KR20190057746A - Super absorbent polymer and preparation method for the same - Google Patents

Abstract

The present invention relates to a super-absorbent resin having improved gel stability, and a preparing method thereof. The super-absorbent resin of the present invention comprises: a crosslinking polymer; and a surface cross-linked layer formed on a surface of the crosslinking polymer.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a super absorbent resin,

The present invention relates to a superabsorbent resin and a method for producing the same. More particularly, the present invention relates to a superabsorbent resin having improved gel stability and a method for producing the same.

Super Absorbent Polymer (SAP) is a synthetic polymer material capable of absorbing moisture of about 500 to 1,000 times its own weight. As a result, it is possible to develop a super absorbent polymer (SAM), an absorbent gel Material), and so on. Such a superabsorbent resin has started to be put into practical use as a sanitary article and is currently being used for sanitary articles such as diapers for children and sanitary napkins as well as soil repair agents for horticultural use, index materials for construction and construction, seedling- , And as a material for fomentation and the like.

In most cases, such a superabsorbent resin is widely used in the field of sanitary materials such as diapers and sanitary napkins. For this purpose, it is necessary to exhibit a high absorption capacity for moisture and the like, In addition, there is a need to exhibit excellent permeability by keeping the form well even when the water is absorbed and expanded in volume (swollen).

However, it is known that it is difficult to improve both the water retention capacity (CRC), which is the physical property showing the basic absorption power and the water holding capacity of the superabsorbent resin, and the absorptive capacity under pressure (AUL), which shows the property of holding the moisture absorbed even to the external pressure well . This is because, when the overall cross-link density of the superabsorbent resin is controlled to be low, the hydrophobic ability can be relatively high, but the cross-linking structure becomes poor and the gel strength becomes low and the absorbability under pressure can be lowered. On the other hand, when the cross-linking density is controlled to be high and the absorption ability under pressure is improved, moisture can not easily be absorbed through the dense cross-linking structure, and the basic water-holding ability can be lowered. For the above reasons, there is a limit to providing a superabsorbent resin having both a water-repellent ability and an ability to absorb under pressure.

However, recently, a hygroscopic material such as a diaper or a sanitary napkin is becoming thinner, so that a higher absorption performance is required for a superabsorbent resin. Among them, improvement of the water retention ability and the pressure absorption ability, which are opposite physical properties, and improvement of the liquid permeability are becoming important tasks.

In addition, pressure may be applied to sanitary materials such as diapers and sanitary napkins by the weight of the user. In particular, when a superabsorbent resin applied to a sanitary material such as a diaper or sanitary napkin absorbs a liquid and then a pressure due to the weight of the user is applied to the absorbent resin, rewet phenomenon And leakage of urine may occur.

There are many reasons for this re-wetting phenomenon, but one of the main causes is that the polymer structure is decomposed as the super absorbent resin absorbs urine. That is, the polymer structure of the superabsorbent resin can be decomposed by various components contained in the urine during the process of gelling the superabsorbent resin by absorbing the urine. Such a degradation process weakens the stability of the superabsorbent resin, The speed is lowered and may lead to re-wetting phenomenon.

Therefore, various attempts have been made to suppress such re-wetting phenomenon. However, there is no concrete method to effectively inhibit the re-wetting phenomenon.

In order to solve the problems of the prior art as described above, it is an object of the present invention to provide a superabsorbent resin which exhibits a stable absorbing ability even in a use environment of a sanitary material and suppresses rewetting and leakage of urine, and a method for producing the same.

In order to achieve the above object, according to an aspect of the present invention,

A crosslinked polymer obtained by polymerizing and cross-linking a monomer composition having an acidic group and at least a part of the acidic groups neutralized, the acrylic acid-based monomer, the internal crosslinking agent, the polymerization initiator, and the ionic binder; And a surface cross-linked layer formed on the surface of the cross-linked polymer, wherein the increase amount of the water-soluble component measured by the following formula 1 is 30% or less:

[Formula 1]

(%) = (Content of water after swelling ascorbic acid - content of water for initial water) / amount of water for initial water * 100

According to another aspect of the present invention,

Polymerizing a monomer composition having an acidic group and at least a part of the acidic groups neutralized with an acrylic acid-based monomer, an internal cross-linker, a polymerization initiator, and an ionic binder to form a hydrogel-like polymer;

Drying the hydrogel polymer;

Pulverizing the dried polymer; And

Mixing a surface cross-linking solution containing a surface cross-linking agent with the pulverized polymer and performing a surface cross-linking reaction;

And a method of producing a superabsorbent resin.

INDUSTRIAL APPLICABILITY According to the superabsorbent resin and the method for producing the same of the present invention, it is possible to provide a superabsorbent resin exhibiting a stable absorption capacity even in a use environment of actual sanitary materials.

Accordingly, in a product such as a diaper comprising the superabsorbent resin of the present invention, rehydration phenomenon and urine leakage phenomenon can be suppressed while exhibiting excellent various absorbent properties.

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the invention is not intended to be limited to the particular forms disclosed, but includes all modifications, equivalents, and alternatives falling within the spirit and scope of the invention.

Hereinafter, a superabsorbent resin according to an embodiment of the present invention and a method for producing the same will be described in detail.

