KR102665835B1 - Preparation method of super absorbent polymer - Google Patents

Abstract

The present invention relates to a method for producing a superabsorbent polymer that exhibits improved liquid permeability while maintaining excellent water absorption performance. The method for producing the superabsorbent polymer includes forming a monomer composition comprising a water-soluble ethylenically unsaturated monomer having at least a partially neutralized acidic group, an encapsulated foaming agent, an internal crosslinking agent, and a polymerization initiator; Cross-polymerizing the monomer composition to form a water-containing gel polymer; Forming a base resin in powder form by drying, pulverizing and classifying the hydrogel polymer; And in the presence of a surface cross-linking agent, further cross-linking the surface of the base resin to form a surface cross-linking layer, wherein the encapsulated foaming agent comprises a core containing a hydrocarbon and a shell surrounding the core and formed of a thermoplastic resin. It has a structure that includes an average diameter before expansion of 5 to 50 ㎛, a maximum expansion size in air of 20 to 190 ㎛, and the step of forming the surface cross-linking layer includes a surface cross-linking agent, propylene glycol, and a solvent containing water. and a surface cross-linking agent composition containing an aluminum sulfate salt.

Description

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

The present invention relates to a method for producing a superabsorbent polymer that exhibits improved liquid permeability while maintaining excellent water absorption performance.

Super Absorbent Polymer (SAP) is a synthetic polymer material that has the ability to absorb moisture 500 to 1,000 times its own weight. Each developer produces SAM (Super Absorbency Material) and AGM (Absorbent Gel). They are named with different names, such as Material). The above-mentioned superabsorbent resins have begun to be commercialized as sanitary products, and are currently being used in sanitary products such as children's paper diapers, as well as soil reservatives for horticulture, water-stop materials for civil engineering and construction, sheets for seedlings, freshness maintainers in the food distribution field, and It is widely used as a material for poultices, etc.

In most cases, these superabsorbent polymers are widely used in the field of sanitary products such as diapers and sanitary napkins. For these purposes, they need to exhibit high absorption capacity for moisture, etc., and the absorbed moisture must not escape even under external pressure. In addition, it is necessary to maintain its shape well and exhibit excellent permeability even when it absorbs water and expands (swells).

Recently, as the demand for thin diapers increases, the content of fiber materials such as pulp in the diaper tends to decrease, and the proportion of superabsorbent resin relatively increases. Therefore, there is a need for a superabsorbent polymer to provide the performance previously reserved for diaper fiber materials, and for this purpose, the superabsorbent polymer must have not only high absorption performance but also high absorption speed and liquid permeability. In particular, as the diaper becomes thinner, the superabsorbent polymer must effectively pass liquid such as urine, and the liquid must be evenly absorbed throughout the multilayer distributed superabsorbent polymer particles, so the superabsorbent polymer exhibits improved liquid permeability. The need is increasing.

In this way, in order for the superabsorbent polymer to exhibit high liquid permeability, it is basically necessary to maintain its shape even after the superabsorbent polymer particles absorb moisture and swell, thereby maintaining the pores between the particles. This is because the pores between particles act as flow paths to ensure excellent liquid permeability of the superabsorbent polymer. For this reason, in order to provide a superabsorbent polymer that exhibits improved liquid permeability and other excellent physical properties, it is necessary to manufacture such a superabsorbent polymer to exhibit higher gel strength.

Therefore, in the past, in order to improve the liquid permeability of the superabsorbent polymer, a method of increasing the gel strength was mainly applied by increasing the internal crosslinking density and/or the surface crosslinking density. However, when the liquid permeability of the superabsorbent polymer is improved by increasing the crosslink density, it becomes difficult for the superabsorbent polymer particles to absorb and retain moisture inside, which has the disadvantage of lowering the water absorption performance, such as water retention capacity, of the superabsorbent polymer. It existed.

Accordingly, there is a continued need for the development of a superabsorbent polymer that can be preferably applied to sanitary materials with a reduced content of fibers by improving liquid permeability while maintaining the excellent absorption performance of the superabsorbent polymer.

Accordingly, the present invention provides a method for producing a superabsorbent polymer that exhibits improved liquid permeability while maintaining excellent absorption performance.

Accordingly, the present invention includes the steps of forming a monomer composition comprising a water-soluble ethylenically unsaturated monomer having at least a partially neutralized acidic group, an encapsulated blowing agent, an internal crosslinking agent, and a polymerization initiator;

Cross-polymerizing the monomer composition to form a water-containing gel polymer;

Forming a base resin in powder form by drying, pulverizing and classifying the hydrogel polymer; and

In the presence of a surface cross-linking agent, further cross-linking the surface of the base resin to form a surface cross-linking layer,

The encapsulated foaming agent has a structure including a core containing hydrocarbon and a shell surrounding the core and formed of a thermoplastic resin, has an average diameter before expansion of 5 to 50 ㎛, and has a maximum expansion size in air of 20 to 190 ㎛. ㎛,

The step of forming the surface cross-linking layer provides a method for producing a superabsorbent polymer, which is performed in the presence of a surface cross-linking agent, a solvent containing propylene glycol and water, and a surface cross-linking agent composition containing aluminum sulfate.

According to the method for producing a superabsorbent polymer of the present invention, a solvent containing propylene glycol and aluminum sulfate are applied during surface crosslinking, thereby making it possible to secure an inter-particle flow path on the surface of the superabsorbent polymer particles and provide a hydrophobic environment. . Therefore, the liquid permeability of the superabsorbent polymer can be improved without a significant increase in internal crosslinking density.

Furthermore, in the above manufacturing method, the crosslinking density inside the superabsorbent polymer particles can be controlled to an appropriate level by using an encapsulated foaming agent during crosslinking polymerization, and as a result, the superabsorbent polymer can maintain excellent absorption performance.

Therefore, the superabsorbent polymer prepared according to the present invention maintains excellent absorption performance while exhibiting improved liquid permeability, and can be preferably applied to sanitary materials with reduced fiber content.

Since the present invention can make various changes and take various forms, specific embodiments will be illustrated and described in detail below. However, this is not intended to limit the present invention to a specific disclosed form, and should be understood to include all changes, equivalents, and substitutes included in the spirit and technical scope of the present invention.

Hereinafter, a method for manufacturing a superabsorbent polymer according to specific embodiments of the invention will be described in more detail.

Prior to this, the terminology used in this specification is only for referring to specific implementation examples and is not intended to limit the invention. And, as used herein, singular forms include plural forms unless phrases clearly indicate the contrary.

The term “polymer” or “polymer” used in the specification of the present invention refers to a state in which water-soluble ethylenically unsaturated monomers are polymerized, and may encompass all moisture content ranges or particle size ranges. Among the above polymers, a polymer having a moisture content (moisture content) of about 40% by weight or more in the state after polymerization but before drying may be referred to as a water-containing gel polymer.

