acid-based monomer having an acidic group and at least a portion of the acidic group neutralized, a polymerization initiator, and a first crosslinking agent,
A method for producing a superabsorbent polymer.
According to claim 1,
The primary foaming step and the secondary foaming step are each independently performed at 35 ° C to 50 ° C,
A method for producing a superabsorbent polymer.
According to claim 1,
In the first foaming step, fine cells having a diameter of 100 nm or less are formed,
In the secondary foaming step, bubbles with a diameter of 100 nm to 300 nm are formed,
A method for producing a superabsorbent polymer.
According to claim 1,
The cross-linking polymerization step is carried out at 50 ° C to 100 ° C,
A method for preparing a superabsorbent polymer.
According to claim 1,
The superabsorbent polymer includes a plurality of pores,
The average diameter of the plurality of pores is 20㎛ to 150㎛,
70% or more of the micropores having a diameter of 2 μm to 180 μm among the plurality of pores,
A method for producing a superabsorbent polymer.

Images