Superabsorbent resin

The superabsorbent resin according to one embodiment of the present invention is a crosslinked polymer obtained by polymerizing and cross-linking a monomer composition having an acidic group and at least a part of the acidic group neutralized with an acrylic acid monomer, an internal crosslinking agent, a polymerization initiator, polymer; And a surface crosslinked layer formed on the surface of the crosslinked polymer, characterized in that the amount of increase in the water content measured by the following formula 1 is 30% or less:

[Formula 1]

(%) = (Content of water after swelling ascorbic acid - content of water for initial water) / amount of water for initial water * 100

In Equation (1)

1.0 g of the sample having a particle size of 150 to 850 탆 in the superabsorbent resin was added to 25 g of a salt water (0.9 wt% NaCl aqueous solution), free swelling was carried out at room temperature for 10 minutes, , And 175 ml of saline (0.9 wt% NaCl aqueous solution) was injected thereinto, and the water eluted after free swelling for 1 hour while stirring at 500 rpm at room temperature means the weight percentage of the solvent component,

The content of water-soluble components after swelling of ascorbic acid salt water was determined by adding 1.0 g of a sample having a particle diameter of 150 to 850 탆 in the superabsorbent resin to 25 g of ascorbic acid saline (0.9 wt% NaCl, and 0.05 wt% aqueous ascorbic acid solution) , And then 175 mL of ascorbic acid aqueous solution (0.9 wt% NaCl, and 0.05 wt% ascorbic acid aqueous solution) was added thereto. The mixture was stirred at 500 rpm at room temperature for 1 hour The amount of water eluted after free swelling means the weight percentage of the solvent component.

Throughout the specification of the present invention, " free swelling " means a state in which the superabsorbent resin absorbs brine or ascorbic acid brine and is capable of swelling without restraining load.

By the polymerization reaction of the acrylic acid-based monomer, the hydrogel polymer is subjected to a process such as drying, crushing, classification, surface crosslinking and the like, and is marketed as a superabsorbent resin which is powdery product.

On the other hand, in the process of preparing a superabsorbent resin, a water-soluble polymer that is not crosslinked is produced, and when the water content of the polymer is high, the solution absorption property of the superabsorbent resin is improved. On the other hand, It can easily dissolve on contact, causing the surface to become sticky or to have an unfavorable effect on the skin being contacted.

In recent years, not only the absorption properties such as absorption capacity and liquid permeability but also the degree of dryness of the surface of the swollen super absorbent resin can be maintained in the case where the actual diaper is used, It is becoming a scale. In addition, there are various factors that affect the drying state, but the above-mentioned content of the water-soluble component is pointed out as the main cause.

As a method for evaluating the dry state of the surface of the swollen superabsorbent resin, a method of measuring the content of the water-soluble component after free-swelling the 0.9 wt% salt water in the superabsorbent resin for a predetermined period of time is generally used. However, in a situation where the actual diaper is used, the urine absorbs urine rather than salt water, and the urine contains various components as well as salt, so that the above method does not accurately reflect the physical properties of the superabsorbent resin.

Accordingly, the inventors of the present invention have found that ascorbic acid, which is known as vitamin C among various components contained in urine, reacts with the polymer structure of the highly water-absorbent resin to decompose the polymer structure, thereby increasing water-soluble components, Drying state is deteriorated and re-wetting or leakage of urine is caused, leading to the present invention.

That is, when the superabsorbent resin is swollen under a certain condition by using ascorbic acid-containing brine having the property of decomposing the polymer, the degree of stability of the polymer structure of the superabsorbent resin is not well decomposed, As a result, the change of water content was measured with and without ascorbic acid.

In addition, the inventors of the present invention have found that when the swelling of a superabsorbent resin causes a decomposition of the polymer structure to increase the content of the water-soluble component, the acrylic acid-based monomer contained in the monomer composition and the alkyl used for neutralizing the acrylic acid- It has been noted that the stability of the gel upon swelling can be improved by reducing the residual amount of the metal polyvalent ions derived from the lipid material and the persulfate-based compound used as the polymerization initiator in the superabsorbent resin.

That is, according to the present invention, the monomer composition includes an ionic binder capable of reducing the residual content of polyvalent ions of metals such as Fe, Mg, Ca and the like as a means of minimizing a change in water content, Absorbent resin having little change in the water content component even when the ascorbic acid salt water swells by making it less or substantially not contain the persulfate-based initiator, and a method for producing the same.

In the superabsorbent resin of the present invention, the persulfate-based initiator remaining in the final superabsorbent resin may be about 300 ppm or less, or about 100 ppm or less, or about 50 ppm or less, or about 30 ppm or less And preferably not included.

Further, in the superabsorbent resin of the present invention, the monomer composition includes an ionic binder. The ionic binder is not particularly limited as long as it is a compound capable of capturing polyvalent metal ions. For example, sodium tripolyphosphate can be used.

The ionic binder may be included in the monomer composition at a concentration of about 1 ppm or more, or about 5 ppm or more, or about 10 ppm or more, about 50 ppm or less, or about 40 ppm or less, or about 30 ppm or less.

Therefore, the superabsorbent resin according to one embodiment of the present invention exhibits excellent absorption performance, and even when swollen by ascorbic acid salt water, the content of the water component does not increase, so that the absorbent resin remains dry even under the use environment of the actual diaper. And it was confirmed that it is possible to effectively prevent the rewet phenomenon in which the absorbed urine leaks out again.

Therefore, the superabsorbent resin according to one embodiment of the present invention exhibiting such characteristics can be applied to various sanitary articles such as diapers and exhibits excellent physical properties as a whole.

The superabsorbent resin of the present invention is characterized in that the increase amount of the water-soluble component measured by the following formula 1 is about 30% or less, or about 25% or less, or about 20% or less. The lower limit value is not particularly limited, but may be about 5% or more, or about 3% or more, or about 1% or more.

In addition, the superabsorbent resin of the present invention has a characteristic that the total content of water components measured by the method of EDANA method WSP 270.2 is 20% or less, or about 15% or less, or about 10% or less.