In addition, “superabsorbent polymer” refers, depending on the context, to the polymer or base resin itself, or to the polymer or base resin subjected to additional processing, such as surface cross-linking, fine reassembly, drying, grinding, classification, etc. It is used to encompass everything that has been brought to a state suitable for commercialization.

According to one embodiment of the invention, forming a monomer composition comprising a water-soluble ethylenically unsaturated monomer having at least a partially neutralized acidic group, an encapsulated blowing agent, an internal crosslinking agent, and a polymerization initiator;

Cross-polymerizing the monomer composition to form a water-containing gel polymer;

Forming a base resin in powder form by drying, pulverizing and classifying the hydrogel polymer; and

In the presence of a surface cross-linking agent, further cross-linking the surface of the base resin to form a surface cross-linking layer,

The encapsulated foaming agent has a structure including a core containing hydrocarbon and a shell surrounding the core and formed of a thermoplastic resin, has an average diameter before expansion of 5 to 50 ㎛, and has a maximum expansion size in air of 20 to 190 ㎛. ㎛,

The step of forming the surface cross-linking layer is provided in the presence of a surface cross-linking agent composition containing a surface cross-linking agent, a solvent containing propylene glycol and water, and an aluminum sulfate salt.

In the manufacturing method of one embodiment, a solvent containing propylene glycol and aluminum sulfate are applied during surface crosslinking, and a predetermined encapsulated foaming agent is used during crosslinking polymerization of monomers. First, by using a solvent containing propylene glycol during surface crosslinking and using aluminum sulfate along with a surface crosslinking agent, a channel can be formed between the superabsorbent polymer particles, and also, hydrophobicity can be added to the surface of the superabsorbent particles. It can be granted.

Therefore, liquids such as urine can pass more effectively through the pores or channels between the superabsorbent polymer particles, and thus the liquid permeability of the superabsorbent polymer can be further improved even without increasing the crosslink density of the superabsorbent polymer.

In addition, as will be described later, the encapsulated foaming agent is a component that causes foaming during the cross-linking polymerization process and enables the formation of a porous structure with pores introduced into the superabsorbent polymer particles. In the method of one embodiment, due to the use of these ingredients, the cross-linking density inside the superabsorbent polymer can be controlled to a relatively low level, and furthermore, there is not a high need to further increase the internal cross-linking density to improve liquid permeability. Accordingly, the superabsorbent polymer manufactured by the method of one embodiment can maintain the internal crosslinking density at a low level, and thus can maintain absorbent performance, such as excellent water retention capacity.

As a result, the superabsorbent polymer manufactured by the method of one embodiment exhibits improved liquid permeability while maintaining excellent absorption performance, and can be preferably applied to sanitary materials with reduced fiber content.

Hereinafter, the method for manufacturing the superabsorbent polymer of one embodiment will be described in more detail for each step.

In the method for producing a superabsorbent polymer according to one embodiment, each component of a water-soluble ethylenically unsaturated monomer having at least a partially neutralized acidic group, an encapsulated foaming agent, an internal crosslinking agent, and a polymerization initiator is mixed to form a monomer composition.

The water-soluble ethylenically unsaturated monomer may be any monomer commonly used in the production of superabsorbent resin. As a non-limiting example, the water-soluble ethylenically unsaturated monomer may be a compound represented by the following formula (1):

[Formula 1]

R ₁ -COOM ¹

In Formula 1,

R ₁ is an alkyl group having 2 to 5 carbon atoms containing an unsaturated bond,

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

Suitably, the monomer may be 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 this way, when acrylic acid or its salt is used as a water-soluble ethylenically unsaturated monomer, it is advantageous to obtain a superabsorbent polymer with improved water absorption. In addition, the monomers include maleic anhydride, fumaric acid, crotonic acid, itaconic acid, 2-acryloylethane sulfonic acid, 2-methacryloyl ethane sulfonic acid, 2-(meth)acryloylpropane sulfonic acid, or 2-( Meth)acrylamide-2-methyl propane sulfonic acid anionic monomer and salts thereof; (meth)acrylamide, N-substituted (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, methoxypolyethylene glycol (meth)acrylate or polyethylene glycol ( A nonionic hydrophilic-containing monomer of meth)acrylate; and amino group-containing unsaturated monomers of (N,N)-dimethylaminoethyl (meth)acrylate or (N,N)-dimethylaminopropyl (meth)acrylamide and quaternaries thereof; at least one selected from the group consisting of You can use it.

Here, the water-soluble ethylenically unsaturated monomer has an acidic group, and at least a portion of the acidic group may be neutralized. Preferably, the monomer partially neutralized with an alkaline substance such as sodium hydroxide, potassium hydroxide, or ammonium hydroxide can be used.

At this time, the degree of neutralization of the monomer may be 40 to 95 mol%, or 40 to 80 mol%, or 45 to 75 mol%. The range of the degree of neutralization may vary depending on the final physical properties, but if the degree of neutralization is too high, the neutralized monomer may precipitate, making it difficult for polymerization to proceed smoothly. Conversely, if the degree of neutralization is too low, the water absorption of the polymer will be greatly reduced. It can exhibit properties similar to elastic rubber that are difficult to handle.

In addition, the encapsulated foaming agent refers to a thermally expandable microcapsule foaming agent having a core-shell structure, which includes a core containing hydrocarbon and a shell made of a thermoplastic resin formed on the core as described above. has Specifically, the hydrocarbon constituting the core is a liquid hydrocarbon with a low boiling point and is easily vaporized by heat. Therefore, when heat is applied to the encapsulated foaming agent, the thermoplastic resin forming the shell softens and at the same time the liquid hydrocarbon in the core vaporizes, causing the capsule to expand as the pressure inside increases, resulting in an increased size compared to the existing size. bubbles are formed.

Therefore, the encapsulated foaming agent generates hydrocarbon gas, and includes an organic foaming agent that generates nitrogen gas through an exothermic decomposition reaction between monomers participating in the production of polymers, and an inorganic foaming agent that absorbs heat generated during polymer production to foam carbon dioxide gas. It is distinguished from foaming agents.

This encapsulated foaming agent may have different expansion characteristics depending on the components that make up the core and shell, as well as the weight and diameter of each component, and can be expanded to a desired size by adjusting this, thereby increasing the porosity and/or Internal crosslinking density can be controlled.

Specifically, the encapsulated foaming agent has a particle form with an average diameter of 5 to 50 ㎛ before expansion. For example, the average diameter of the encapsulated blowing agent before expansion may be 5 to 30 μm, or 5 to 20 μm, or 7 to 17 μm, or 10 to 16 μm, and the capsule thickness of the encapsulated blowing agent may be 2 to 16 μm. It may be 15 μm. It is difficult to manufacture the encapsulated foaming agent to have an average diameter of less than 5 μm, and if the average diameter of the encapsulated foaming agent exceeds 20 μm, it may be difficult to efficiently increase the surface area because the pore size is too large. there is. Therefore, when the encapsulated foaming agent has the above average diameter, an appropriate degree of pore structure can be introduced into the resin, and the crosslinking density within the superabsorbent polymer is controlled to an appropriate level, thereby increasing the absorption rate and absorption performance of the superabsorbent polymer. can be optimized. The average diameter can be measured by measuring the diameter of each encapsulated foaming agent particle as the average Feret diameter and then calculating the average value.