 [Formula 1]

(%) = (Content of water after swelling ascorbic acid - content of water for initial water) / amount of water for initial water * 100

In Equation (1)

1.0 g of the sample having a particle size of 150 to 850 탆 in the superabsorbent resin was added to 25 g of a salt water (0.9 wt% NaCl aqueous solution), free swelling was carried out at room temperature for 10 minutes, , And 175 ml of saline (0.9 wt% NaCl aqueous solution) was injected thereinto, and the water eluted after free swelling for 1 hour while stirring at 500 rpm at room temperature means the weight percentage of the solvent component,

The content of water-soluble components after swelling of ascorbic acid salt water was determined by adding 1.0 g of a sample having a particle diameter of 150 to 850 탆 in the superabsorbent resin to 25 g of ascorbic acid saline (0.9 wt% NaCl, and 0.05 wt% aqueous ascorbic acid solution) , And then 175 mL of ascorbic acid aqueous solution (0.9 wt% NaCl, and 0.05 wt% ascorbic acid aqueous solution) was added thereto. The mixture was stirred at 500 rpm at room temperature for 1 hour The amount of water eluted after free swelling means the weight percentage of the solvent component.

As previously described, the previous method of measuring the water content generally measures components eluted after swelling with salt water (0.9 wt% NaCl solution), since urine is not the same as salt water, There was a problem. In addition, in order to evaluate the change of the drying state due to the absorption of urine, it is necessary to measure the increase amount of the water content with time.

Therefore, in the present invention, ascorbic acid contained in the urine is found to be the main cause of the increase in the water content component, that is, decomposition of the superabsorbent resin. Thus, swelling of the superabsorbent resin with the salt water containing ascorbic acid, Based on the fact that it can better reflect the usage environment of the user.

In the formula 1, 1.0 g of a sample having a particle size of 150 to 850 탆 in the superabsorbent resin was added to 25 g of a saline solution (0.9 wt% NaCl aqueous solution) and free swelling was performed at room temperature for 10 minutes. And 60 for 1 hour, 175 ml of saline (0.9 wt% NaCl aqueous solution) is injected thereto again, and the water eluted after free swelling for 1 hour while stirring at 500 rpm at room temperature means the weight percentage of the solvent component .

In order to measure the water content after swelling of ascorbic acid salt water, 1.0 g of a sample having a particle diameter of 150 to 850 탆 in a superabsorbent resin was added to 25 g of ascorbic acid aqueous solution (0.9 wt% NaCl, and 0.05 wt% aqueous ascorbic acid solution) 175 mL of ascorbic acid (0.9 wt% NaCl, and 0.05 wt% of ascorbic acid solution) was added thereto, and the mixture was stirred at 500 rpm at room temperature for 1 hour Refers to the weight percent of the constituents of the eluent after free swelling.

The method for measuring the content of the water-soluble component is as follows. The solution swollen in a freely swollen state is filtered with a filter paper, and the filtered solution is firstly titrated to pH 10 with 0.1 N caustic soda solution. 2.7, the content (% by weight) of the water-soluble component in the superabsorbent resin can be calculated from the obtained amount.

When the increase amount of the water content before and after the swelling of the ascorbic acid salt water measured by the above-described method is 30% or less, excellent dryness characteristics are exhibited in the actual use environment of the diaper and the rewet phenomenon is suppressed have.

The superabsorbent resin preferably has a CRC of at least about 28 g / g, or at least about 30 g / g, or at least about 32 g / g, measured according to EDANA method WSP 241.3, g or less, or about 40 g / g or less, or about 38 g / g or less.

The superabsorbent resin preferably has a compressive absorption capacity (AUL) of about 0.9 g / g or more, about 19 g / g or more, or about 21 g / g or more as measured according to EDANA method WSP 242.3 25 g / g or less, or about 24 g / g or less, or about 23 g / g or less.

As described above, the superabsorbent resin of the present invention has excellent physical properties such as water retention capacity and pressure absorption ability, and even when the urine is absorbed, the increase in the amount of water component is small and excellent stability can be restrained so that rewetting and leakage of urine can be suppressed.

Method for producing superabsorbent resin

The superabsorbent resin having the above characteristics can be produced by the following production method.

That is, a method of producing a superabsorbent resin according to another embodiment of the present invention comprises: preparing a monomer composition comprising an acrylic acid-based monomer having an acidic group and at least a part of the acidic groups neutralized, an internal cross-linking agent, Polymerizing to form a hydrogel polymer; Drying the hydrogel polymer; Pulverizing the dried polymer; And mixing the ground polymer with a surface cross-linking solution containing a surface cross-linking agent and performing a surface cross-linking reaction.

In the method for producing a superabsorbent resin of the present invention, the monomer composition which is a raw material of the superabsorbent resin includes an acrylic acid-based monomer having an acidic group and at least a part of the acidic groups neutralized, an internal crosslinking agent, a polymerization initiator, and an ionic binder .

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

[Chemical 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.

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

Here, the acrylic acid-based monomer may have an acidic group and at least a part of the acidic group may be neutralized. Preferably, the monomer is partially neutralized with an alkylene material such as sodium hydroxide, potassium hydroxide, ammonium hydroxide or the like. At this time, the neutralization degree of the acrylic acid monomer may be 40 to 95 mol%, or 40 to 80 mol%, or 45 to 75 mol%. The range of the degree of neutralization can be adjusted according to the final properties. However, if the degree of neutralization is too high, neutralized monomers may precipitate and polymerization may not be smoothly proceeded. On the other hand, if the degree of neutralization is too low, the absorption capacity of the polymer is greatly lowered, have.

The concentration of the acrylic acid-based monomer may be about 20 to about 60% by weight, preferably about 40 to about 50% by weight, based on the monomer composition including the raw material and the solvent of the superabsorbent resin, It may be an appropriate concentration considering the reaction conditions and the like. However, if the concentration of the monomer is excessively low, the yield of the superabsorbent resin may be low and economical efficiency may be deteriorated. On the other hand, if the concentration is excessively high, a part of the monomer may precipitate or the pulverization efficiency may be low Problems such as the like may occur and the physical properties of the superabsorbent resin may be deteriorated.