Additionally, the encapsulated foaming agent has a maximum expansion size in air of 20 to 190 μm. For example, the encapsulated blowing agent may have a maximum expansion size in air of 50 to 190 μm, or 70 to 190 μm, or 75 to 190 μm. It is difficult in manufacturing to ensure that the maximum expansion size of the encapsulated foaming agent in the air is less than 20 ㎛, and if the maximum expansion size in the air exceeds 190 ㎛, the absorption rate and/or absorption performance of the superabsorbent polymer may decrease. Rather, it may deteriorate.

And, the encapsulated foaming agent may have a maximum expansion ratio in air of 3 to 15 times, or 5 to 15 times, or 8.5 to 10 times. It can be determined that the encapsulated foaming agent having the maximum expansion ratio in the above-mentioned range is suitable for forming an appropriate pore structure in the superabsorbent polymer and achieving an appropriate level of crosslinking density.

The reason for measuring the maximum expansion size and maximum expansion ratio of the encapsulated foaming agent in air is to determine whether pores of a desired size are formed in the superabsorbent polymer manufactured using the encapsulated foaming agent. Specifically, the form in which the foaming agent is foamed may vary depending on the manufacturing conditions of the superabsorbent polymer, so it is difficult to define it as one form. Therefore, by first foaming the encapsulated foaming agent in the air and checking the expansion size and expansion ratio, it can be confirmed whether it is suitable for forming the desired pores.

At this time, the maximum expansion size of the encapsulated foaming agent in the air is determined by applying a certain amount of the encapsulated foaming agent on a glass Petri dish, expanding the encapsulated foaming agent by applying heat at 150° C. in the air for 10 minutes, and then examining the encapsulated foaming agent under an optical microscope. By observation, the diameter of the top 10% by weight in the order of most expanded particles is obtained as the average value of the measured values. In addition, the maximum expansion rate of the encapsulated foaming agent in air is the upper part of the highly expanded particle after expanding the encapsulated foaming agent in the same manner as above and then applying heat for the average diameter (D0) measured before applying heat. It is obtained as the ratio (DM/D0) of the average diameter (DM) of 10% by weight. The maximum expansion rate and maximum expansion diameter of the encapsulated foaming agent in air will be described in more detail in the manufacturing example described later.

In addition, the hydrocarbons constituting the core of the encapsulated blowing agent include n-propane, n-butane, iso-butane, cyclobutane, n-pentane, iso-pentane, cyclopentane, n-hexane, iso-hexane, cyclohexane, It may be one or more selected from the group consisting of n-heptane, iso-heptane, cycloheptane, n-octane, iso-octane, and cyclooctane. Among these, hydrocarbons having 3 to 5 carbon atoms (n-propane, n-butane, iso-butane, cyclobutane, n-pentane, iso-pentane, cyclopentane) are suitable for forming pores of the above-mentioned sizes, and iso -Bhutan may be the best fit.

In addition, the thermoplastic resin constituting the shell of the encapsulated foaming agent is formed from one or more monomers selected from the group consisting of (meth)acrylate, (meth)acrylonitrile, aromatic vinyl, vinyl acetate, vinyl halide, and vinylidene halide. It may be a polymer that Among these, copolymers of (meth)acrylate and (meth)acrylonitrile may be most suitable for forming pores of the above-mentioned sizes.

In addition, the foaming start temperature (T _start ) of the encapsulated foaming agent may be 60°C to 120°C, or 65°C to 120°C, or 70°C to 80°C, and the maximum foaming temperature (T _max ) may be 100°C to 150°C. °C, or 105°C to 135°C, or 110°C to 120°C. In the case of the above-mentioned range, foaming can easily occur in the subsequent cross-linking polymerization process or drying process to properly introduce a pore structure into the polymer, and effective control of the cross-linking density in the cross-linking polymerization process is possible. This foaming start temperature and maximum foaming temperature can be measured using a thermomechanical analyzer.

In addition, the encapsulated foaming agent may include 10 to 30% by weight of a core made of hydrocarbon based on the total weight of the encapsulated foaming agent. Within this range, it may be most suitable for forming the pore structure of the superabsorbent polymer.

The encapsulated foaming agent can be manufactured and used, or a commercially available foaming agent that satisfies the above-mentioned conditions can be used.

Additionally, the encapsulated foaming agent may be used in an amount of 0.01 to 1.0 parts by weight based on 100 parts by weight of the water-soluble ethylenically unsaturated monomer. For example, the encapsulated foaming agent is present in an amount of 0.01 part by weight or more, 0.05 part by weight, or 0.1 part by weight and 1.0 part by weight or less, or 0.8 part by weight or less, or 0.5 part by weight, based on 100 parts by weight of the water-soluble ethylenically unsaturated monomer. It can be used below. If the content of the encapsulated foaming agent is too small, the pore structure in the resin may not be formed properly, and if it is included in too much, the porosity of the resin may be too high and the strength of the superabsorbent polymer may be weakened. From this point of view, the content A range may be desirable.

In addition, the term 'internal crosslinking agent' used in this specification is a term used to distinguish it from a surface crosslinking agent for crosslinking the surface of the base resin described later, and serves to polymerize the water-soluble ethylenically unsaturated monomers by crosslinking the unsaturated bonds described above. Do it. The crosslinking in the above step is carried out without distinction between surface or interior, but by the surface crosslinking process of the base resin described later, the surface of the final manufactured superabsorbent polymer has a structure crosslinked by a surface crosslinking agent, and the interior is composed of the internal crosslinking agent. It consists of a cross-linked structure.

As the internal cross-linking agent, any compound can be used as long as it enables the introduction of cross-linking bonds during polymerization of the water-soluble ethylenically unsaturated monomer. As a non-limiting example, the internal crosslinking agent is N,N'-methylenebisacrylamide, trimethylolpropane tri(meth)acrylate, ethylene glycol di(meth)acrylate, polyethylene glycol (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 ) Acrylate, triethylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, dipentaerythritol pentaacrylate, glycerin tri(meth)acrylate, penta Diglycidyl ether compounds of alkylene glycols such as erythristol tetraacrylate, triarylamine, ethylene glycol diglycidyl ether, polyfunctional crosslinking agents such as propylene glycol, glycerin, or ethylene carbonate are used alone or in combination of two or more. It can be used in combination, but is not limited thereto. Preferably, among these, alkylene glycol diglycidyl ether compounds such as ethylene glycol diglycidyl ether can be appropriately used.