In the method for producing a superabsorbent resin of the present invention, the polymerization initiator used in polymerization is not particularly limited as long as it is generally used in the production of a superabsorbent resin.

Specifically, as the polymerization initiator, a thermal polymerization initiator or a photopolymerization initiator based on UV irradiation may be used depending on the polymerization method. However, even when the photopolymerization method is employed, a certain amount of heat is generated by irradiation of ultraviolet light or the like, and a certain amount of heat is generated as the polymerization reaction, which is an exothermic reaction, proceeds.

The photopolymerization initiator can be used without limitation in the constitution as long as it is a compound capable of forming a radical by light such as ultraviolet rays.

Examples of the photopolymerization initiator include benzoin ether, dialkyl acetophenone, hydroxyl alkylketone, phenyl glyoxylate, benzyl dimethyl ketal Ketal, acyl phosphine, and alpha-aminoketone may be used. On the other hand, as a specific example of the acylphosphine, a commonly used lucyrin TPO, i.e., 2,4,6-trimethyl-benzoyl-trimethyl phosphine oxide can be used . More photoinitiators are well described in Reinhold Schwalm, UV Coatings: Basics, Recent Developments and New Applications (Elsevier 2007), p. 115, and are not limited to the above examples.

The photopolymerization initiator may be included in the monomer composition at a concentration of about 0.01 to about 1.0 wt%. If the concentration of such a photopolymerization initiator is too low, the polymerization rate may be slowed. If the concentration of the photopolymerization initiator is too high, the molecular weight of the superabsorbent resin may be small and the physical properties may become uneven.

As the thermal polymerization initiator, at least one selected from persulfate-based initiators, azo-based initiators, initiators consisting of hydrogen peroxide and ascorbic acid can be used. Specifically, examples of the persulfate-based initiator include sodium persulfate (Na ₂ S ₂ O ₈ ), potassium persulfate (K ₂ S ₂ O ₈ ), ammonium persulfate (NH ₄ ) ₂ S ₂ O ₈ ). Examples of the azo-based initiator include 2, 2-azobis (2-amidinopropane) dihydrochloride, 2 , 2-azobis- (N, N-dimethylene) isobutyramidine dihydrochloride, 2- (carbamoyl azo) isobutyronitrile Azobis [2- (2-imidazolin-2-yl) propane] dihydrochloride, yl) propane] dihydrochloride, and 4,4-azobis- (4-cyanovaleric acid). A variety of thermal polymerization initiators are well described in the Odian book, Principle of Polymerization (Wiley, 1981), p. 203, and are not limited to the above examples.

On the other hand, the persulfate-based initiator remaining in the final superabsorbent resin in the thermal polymerization initiator may be about 300 ppm or less, or about 100 ppm or less, or about 50 ppm or less, or about 30 ppm or less, But may preferably not include it. The persulfate-based initiator is often used as a thermal polymerization initiator in polymerization of the monomer composition after polymerization. However, when the polymerization initiator remains in the final superabsorbent resin after polymerization to a certain amount or more, it causes discoloration or causes gel stability in an environment where the superabsorbent resin swells do. Thus, in the present invention, the stability of the gel can be increased by not containing the persulfate-based initiator in a small amount or substantially not.

According to an embodiment of the present invention, a sulfite additive may be added after polymerization to replace the residual persulfate-based initiator in the final superabsorbent resin to replace the persulfate ions with sulfite ions. The sulfite-based additive may be added at any stage after the polymerization, but may be added to the surface cross-linking solution, preferably at the surface cross-linking step.

The monomer composition of the present invention comprises an ionic binder.

Examples of the alkylene monomer such as sodium hydroxide, potassium hydroxide, ammonium hydroxide and the like used for neutralization of the acrylic acid-based monomer and the acrylic acid-based monomer contained in the monomer composition include polyvalent ions of metals such as Fe, Mg, Ca and the like. When such a metal multi-valent ion remains in the superabsorbent resin, the polymer structure is decomposed upon swelling of the superabsorbent resin, thereby interfering with the gel stability. Therefore, according to the preparation method of the present invention, gel stability can be maintained by incorporating an ionic binder into the monomer composition in order to minimize the residual of the metal polyvalent ions.

The ionic binder is not particularly limited as long as it is a compound capable of capturing polyvalent metal ions. For example, sodium tripolyphosphate can be used.

The ionic binder may be included in the monomer composition at a concentration of about 1 ppm or more, or about 5 ppm or more, or about 10 ppm or more, about 50 ppm or less, or about 40 ppm or less, or about 30 ppm or less.

In the production method of the present invention, the concentration of the metal polyvalent ions may be included in the monomer composition at a concentration of about 10 ppm or less, or about 7 ppm or less, or about 5 ppm or less, May not be included.

According to one embodiment of the present invention, the monomer composition includes an internal cross-linking agent as a raw material for a superabsorbent resin. Examples of the internal crosslinking agent include a crosslinking agent having at least one functional group capable of reacting with the acrylic acid-based monomer and having at least one ethylenic unsaturated group; Or a crosslinking agent having two or more functional groups capable of reacting with a substituent formed by hydrolysis of a substituent and / or a monomer of the acrylic acid-based monomer may be used.

Specific examples of the internal crosslinking agent include N, N'-methylenebisacrylamide, trimethylolpropane tri (meth) acrylate, ethylene glycol di (meth) acrylate, polyethylene glycol (meth) acrylate, propylene glycol di Butylene diol di (meth) acrylate, butylene glycol di (meth) acrylate, diethylene glycol di (meth) acrylate, hexane diol di (meth) acrylate, Acrylate, triethylene glycol di (meth) acrylate, tripropylene glycol di (meth) acrylate, tetraethylene glycol di (meth) acrylate, dipentaerythritol pentaacrylate, glycerin tri Styrene tetraacrylate, triarylamine, ethylene glycol diglycidyl ether, propylene glycol, Lee serine, and it can be used at least one selected from the group consisting of ethylene carbonate.

Such an internal crosslinking agent may be included at a concentration of about 0.01 to about 0.5% by weight based on the monomer composition to crosslink the polymerized polymer.

In the production method of the present invention, the monomer composition of the superabsorbent resin may further contain additives such as a thickener, a plasticizer, a storage stabilizer, and an antioxidant, if necessary.

Raw materials such as acrylic acid-based monomer, photopolymerization initiator, thermal polymerization initiator, internal cross-linking agent and additive having the above-mentioned acid group and at least part of which is neutralized can be prepared in the form of a monomer composition solution dissolved in a solvent.

The solvent which can be used at this time can be used without limitation of its constitution as long as it can dissolve the above-mentioned components. Examples thereof include water, ethanol, ethylene glycol, diethylene glycol, triethylene glycol, Propylene glycol monomethyl ether, propylene glycol monomethyl ether acetate, methyl ethyl ketone, acetone, methyl amyl ketone, cyclohexanone, cyclopentanone, diethylene glycol monomethyl ether, diethylene glycol At least one selected from ethyl ether, toluene, xylene, butylolactone, carbitol, methylcellosolve acetate and N, N-dimethylacetamide can be used in combination.

The solvent may be included in the remaining amount of the monomer composition excluding the components described above.

On the other hand, the method of forming a hydrogel polymer by thermal polymerization or photopolymerization of such a monomer composition is not particularly limited as long as it is a commonly used polymerization method.

Specifically, the polymerization method is largely divided into thermal polymerization and photopolymerization depending on the polymerization energy source. In general, when thermal polymerization is carried out, it may proceed in a reactor having a stirring axis such as a kneader. In the case where light polymerization is proceeded, The polymerization method described above is merely an example, and the present invention is not limited to the polymerization method described above.

For example, the hydrogel polymer obtained by supplying hot air or heating the reactor to a reactor such as a kneader having an agitating shaft as described above and thermally polymerizing the reactor may be supplied to the reactor outlet The discharged hydrogel polymer may be in the range of a few centimeters to a few millimeters. Specifically, the size of the obtained hydrogel polymer may vary depending on the concentration of the monomer composition to be injected, the injection rate, etc. In general, a hydrogel polymer having a weight average particle diameter of 2 to 50 mm can be obtained.

In addition, when photopolymerization proceeds in a reactor equipped with a movable conveyor belt as described above, the form of the hydrogel polymer that is usually obtained may be a hydrogel polymer on a sheet having a belt width. At this time, the thickness of the polymer sheet varies depending on the concentration of the monomer composition to be injected and the injection rate, but it is preferable to supply the monomer composition so that a polymer in the form of a sheet having a thickness of usually 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 in the sheet is too thin, the production efficiency is low, which is undesirable. When the thickness of the polymer on the sheet exceeds 5 cm, the polymerization reaction occurs evenly over the entire thickness due to the excessively thick thickness I can not.

The normal water content of the hydrogel polymer obtained by this method may be about 40 to about 80 wt%. On the other hand, throughout the present specification, the term "moisture content" means the moisture content of the total functional gelated polymer weight minus the weight of the hydrogel polymer in dry state. Specifically, it is defined as a value calculated by measuring the weight loss due to moisture evaporation in the polymer in the process of 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 keeping it at 180 ° C, and the total drying time is set to 20 minutes including 5 minutes of the temperature raising step, and water content is measured.

Next, the step of drying the obtained hydrogel polymer is carried out.

At this time, if necessary, the step of coarse grinding may be further carried out before drying in order to increase the efficiency of the drying step.

In this case, the pulverizer to be used is not limited in its constitution, but may be a vertical pulverizer, a turbo cutter, a turbo grinder, a rotary cutter mill, A crusher, a disc mill, a disc mill, a shred crusher, a crusher, a chopper, and a disc cutter. However, the present invention is not limited to the above-described example.

Wherein the milling step may be milled so that the size of the hydrogel polymer is from about 2 to about 10 mm.

It is technically not easy to crush to less than 2 mm in diameter due to the high moisture content of the hydrogel polymer, and there may be a phenomenon that the crushed particles cohere to each other. On the other hand, when the particle size is larger than 10 mm, the effect of increasing the efficiency of the subsequent drying step is insignificant.

Drying is carried out on the hydrogel polymer immediately after polymerization as described above, or without the pulverization step. The drying temperature of the drying step may be about 150 to about 250 ° C. If the drying temperature is lower than 150 ° C, the drying time becomes excessively long and the physical properties of the superabsorbent resin to be finally formed may deteriorate. When the drying temperature exceeds 250 ° C, only the polymer surface is excessively dried, And the physical properties of the finally formed superabsorbent resin may be deteriorated. Thus, preferably, the drying can proceed at a temperature of from about 150 to about 200 < 0 > C, more preferably from about 160 to about 180 < 0 > C.

On the other hand, in the case of drying time, it may proceed for about 20 to about 90 minutes, but is not limited thereto, considering process efficiency and the like.

The drying method in the drying step may be selected and used as long as it is usually used as a drying step of the hydrogel polymer. Specifically, the drying step can be carried out by hot air supply, infrared irradiation, microwave irradiation, ultraviolet irradiation, or the like. The water content of the polymer after such a drying step may be from about 0.1 to about 10% by weight.