This internal cross-linking 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 internal cross-linking agent is 0.01 parts by weight or more, 0.03 parts by weight, or 0.05 parts by weight or less, and 5 parts by weight or less, 3 parts by weight or less, 2 parts by weight or less, or It can be used in an amount of 1 part by weight or less. If the content of the upper internal cross-linking agent is too low, cross-linking may not occur sufficiently, making it difficult to achieve an appropriate level of strength. If the content of the upper internal cross-linking agent is too high, the internal cross-linking density increases, making it difficult to achieve the desired water retention performance.

In addition, the polymerization initiator may be appropriately selected depending on the polymerization method. When using a thermal polymerization method, a thermal polymerization initiator is used, when using a photopolymerization method, a photopolymerization initiator is used, and when using a photopolymerization method, a photopolymerization initiator is used, and a hybrid polymerization method (thermal and light When using a method using both, both a thermal polymerization initiator and a photo polymerization initiator can be used. However, even with the photopolymerization method, a certain amount of heat is generated by light irradiation such as ultraviolet ray irradiation, and a certain amount of heat is also generated as the polymerization reaction, which is an exothermic reaction, progresses, so a thermal polymerization initiator may be additionally used.

The photopolymerization initiator can be used without limitation in composition as long as it is a compound that can form radicals by light such as ultraviolet rays.

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) can be used. Meanwhile, specific examples of acylphosphine include diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide, phenylbis (2,4,6-trimethylbenzoyl) phosphine oxide, and ethyl (2,4,6- Trimethylbenzoyl)phenylphosphineate, etc. can be mentioned. More diverse photoinitiators are well described in Reinhold Schwalm's book "UV Coatings: Basics, Recent Developments and New Application (Elsevier 2007)" p115, and are not limited to the above-mentioned examples.

The photopolymerization initiator may be included at a concentration of 0.0001 to 2.0% by weight based on the monomer composition. If the concentration of the photopolymerization initiator is too low, the polymerization rate may be slow, and if the concentration of the photopolymerization initiator is too high, the molecular weight of the superabsorbent polymer may be small and the physical properties may become non-uniform.

Additionally, the thermal polymerization initiator may be one or more selected from the group of initiators consisting of persulfate-based initiator, azo-based initiator, hydrogen peroxide, and ascorbic acid. Specifically, examples of persulfate-based initiators include sodium persulfate (Na ₂ S ₂ O ₈ ), potassium persulfate (K ₂ S ₂ O ₈ ), and ammonium persulfate (NH ₄ ). ₂ S ₂ O ₈ ), etc., and examples of azo-based initiators include 2,2-azobis(2-amidinopropane) dihydrochloride, 2 ,2-azobis-(N,N-dimethylene)isobutyramidine dihydrochloride (2,2-azobis-(N,N-dimethylene)isobutyramidine dihydrochloride), 2-(carbamoylazo)isobutyronitrile (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), etc. A variety of thermal polymerization initiators are well described in Odian's book, 'Principle of Polymerization (Wiley, 1981)', p203, and are not limited to the above-mentioned examples.

The thermal polymerization initiator may be included at a concentration of 0.001 to 2.0% by weight based on the monomer composition. If the concentration of the thermal polymerization initiator is too low, no additional thermal polymerization occurs, so the effect of adding the thermal polymerization initiator may be minimal. If the concentration of the thermal polymerization initiator is too high, the molecular weight of the superabsorbent polymer may be small and the physical properties may become uneven. there is.

The monomer composition may further include additives such as thickeners, plasticizers, storage stabilizers, antioxidants, or surfactants, if necessary.

In addition, the water-soluble ethylenically unsaturated monomer described above may be mixed with a solvent and a basic material including potassium hydroxide, an encapsulated foaming agent, an internal crosslinking agent, and a polymerization initiator. Therefore, the monomer composition prepared in the above step is dissolved in the solvent, and the solid content in the monomer composition may be 20 to 60% by weight.

The solvent that can be used at this time can be used without limitation 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. One or more selected from ether, toluene, xylene, butyrolactone, carbitol, methylcellosolveacetate, and N,N-dimethylacetamide can be used in combination.

Meanwhile, mixing of each of the above-mentioned components is not particularly limited, and may be performed using a method commonly used in the art, for example, stirring.

Next, a step of cross-polymerizing the monomer composition is performed to form a water-containing gel polymer.

The above step can be carried out without particular limitations in composition as long as the prepared monomer composition can be cross-polymerized by thermal polymerization, photo polymerization, or co-polymerization to form a water-containing gel polymer.

Specifically, when thermal polymerization is performed, it may be performed in a reactor with a stirring shaft such as a kneader. Additionally, when thermal polymerization is performed, it may be carried out at a temperature of about 80°C or higher and less than about 110°C. The means for achieving the polymerization temperature in the above-mentioned range is not particularly limited, and heating can be performed by supplying a heat medium to the reactor or directly supplying a heat source. Types of heat medium that can be used include, but are not limited to, steam, hot air, and heated fluids such as hot oil. Additionally, the temperature of the supplied heat medium is determined by taking into account the means of the heat medium, the temperature increase rate, and the temperature increase target temperature. You can choose appropriately. Meanwhile, directly supplied heat sources include heating through electricity and heating through gas, but are not limited to the above-mentioned examples.

On the other hand, when photopolymerization is carried out, it may be carried out in a reactor equipped with a movable conveyor belt, but the above-described polymerization method is an example, and the present invention is not limited to the above-described polymerization method.

For example, when thermal polymerization is performed by supplying a heat medium or heating the reactor to a reactor such as a kneader equipped with a stirring shaft as described above, a water-containing gel polymer discharged from the reactor outlet can be obtained. The water-containing gel polymer obtained in this way can be obtained in a size of several centimeters to several millimeters, depending on the type of stirring shaft provided in the reactor. Specifically, the size of the obtained hydrogel polymer may vary depending on the concentration and injection speed of the injected monomer composition.

In addition, when photopolymerization is carried out in a reactor equipped with a movable conveyor belt as described above, the form of the hydrogel polymer usually obtained may be a sheet-like hydrogel polymer with the width of a belt. At this time, the thickness of the polymer sheet varies depending on the concentration and injection speed of the injected monomer composition, but it is generally preferable to supply the monomer composition so that a sheet-like polymer with a thickness of about 0.5 to about 10 cm can be obtained. If the monomer composition is supplied so that the thickness of the polymer on the sheet is too thin, it is undesirable due to low production efficiency, and if the thickness of the polymer on the sheet exceeds 10 cm, the polymerization reaction is not carried out evenly over the entire thickness due to the excessively thick thickness. It may not happen.

The polymerization time of the monomer composition is not particularly limited and may be adjusted to about 30 seconds to 60 minutes.