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

The polymer powder obtained after the milling step may have a particle diameter of from about 150 to about 850 탆. The pulverizer used for crushing with such a particle size is specifically a pin mill, a hammer mill, a screw mill, a roll mill, a disc mill, A jog mill, or the like may be used, but the present invention is not limited to the above examples.

Further, in order to control the physical properties of the superabsorbent resin powder which is finally produced after the pulverization step, a separate process of classifying the polymer powder obtained after pulverization according to the particle size may be performed. Preferably, the polymer having a particle diameter of about 150 to about 850 탆 is classified, and the polymer powder having such a particle diameter can be commercialized through a surface modification step only.

Next, the surface cross-linking step is carried out for the ground polymer by adding a surface cross-linking agent to the ground polymer, mixing the mixture, and heating the mixture by heating.

The surface crosslinking step is a step of inducing a crosslinking reaction on the surface of the pulverized polymer in the presence of a surface crosslinking agent to form a superabsorbent resin having improved physical properties. Through such surface crosslinking, a surface crosslinked layer (surface modifying layer) is formed on the surface of the pulverized polymer particles.

The surface cross-linking may be carried out by a conventional method for increasing the cross-linking density of the polymer particle surface, and may be performed, for example, by a method in which a solution containing a surface cross-linking agent and the ground polymer are mixed and crosslinked .

Also, in general, the surface cross-linking agent is applied to the surface of the superabsorbent resin particles. Thus, this reaction takes place on the surface of the superabsorbent resin particles, which improves the crosslinkability on the surface of the particles without substantially affecting the interior of the particles. Thus, the surface cross-linked superabsorbent resin particles have a higher degree of crosslinking in the vicinity of the surface than in the interior.

When the surface cross-linking agent is added, water may be further mixed together and added in the form of a surface cross-linking solution. When water is added, there is an advantage that the surface cross-linking agent can be uniformly dispersed in the polymer. At this time, the added water content is preferably from about 1 to about 10 wt. Parts per 100 parts by weight of the polymer for the purpose of inducing uniform dispersion of the surface cross-linking agent and preventing the polymer powder from aggregating and optimizing the surface penetration depth of the surface cross- By weight.

According to an embodiment of the present invention, a sulfite additive may be mixed and added to the surface cross-linking solution to reduce the content of the persulfate-based initiator remaining in the final superabsorbent resin.

The sulfite-based additive may be added in an amount of about 0.1 to about 10 parts by weight, or about 0.1 to about 5 parts by weight, based on 100 parts by weight of the ground polymer.

The surface cross-linking agent is not limited in its constitution as long as it is a compound capable of reacting with a functional group contained in the polymer.

Preferably, in order to improve the properties of the resulting superabsorbent resin, a polyhydric alcohol compound as the surface crosslinking agent; Epoxy compounds; Polyamine compounds; Halo epoxy compounds; A condensation product of a haloepoxy compound; Oxazoline compounds; Mono-, di- or polyoxazolidinone compounds; Cyclic urea compounds; Polyvalent metal salts; And an alkylene carbonate compound can be used.

Specific examples of the polyhydric alcohol compound include mono-, di-, tri-, tetra- or polyethylene glycol, monopropylene glycol, 1,3-propanediol, dipropylene glycol, 2,3,4-trimethyl- - pentanediol, polypropylene glycol, glycerol, polyglycerol, 2-butene-1,4-diol, 1,4-butanediol, 1,3-butanediol, 1,5-pentanediol, 1,2-cyclohexane dimethanol, and the like.

Examples of the epoxy compounds include ethylene glycol diglycidyl ether and glycidol. Examples of the polyamine compounds include ethylene diamine, diethylene triamine, triethylene tetramine, tetraethylene pentaamine, pentaethylene hexamine , Polyethyleneimine, and polyamide polyamines can be used.

As the haloepoxy compound, epichlorohydrin, epibromohydrin, and? -Methyl epichlorohydrin may be used. On the other hand, as the mono-, di- or polyoxazolidinone compounds, for example, 2-oxazolidinone and the like can be used.

As the alkylene carbonate compound, ethylene carbonate and the like can be used. These may be used alone or in combination with each other. On the other hand, in order to increase the efficiency of the surface cross-linking step, at least one polyhydric alcohol compound having 2 to 10 carbon atoms among these surface cross-linking agents may be used.

The amount of the surface crosslinking agent to be added may be appropriately selected according to the kind of the surface crosslinking agent to be added and the reaction conditions, but is usually about 0.001 to about 5 parts by weight, preferably about 0.01 to about 5 parts by weight, 3 parts by weight, more preferably about 0.05 to about 2 parts by weight, may be used.

If the content of the surface cross-linking agent is too small, surface cross-linking reaction hardly occurs. If the amount of the surface cross-linking agent is more than 5 parts by weight based on 100 parts by weight of the polymer, excessive absorption of the surface cross- .

 By heating the polymer particles to which the surface cross-linking agent has been added at a temperature of about 160 to about 200 DEG C, preferably about 170 to about 190 DEG C, for about 20 to about 100 minutes, preferably about 30 to about 90 minutes, The surface cross-linking reaction can be performed. If the crosslinking reaction temperature is less than 160 ° C or the reaction time is too short, the surface cross-linking reaction does not occur properly and the permeability may be lowered. If the reaction temperature is more than 200 ° C or the reaction time is too long,

The temperature raising means for the surface cross-linking reaction is not particularly limited. A heating medium can be supplied, or a heating source can be directly supplied and heated. At this time, as the type of usable heat medium, it is possible to use a heated fluid such as steam, hot air or hot oil. However, the present invention is not limited thereto, and the temperature of the heat medium to be supplied can be controlled by means of heat medium, It can be appropriately selected in consideration of the target temperature. On the other hand, as a heat source to be directly supplied, a heating method using electricity or a heating method using gas may be mentioned, but the present invention is not limited to the above-mentioned examples.