The normal moisture content of the hydrogel polymer obtained by this method may be about 30 to about 80% by weight. Meanwhile, throughout this specification, “moisture content” refers to the content of moisture relative to the total weight of the water-containing gel polymer, which is calculated by subtracting the weight of the polymer in a dry state from the weight of the water-containing gel polymer. Specifically, it is defined as a value calculated by measuring the weight loss due to moisture evaporation from the polymer during the drying process by raising the temperature of the polymer through infrared heating. At this time, the drying conditions were to increase the temperature from room temperature to about 180°C and then maintain it at 180°C. The total drying time was set to 40 minutes, including 5 minutes for the temperature increase step, and the moisture content was measured.

Next, the hydrogel polymer is dried, pulverized, and classified to form a base resin in powder form.

As the encapsulated foaming agent is foamed by the heat applied for drying in the above step, the interior of the manufactured base resin may have a structure in which multiple pores are formed and may have a controlled crosslink density. Accordingly, it is possible to manufacture a superabsorbent polymer with improved absorption performance and/or absorption rate compared to the case where the encapsulated foaming agent is not used.

Meanwhile, the step of forming the base resin may include a process of coarsely pulverizing the hydrogel polymer before drying it to increase drying efficiency.

At this time, the pulverizer used is not limited in composition, specifically, vertical pulverizer, turbo cutter, turbo grinder, rotary cutter mill, cutting. Including any one selected from the group of shredding machines consisting of a cutter mill, a disc mill, a shred crusher, a crusher, a chopper, and a disc cutter. It can be done, but it is not limited to the above-described example.

Through this coarse grinding process, the particle size of the hydrogel polymer can be adjusted to about 0.1 to about 10 mm. Grinding the particles to a particle size of less than 0.1 mm is not technically easy due to the high moisture content of the hydrogel polymer, and agglomeration may occur between the crushed particles. On the other hand, when pulverizing the particle size to exceed 10 mm, the effect of increasing the efficiency of the subsequent drying step may be minimal.

Drying is performed on the water-containing gel polymer that undergoes a coarse grinding process as described above or immediately after polymerization without undergoing a coarse grinding process. At this time, the drying temperature may be about 60°C to about 250°C. At this time, if the drying temperature is less than about 70°C, the drying time becomes too long and the thermoplastic resin shell of the encapsulated foaming agent is difficult to soften, so foaming may not occur, and if the drying temperature exceeds about 250°C, too much polymer Since only the surface is dried, fine powder may be generated in the subsequent grinding process, and there is a risk that the physical properties of the finally formed superabsorbent polymer may be deteriorated. Therefore, the drying may preferably be carried out at a temperature of about 100°C to about 240°C, more preferably at a temperature of about 110°C to about 220°C.

Additionally, the drying time may be from about 20 minutes to about 12 hours considering process efficiency, etc. For example, it may be dried for about 10 minutes to about 100 minutes, or about 20 minutes to about 60 minutes.

The drying method of the drying step may also be selected and used without limitation in composition as long as it is commonly used in the drying process of the water-containing gel polymer. Specifically, the drying step can be performed by methods such as hot air supply, infrared irradiation, microwave irradiation, or ultraviolet irradiation. The moisture content of the polymer after this drying step may be about 0.1 to about 10% by weight.

Afterwards, the dried polymer obtained through the drying step is pulverized using a grinder.

Specifically, the grinder used to grind the base resin in powder form into particles having a particle size of about 150 ㎛ to about 850 ㎛ is specifically a pin mill, hammer mill, and screw. It may be a screw mill, roll mill, disc mill, jog mill, etc., but is not limited to the above-mentioned examples.

In addition, in order to manage the physical properties of the superabsorbent polymer powder that is finalized after the grinding step, a separate process may be performed to classify the polymer powder obtained after grinding according to particle size. Preferably, polymers with a particle diameter of 150 to 850 ㎛ are classified, and only polymer powders with this particle diameter can be commercialized through a surface cross-linking reaction step. More specifically, the classified base resin powder may have a particle diameter of 150 to 850 ㎛ and may contain 50% by weight or more of particles with a particle diameter of 300 to 600 ㎛.

Meanwhile, after forming the base resin through the above-described classification process, a step of forming a surface cross-linking layer is performed by further cross-linking the surface of the base resin in the presence of a surface cross-linking agent.

The above step is a step of forming a surface cross-linking layer using a surface cross-linking agent to increase the surface cross-linking density of the base resin, so that the unsaturated bonds of the water-soluble ethylenically unsaturated monomer remaining on the surface without being cross-linked are cross-linked by the surface cross-linking agent. As a result, a superabsorbent polymer with increased surface crosslinking density is formed. This heat treatment process increases the surface cross-link density, that is, the external cross-link density, while the internal cross-link density does not change, so the superabsorbent polymer with the manufactured surface cross-link layer has a structure with a higher cross-link density on the outside than on the inside.

In particular, in the method of one embodiment, in the surface cross-linking layer forming step, a surface cross-linking agent composition containing a surface cross-linking agent, a solvent containing propylene glycol and water, and an aluminum sulfate salt is used. In this way, by using propylene glycol as a solvent and aluminum sulfate salt during surface crosslinking, the liquid permeability of the superabsorbent polymer can be further improved by forming a flow path between the superabsorbent polymer particles.

Meanwhile, as the surface cross-linking agent included in the surface cross-linking agent composition, any surface cross-linking agent that has been conventionally used in the production of superabsorbent polymers can be used without any restrictions. For example, the surface cross-linking agent is ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, 1,2-hexanediol, 1,3-hexanediol, 2- One 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 or more polyols; 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; It may include etc. Preferably, the same internal crosslinking agent as described above can be used, and for example, alkylene glycol diglycidyl ether-based compounds such as ethylene glycol diglycidyl ether can be used.

This surface cross-linking 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. For example, the surface cross-linking agent may be used in an amount of 0.005 parts by weight or more, 0.01 parts by weight or more, or 0.02 parts by weight or more, and 0.5 parts by weight or less, or 0.3 parts by weight or less, based on 100 parts by weight of the base resin. By adjusting the content range of the surface cross-linking agent to the above-mentioned range, a superabsorbent polymer that exhibits various physical properties such as excellent absorption performance and liquid permeability can be manufactured.

Additionally, aluminum sulfate salt may be included in the surface crosslinking agent composition in an amount of 0.02 to 1.5 parts by weight based on 100 parts by weight of the base resin. For example, the aluminum sulfate salt is present in an amount of 0.1 part by weight, 0.2 part by weight, 0.3 part by weight, or 0.5 part by weight, and 1.3 part by weight or less, or 1 part by weight or less, based on 100 parts by weight of the base resin. It can be used as As a result, the liquid permeability of the superabsorbent polymer can be further improved by effectively securing a flow path between the superabsorbent polymer particles, and at the same time, the degradation of other physical properties can be reduced.

Additionally, the surface crosslinking agent composition may include a solvent including water and propylene glycol. This solvent can allow the surface cross-linking agent to be evenly dispersed in the base resin, and furthermore, propylene glycol can further improve the liquid permeability of the superabsorbent polymer by hydrophobizing the surface of the superabsorbent polymer particles. For this purpose, the solvent is 0.05 to 3 parts by weight, or 0.1 to 3 parts by weight, or 0.3 to 2 parts by weight of propylene glycol, and 1 to 10 parts by weight, or 2 to 8 parts by weight of water, based on 100 parts by weight of the base resin. May include wealth. As a result, even dispersion of the surface cross-linking agent can be induced, agglomeration of the base resin can be prevented, and the liquid permeability of the superabsorbent polymer can be further improved.

Meanwhile, the above-described surface cross-linking agent composition further includes one or more inorganic particles selected from the group consisting of silica, clay, alumina, silica-alumina composite, titania, zinc oxide, and aluminum sulfate, and the like to perform a surface cross-linking reaction. can be performed. These inorganic particles can be used as alumina powder, silica-alumina powder, titania powder, hydrophilic silica particles, or nano-silica solution. Additionally, the inorganic particles may be used in an amount of 0.01 to 0.5 parts by weight based on 100 parts by weight of the base resin.

Additionally, there is no limitation on the method of mixing the surface crosslinking agent composition with the base resin. For example, mixing the surface cross-linking agent composition and the base resin in a reaction tank, spraying the surface cross-linking agent composition on the base resin, continuously supplying the base resin and the surface cross-linking agent composition to a continuously operating mixer, etc. can be used.

The surface crosslinking process may be performed at a temperature of about 80°C to about 250°C. More specifically, the surface crosslinking process may be performed at a temperature of about 100°C to about 220°C, or about 120°C to about 200°C, for about 20 minutes to about 2 hours, or about 40 minutes to about 80 minutes. . When the above-mentioned surface cross-linking process conditions are met, the surface of the base resin is sufficiently cross-linked to increase pressure absorbency or liquid permeability.

The temperature raising means for the surface crosslinking reaction is not particularly limited. Heating can be done by supplying a heat medium or directly supplying a heat source. At this time, the type of heat medium that can be used may be steam, hot air, or a heated fluid such as hot oil, but is not limited to this, and the temperature of the supplied heat medium depends on the means of the heat medium, the temperature increase rate, and the temperature increase target temperature. You can choose appropriately by taking this into consideration. Meanwhile, directly supplied heat sources include heating through electricity and heating through gas, but are not limited to the above-mentioned examples.

Meanwhile, the superabsorbent polymer manufactured according to the above-described embodiment can exhibit improved liquid permeability while maintaining excellent absorption performance.

The excellent absorption performance of these superabsorbent polymers can be confirmed by their centrifugal retention capacity (CRC).

Specifically, the base resin powder prepared by the above-described method has a centrifugation retention capacity (CRC) for 30 minutes in physiological saline (0.9 wt% sodium chloride aqueous solution) of 30 to 45 g/g, or 30 to 43 g/g, or It can be 35 to 41 g/g. The water retention capacity can be measured according to the European Disposables and Nonwovens Association (EDANA) standard EDANA WSP 241.3, and can be calculated according to formula A described in the test example of this specification.

Additionally, the improved liquid permeability of the superabsorbent polymer can be defined by permeability.

Specifically, the superabsorbent polymer may have a permeability defined by the following equation 1 of 300 seconds or less, or 250 seconds or less, or 10 to 250 seconds, or 30 to 230 seconds, or 40 to 210 seconds, such as The specific measurement/calculation method of transmittance is described in the test example described below:

[Equation 1]

Transmittance (sec) = T _S - T _O

T _S (unit: seconds) prepares a salt water-absorbing super absorbent polymer by swelling 0.2 g of super absorbent polymer powder with a 0.9 weight % salt water (NaCl) solution for 30 minutes, and then swells 0.9 weight % salt water solution under a pressure of 0.3 psi. This means the time it takes to permeate the salt water-absorbing superabsorbent polymer, and T _O (unit: seconds) means the time it takes for a 0.9% saline solution to permeate under a pressure of 0.3 psi without the salt water-absorbing superabsorbent polymer. .

Below, preferred embodiments are presented to aid understanding of the invention. However, the following examples are only for illustrating the invention and do not limit the invention to these only.

< Example >

Synthesis example : Preparation of encapsulated foaming agent

As the encapsulated blowing agent used in the examples, F-36D manufactured by Matsumoto Company was prepared, where the core is iso-butane and the shell is composed of a copolymer of acrylate and acrylonitrile. At this time, the foaming start temperature (T _start ) of F-36D is 70°C to 80°C, and the maximum foaming temperature (T _max ) is 110°C to 120°C.

The particle size of each encapsulated blowing agent was measured as the average Feret diameter through an optical microscope. Then, the average value of the diameters of the encapsulated foaming agents was calculated and defined as the average diameter of the encapsulated foaming agents.

Additionally, in order to confirm the expansion characteristics of the encapsulated foaming agent, 0.2 g of the prepared encapsulated foaming agent was applied on a glass Petri dish and left on a hot plate preheated to 150°C for 10 minutes. The encapsulated foaming agent slowly expanded by heat, and this was observed under an optical microscope to measure the maximum expansion rate and maximum expansion size of the encapsulated foaming agent in air. After applying heat to the encapsulated foaming agent, the diameter of the top 10% by weight of the most expanded particles was measured and defined as the maximum expansion size. Heat was applied to the average diameter (D0) measured before applying heat to the encapsulated foaming agent. Afterwards, the ratio (DM/D0) of the average diameter (DM) of the top 10% by weight of highly expanded particles was calculated and defined as the maximum expansion ratio.

The average diameter of the prepared encapsulated foam before expansion was 13 μm, the maximum expansion ratio in air was about 9 times, and the maximum expansion size was about 80 to 150 μm.

Example One

In a glass reactor, 100 g of acrylic acid, 123.5 g of 32% by weight caustic soda (NaOH) solution, 0.1 g of the encapsulated blowing agent F-36D prepared above, 0.06 g of ethylene glycol diglycidyl ether as an internal crosslinker, and 0.2 g of sodium persulfate as a thermal polymerization initiator. , 0.008 g of photopolymerization initiator (diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide), 0.09 g of 28% by weight sodium dodecyl sulfate aqueous solution, and 59.0 g of water were mixed to form a monomer composition with a total solid concentration of 43% by weight. was manufactured.

The monomer composition was supplied at a rate of 500 to 2000 mL/min on a conveyor belt with a width of 10 cm and a length of 2 m rotating at a rate of 50 cm/min. Then, at the same time as the monomer composition was supplied, ultraviolet rays with an intensity of 10 mW/cm ² were irradiated to proceed with the polymerization reaction for 60 seconds.