The superabsorbent resin produced by the production method of the present invention can have improved gel stability without deteriorating physical properties such as water retention capacity and pressure absorption ability.

For example, the water content increment measured by the following formula 1 may be 30% or less:

[Formula 1]

(%) = (Content of water after swelling ascorbic acid - content of water for initial water) / amount of water for initial water * 100

In Equation (1)

1.0 g of the sample having a particle size of 150 to 850 탆 in the superabsorbent resin was added to 25 g of a salt water (0.9 wt% NaCl aqueous solution), free swelling was carried out at room temperature for 10 minutes, , And 175 ml of saline (0.9 wt% NaCl aqueous solution) was injected thereinto, and the water eluted after free swelling for 1 hour while stirring at 500 rpm at room temperature means the weight percentage of the solvent component,

The content of water-soluble components after swelling of ascorbic acid salt water was determined by adding 1.0 g of a sample having a particle diameter of 150 to 850 탆 in the superabsorbent resin to 25 g of ascorbic acid saline (0.9 wt% NaCl, and 0.05 wt% aqueous ascorbic acid solution) , And then 175 mL of ascorbic acid aqueous solution (0.9 wt% NaCl, and 0.05 wt% ascorbic acid aqueous solution) was added thereto. The mixture was stirred at 500 rpm at room temperature for 1 hour The amount of water eluted after free swelling means the weight percentage of the solvent component.

The superabsorbent resin preferably has a CRC of at least about 28 g / g, or at least about 30 g / g, or at least about 32 g / g, measured according to EDANA method WSP 241.3, g or less, or about 40 g / g or less, or about 38 g / g or less.

The superabsorbent resin preferably has a compressive absorption capacity (AUL) of about 0.9 g / g or more, about 19 g / g or more, or about 21 g / g or more as measured according to EDANA method WSP 242.3 25 g / g or less, or about 24 g / g or less, or about 23 g / g or less.

As described above, the superabsorbent resin of the present invention has excellent absorbing ability and has excellent gel stability even when absorbing urine, and rewetting and leakage of urine can be suppressed.

The present invention will be described in more detail in the following Examples. However, the following examples are illustrative of the present invention, and the present invention is not limited by the following examples.

<Examples>

Preparation of superabsorbent resin

Example 1

To a 2 L glass reactor surrounded by a jacket in which a heating medium precooled to 25 DEG C was circulated was added 11 g of a 1% IRGACURE 819 initiator diluted in acrylic acid to 480 g of acrylic acid, and 5% polyethylene glycol diacrylate (PEGDA, Molecular weight 400) solution (solution A), and 860 g of a 24% caustic soda solution (solution B) were slowly added dropwise.

After mixing the two solutions, it was confirmed that the temperature of the mixture rose to 80 ° C or more by neutralizing heat. After the temperature was cooled to 40 ° C, when the reaction temperature reached 40 ° C, 29g of 2% sodium persulfate solution 415 ppm relative to the monomer composition). In addition, 0.5 g (7.1 ppm relative to the monomer composition) of 2% sodium tripolyphosphate diluted in water was injected.

The solution was poured into a Vat-shaped tray (15 cm.times.15 cm long) installed in a square polymerization reactor equipped with a light irradiation device at the top and preheated at 80.degree. C., and light irradiation was initiated by photoirradiation. After about 25 seconds after light irradiation, the gel is generated from the surface. After about 50 seconds, it is confirmed that the polymerization reaction occurs simultaneously with foaming. After that, after 3 minutes is further reacted, the polymerized sheet is taken out and cut into 3x3 cm The crumb was chopped using a Meat chopper.

The crumb was dried in an oven capable of airflow transfer up and down. 180 ℃ hot air was flowed from bottom to top for 15 minutes and from bottom to top for 15 minutes. After drying, the water content of the resin became less than 2%.

After drying, the mixture was pulverized by a pulverizer, and classified into 150 to 850 mu m size to prepare a base resin. To 100 g of the obtained base resin, 5.3 g of a surface cross-linking liquid (42 g of water, 10 g of ethylene carbonate, 1 g of Aerosil 200 and 0.5 g of sodium hydrogen sulfite) was uniformly dispersed and stirred at 100 rpm for 30 seconds using a high- heat jacket, and reacted at 185 ° C for 50 minutes. After the reaction, the superabsorbent resin was pulverized and classified with a particle size of 150 to 850 탆 by using a sieve to obtain a surface-treated superabsorbent resin.

Example 2

Except that 0.5 g of sodium metabisulfite was used instead of 0.5 g of sodium hydrogen sulfite as the surface cross-linking agent in the surface cross-linking using the base resin obtained in Example 1, a superabsorbent resin .

Example 3

The surface crosslinking using the base resin obtained in Example 1 was carried out in the same manner as in Example 1, except that 0.5 g of sodium hydrogen sulfite was used as the surface cross-linking solution, and 0.3 g of Blancolen HP of Germany was used instead Thereby obtaining an absorbent resin.

Comparative Example 1

A superabsorbent resin was obtained in the same manner as in Example 1, except that 0.5 g of 2% sodium tripolyphosphate diluted in water at the time of polymerization in Example 1 was not used.

Comparative Example 2

In Comparative Example 1, a superabsorbent resin was obtained in the same manner as in Comparative Example 1, except that sodium hydrogensulfite was not used as the surface cross-linking solution.

Comparative Example 3

A superabsorbent resin was obtained in the same manner as in Example 2, except that 0.5 g of 2% sodium tripolyphosphate diluted in water in Example 2 was not used and sodium metabisulfite was not used in the surface cross-linking liquid.

<Experimental Example>

The properties of the superabsorbent resin prepared in the above Examples and Comparative Examples were evaluated by the following methods.