Then, the water-containing gel polymer obtained through the polymerization reaction was passed through a hole with a diameter of 10 mm using a meat chopper to make powder (crump). Next, using a convection oven capable of moving air volume up and down, hot air at 185°C was flowed from downward to upward for 20 minutes, and then flowed from upward to downward for 20 minutes to dry the crumb evenly. I ordered it. The dried powder was pulverized, and powders having a diameter of 150 to 850 ㎛ were classified to prepare a base resin in powder form.

A surface cross-linking agent prepared by mixing 100 g of the base resin prepared above with 7 g of ultrapure water, 0.5 g of propylene glycol, 0.1 g of surface cross-linking agent ethylene glycol diglycidyl ether, 0.8 g of aluminum sulfate, and 0.04 g of Aerosil 200 (hydrophilic silica particles). An aqueous solution was added and stirred to mix for 2 minutes. Afterwards, surface crosslinking reaction was performed by heat treatment at 130°C for 50 minutes. Afterwards, it was classified to obtain a superabsorbent polymer composed of particles with a particle size of 150 to 850 ㎛.

Example 2

A surface cross-linking agent aqueous solution was used by mixing 100 g of base resin with 7 g of ultrapure water, 0.5 g of propylene glycol, 0.14 g of surface cross-linking agent ethylene glycol diglycidyl ether, 0.8 g of aluminum sulfate, and 0.04 g of Aerosil 200 (hydrophilic silica particles). A superabsorbent polymer was obtained in the same manner as in Example 1, except that.

Example 3

Use a surface cross-linking agent aqueous solution of 100 g of base resin, 7 g of ultrapure water, 0.5 g of propylene glycol, 0.12 g of surface cross-linking agent ethylene glycol diglycidyl ether, 0.8 g of aluminum sulfate, and 0.04 g of Aerosil 200 (hydrophilic silica particles). , A superabsorbent polymer was obtained in the same manner as in Example 1, except that it was heat treated at 160°C for 50 minutes to perform a surface crosslinking reaction.

Example 4

Use an aqueous surface cross-linking agent solution in which 7 g of ultrapure water, 0.5 g of propylene glycol, 0.14 g of surface cross-linking agent ethylene glycol diglycidyl ether, 0.8 g of aluminum sulfate, and 0.04 g of Aerosil 200 (hydrophilic silica particles) are mixed with 100 g of base resin. , A superabsorbent polymer was obtained in the same manner as in Example 1, except that it was heat treated at 160°C for 50 minutes to perform a surface crosslinking reaction.

Comparative example One

A surface cross-linking agent aqueous solution was used in which 7 g of ultrapure water, 0.1 g of surface cross-linking agent ethylene glycol diglycidyl ether, and 0.04 g of Aerosil 200 (hydrophilic silica particles) were mixed with 100 g of base resin, and the surface cross-linking reaction was carried out at 160°C for 50 minutes. A superabsorbent polymer was obtained in the same manner as in Example 1, except that it was heat treated for 10 minutes.

Comparative example 2

A surface cross-linking agent aqueous solution containing 100 g of base resin, 7 g of ultrapure water, 0.12 g of surface cross-linking agent ethylene glycol diglycidyl ether, and 0.04 g of Aerosil 200 (hydrophilic silica particles) was used, and the surface cross-linking reaction was carried out at 160°C for 50 minutes. A superabsorbent polymer was obtained in the same manner as in Example 1, except that it was heat treated for 10 minutes.

Comparative example 3

A surface cross-linking agent aqueous solution was used in which 7 g of ultrapure water, 0.14 g of surface cross-linking agent ethylene glycol diglycidyl ether, and 0.04 g of Aerosil 200 (hydrophilic silica particles) were mixed with 100 g of base resin, and the surface cross-linking reaction was carried out at 160°C for 50 minutes. A superabsorbent polymer was obtained in the same manner as in Example 1, except that it was heat treated for 10 minutes.

Comparative example 4

In a glass reactor, 100 g of acrylic acid, 123.5 g of 32% by weight caustic soda (NaOH) solution, 0.06 g of internal cross-linker ethylene glycol diglycidyl ether, 0.2 g of sodium persulfate as a thermal polymerization initiator, and a photopolymerization initiator (diphenyl (2,4, A monomer composition having a total solid concentration of 43% by weight was prepared by mixing 0.008 g of 6-trimethylbenzoyl)phosphine oxide and 59.0 g of water.

The monomer composition was supplied at a rate of 500 to 2000 mL/min on a conveyor belt with a width of 10 cm and a length of 2 m rotating at a rate of 50 cm/min. Then, at the same time as the monomer composition was supplied, ultraviolet rays with an intensity of 10 mW/cm ² were irradiated to proceed with the polymerization reaction for 60 seconds.

Then, the water-containing gel polymer obtained through the polymerization reaction was passed through a hole with a diameter of 10 mm using a meat chopper to make powder (crump). Next, using a convection oven capable of moving air volume up and down, hot air at 185°C was flowed from downward to upward for 20 minutes, and then flowed from upward to downward for 20 minutes to dry the crumb evenly. I ordered it. The dried powder was pulverized, and powders having a diameter of 150 to 850 ㎛ were classified to prepare a base resin in powder form.

To 100 g of the base resin prepared above, an aqueous surface cross-linking agent solution containing 3.2 g of ultrapure water, 4 g of methanol, 0.13 g of surface cross-linking agent ethylene carbonate, 0.08 g of aluminum sulfate, and 0.01 g of DM-30S (silica particles) was added and stirred. Mixed for 2 minutes. Afterwards, surface crosslinking reaction was performed by heat treatment at 185°C for 60 minutes. Afterwards, it was classified to obtain a superabsorbent polymer composed of particles with a particle size of 150 to 850 ㎛.

Comparative example 5

A surface cross-linking agent aqueous solution containing 100 g of base resin mixed with 3.2 g of ultrapure water, 4 g of methanol, 0.13 g of surface cross-linking agent ethylene carbonate, 0.08 g of aluminum sulfate, and 0.01 g of DM-30S (silica particles) was used to perform the surface cross-linking reaction. A superabsorbent polymer was obtained in the same manner as in Example 1, except that it was heat treated at 185°C for 90 minutes.

Test example

For the superabsorbent polymers prepared in the above Examples and Comparative Examples, centrifugation retention capacity (CRC) and permeability were measured in the following manner, and the results are shown in Table 1 below.

(1) Centrifuge retention capacity (CRC)

Centrifuge retention capacity (CRC) was measured by no-load absorption ratio according to the European Disposables and Nonwovens Association (EDANA) standard EDANA WSP 241.3.

Specifically, superabsorbent polymer (or base resin powder; hereinafter the same) W ₀ (g, about 0.2 g) is uniformly placed in a non-woven bag and sealed, and then the physiological state of a 0.9% by weight sodium chloride aqueous solution is placed at room temperature. Submerged in saline solution. After 30 minutes, the bag was centrifuged, drained at 250G for 3 minutes, and the mass W ₂ (g) of the bag was measured. Additionally, after performing the same operation without using the superabsorbent polymer, the mass W ₁ (g) was measured. Using each mass obtained in this way, CRC (g/g) was calculated according to the following calculation formula A to confirm the water retention capacity.

[Calculation Formula A]

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

(2) Transmittance

Permeability was measured under a 0.3 psi load using a 0.9% brine solution by the method described in Buchholz, F.L. and Graham, A.T., "Modern Superabsorbent Polymer Technology," John Wiley & Sons (1998), page 161.

To describe a more specific measurement method, 0.2 g of particles with a particle diameter of 300 to 600㎛ were taken from the superabsorbent polymers (hereinafter referred to as samples) prepared in Examples and Comparative Examples and placed in a cylinder (Φ20mm). . At this time, a stopcock is included at one end of the cylinder, and the upper and lower limits are marked. The upper limit of the cylinder is displayed at the position when 40mL of (salt water) solution is filled, and the lower limit is displayed at the position when 20mL of (salt water) solution is filled. It is displayed in the position it was in when filled.

50 g of 0.9% saline solution (NaCl) was added to the cylinder with the stopcock locked and left for 30 minutes. Next, if necessary, additional brine solution is added to bring the level of the brine solution to the upper limit of the cylinder. Next, a load of 0.3 psi was applied to the cylinder now containing the salt water-absorbing superabsorbent polymer and left for 1 minute. Afterwards, the stopcock located below the cylinder was opened to measure the time for the 0.9% saline solution to pass from the upper limit indicated on the cylinder to the lower limit. All measurements were conducted at a temperature of 24 ± 1°C and a relative humidity of 50 ± 10%.

The time passing from the upper limit to the lower limit was measured for each sample (Ts) and without the addition of superabsorbent polymer (T ₀ ), and the transmittance was calculated according to Equation 1 below.

[Equation 1]

Transmittance (sec) = T _S - T ₀

Water retention capacity (g/g) Transmittance (seconds) Example 1 40.5 205 Example 2 38.7 155 Example 3 36.4 91 Example 4 35.1 58 Comparative Example 1 40.3 1587 Comparative Example 2 38.1 820 Comparative Example 3 35.3 306 Comparative Example 4 38.2 778 Comparative Example 5 35.8 336

Referring to Table 1, it was confirmed that the superabsorbent polymer of the example showed water retention capacity (absorbent performance) at an equivalent level or higher and significantly improved permeability (liquid permeability) compared to the comparative example. In contrast, Comparative Examples 1 to 3 and 5 in which the surface cross-linking agent aqueous solution did not contain propylene glycol and aluminum sulfate salt, or Comparative Example 4 in which an encapsulated foaming agent was not applied, were intended to exhibit absorption performance at the same level as the Example. , it was confirmed that liquid permeability was inevitably greatly reduced.

Claims (12)

Forming a monomer composition comprising a water-soluble ethylenically unsaturated monomer having at least a partially neutralized acidic group, an encapsulated blowing agent, an internal crosslinking agent, and a polymerization initiator;
Cross-polymerizing the monomer composition to form a water-containing gel polymer;
Forming a base resin in powder form by drying, pulverizing and classifying the hydrogel polymer; and
In the presence of a surface cross-linking agent, further cross-linking the surface of the base resin to form a surface cross-linking layer,
The encapsulated foaming agent has a structure including a core containing hydrocarbon and a shell surrounding the core and formed of a thermoplastic resin, has an average diameter before expansion of 5 to 50 ㎛, and has a maximum expansion size in air of 20 to 190 ㎛. ㎛,
The step of forming the surface cross-linking layer is a method of producing a superabsorbent polymer that is carried out in the presence of a surface cross-linking agent, a solvent containing propylene glycol and water, and a surface cross-linking agent composition containing aluminum sulfate,
The superabsorbent polymer has a centrifugation retention capacity (CRC) of 35.1 to 45 g/g for 30 minutes in physiological saline solution (0.9% by weight sodium chloride aqueous solution).
Method for producing superabsorbent polymer.
The method of claim 1, wherein the encapsulated foaming agent has a maximum expansion ratio in air of 3 to 15 times.

The method of claim 1, wherein the encapsulated foaming agent has a foaming start temperature (T _start ) of 60°C to 120°C and a maximum foaming temperature (T _max ) of 100°C to 150°C.
The method of claim 1, wherein the hydrocarbon is n-propane, n-butane, iso-butane, cyclobutane, n-pentane, iso-pentane, cyclopentane, n-hexane, iso-hexane, cyclohexane, n-heptane, A method of producing a superabsorbent polymer selected from the group consisting of iso-heptane, cycloheptane, n-octane, iso-octane, and cyclooctane.
The method of claim 1, wherein the thermoplastic resin is a polymer formed from one or more monomers selected from the group consisting of (meth)acrylate, (meth)acrylonitrile, aromatic vinyl, vinyl acetate, vinyl halide, and vinylidene halide. Method for producing water absorbent resin.
The method of claim 1, wherein the encapsulated foaming agent is used in an amount of 0.01 to 1.0 parts by weight based on 100 parts by weight of the monomer.
The method of claim 1, wherein the internal crosslinking agent includes a diglycidyl ether-based compound of alkylene glycol.
The method of claim 1, wherein the surface cross-linking agent composition contains 0.001 to 2 parts by weight of a surface cross-linking agent, 0.05 to 3 parts by weight of propylene glycol, 1 to 10 parts by weight of water, and 0.02 parts by weight of aluminum sulfate, based on 100 parts by weight of the base resin. A method for producing a superabsorbent polymer comprising from 1.5 parts by weight.
The method of claim 1, wherein the surface crosslinking agent includes a diglycidyl ether-based compound of alkylene glycol.
The method of claim 1, wherein the surface cross-linking agent composition further comprises 0.01 to 0.5 parts by weight of inorganic particles based on 100 parts by weight of the base resin.
The method of claim 1, wherein the superabsorbent polymer has a centrifugation retention capacity (CRC) of 35.1 to 40.5 g/g in physiological saline (0.9% by weight aqueous sodium chloride solution) for 30 minutes.
The method of claim 1, wherein the superabsorbent polymer has a permeability of 300 seconds or less, defined by the following formula 1:
[Equation 1]
Transmittance (sec) = T _S - T _O
T _S (unit: seconds) prepares a salt water-absorbing super absorbent polymer by swelling 0.2 g of super absorbent polymer powder with a 0.9 weight % salt water (NaCl) solution for 30 minutes, and then swells 0.9 weight % salt water solution under a pressure of 0.3 psi. This means the time it takes to permeate the salt water-absorbing superabsorbent polymer, and T _O (unit: seconds) means the time it takes for a 0.9% saline solution to permeate under a pressure of 0.3 psi without the salt water-absorbing superabsorbent polymer. .