 (1) Determination of the content of the initial water component

1.0 g of a sample having a particle diameter of 150 to 850 탆 in a superabsorbent resin was added to 25 g of a saline solution (0.9 wt% NaCl aqueous solution), free swelling was carried out at room temperature for 10 minutes and the sample was left at 60 캜 for 1 hour. 175 ml of a saline solution (0.9 wt% NaCl aqueous solution) was poured into the flask, and the flask was freely swollen for 1 hour while stirring at 500 rpm at room temperature. Then, the aqueous solution was filtered with a filter paper.

50 ml of the filtered solution was first titrated to pH 10 with 0.1 N caustic soda solution and then back titrated to pH 2.7 with 0.1 N hydrogen chloride solution to calculate the content (wt.%) Of the water soluble component in the superabsorbent resin .

(2) Determination of content of water component after swelling ascorbic acid salt water

1.0 g of a sample having a particle diameter of 150 to 850 탆 in a superabsorbent resin was added to 25 g of ascorbic acid brine (0.9 wt% NaCl, and 0.05 wt% of ascorbic acid aqueous solution) and subjected to free swelling at room temperature for 10 minutes and 1 hour at 60 캜 175 mL of ascorbic acid aqueous solution (0.9 wt% NaCl, and 0.05 wt% ascorbic acid aqueous solution) was added thereto, and the mixture was freely swollen for 1 hour while stirring at 500 rpm at room temperature. Then, filter paper The aqueous solution was squeezed.

50 ml of the filtered solution was first titrated to pH 10 with 0.1 N caustic soda solution and then back titrated to pH 2.7 with 0.1 N hydrogen chloride solution to calculate the content (wt.%) Of the water soluble component in the superabsorbent resin .

(3) Analysis of increase in water content

The amount of water-soluble component (%) was calculated by substituting the content of the initial water component and the content of the water component after swelling ascorbic acid salt as described above into the formula 1 below.

[Formula 1]

(%) = (Content of water after swelling ascorbic acid - content of water for initial water) / amount of water for initial water * 100

The physical properties of the above examples and comparative examples are shown in Table 1 below.

The content (wt%) of the initial water component Contents of water soluble component (wt%) after swelling of ascorbic acid salt water Water content increase (%) Residual thermal initiator (sodium persulfate) content (ppm) Example 1 12.6 13.9 10 9 Example 2 12.5 14.0 12 14 Example 3 12.7 14.6 15 17 Comparative Example 1 12.8 21.4 67 43 Comparative Example 2 12.8 28.3 121 57 Comparative Example 3 12.8 26.8 109 48

The results are shown in Table 1. Referring to Table 1, the superabsorbent resin of Examples 1 to 3 of the present invention had an increase in water content of 30% or less by the predetermined formula 1, It can be evaluated that the dry state of the surface is well maintained and the rewet phenomenon can be suppressed.

Claims (9)

A crosslinked polymer obtained by polymerizing and cross-linking a monomer composition having an acidic group and at least a part of the acidic groups neutralized, the acrylic acid-based monomer, the internal crosslinking agent, the polymerization initiator, and the ionic binder; And
And a surface cross-linked layer formed on the surface of the cross-linked polymer,
Wherein a water content increase amount measured by the following formula (1) is 30% or less:
[Formula 1]
(%) = (Content of water after swelling ascorbic acid - content of water for initial water) / amount of water for initial water * 100
In Equation (1)
1.0 g of the sample having a particle size of 150 to 850 탆 in the superabsorbent resin was added to 25 g of a salt water (0.9 wt% NaCl aqueous solution), free swelling was carried out at room temperature for 10 minutes, , And 175 ml of saline (0.9 wt% NaCl aqueous solution) was injected thereinto, and the water eluted after free swelling for 1 hour while stirring at 500 rpm at room temperature means the weight percentage of the solvent component,
The content of water-soluble components after swelling of ascorbic acid salt water was determined by adding 1.0 g of a sample having a particle diameter of 150 to 850 탆 in the superabsorbent resin to 25 g of ascorbic acid saline (0.9 wt% NaCl, and 0.05 wt% aqueous ascorbic acid solution) , And then 175 mL of ascorbic acid aqueous solution (0.9 wt% NaCl, and 0.05 wt% ascorbic acid aqueous solution) was added thereto. The mixture was stirred at 500 rpm at room temperature for 1 hour The amount of water eluted after free swelling means the weight percentage of the solvent component.
The method according to claim 1,
Wherein the persulfate-based polymerization initiator remaining in the superabsorbent resin is 300 ppm or less.
The method according to claim 1,
Wherein the ionic binder comprises sodium tripolyphosphate.
The method according to claim 1,
Wherein the ionic binder is contained in an amount of 1 to 50 ppm based on the monomer composition.
Polymerizing a monomer composition having an acidic group and at least a part of the acidic groups neutralized with an acrylic acid-based monomer, an internal cross-linker, a polymerization initiator, and an ionic binder to form a hydrogel-like polymer;
Drying the hydrogel polymer;
Pulverizing the dried polymer; And
Mixing a surface cross-linking solution containing a surface cross-linking agent with the pulverized polymer and performing a surface cross-linking reaction;
Absorbent resin.
6. The method of claim 5,
Wherein the persulfate-based polymerization initiator remaining in the superabsorbent resin is 300 ppm or less.
6. The method of claim 5,
Wherein the ionic binder comprises sodium tripolyphosphate. &Lt; RTI ID = 0.0 &gt; 11. &lt; / RTI &gt;
6. The method of claim 5,
Wherein the ionic binder is contained in an amount of 1 to 50 ppm based on the monomer composition.
6. The method of claim 5,
Wherein the surface cross-linking solution further comprises a sulfite additive.