阅读说明：本技术 超吸收性聚合物组合物及其制备方法 (Superabsorbent polymer composition and method of making the same ) 是由 许荣宰 南大祐 朴宝熙 许成范 于 2018-11-28 设计创作，主要内容包括：本发明涉及超吸收性聚合物及其制备方法。本发明可以提供其中基础树脂中混合有基于环氧化物的表面交联剂和HLB为0至6的疏水性材料,从而通过基础树脂的表面改性而具有改善的再润湿特性和渗透率的超吸收性聚合物。(The present invention may provide a superabsorbent polymer in which an epoxy-based surface cross-linking agent and a hydrophobic material having H L B of 0 to 6 are mixed in a base resin, thereby having improved rewetting characteristics and permeability through surface modification of the base resin.)

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

preparing a base resin in which an acrylic acid-based monomer having at least partially neutralized acid groups is crosslinked with an internal crosslinking agent (step 1);

mixing a hydrophobic material H L B of 0 or more and 6 or less and an epoxide-based surface cross-linking agent with the base resin (step 2), and

the temperature of the mixture of step 2 is raised to perform surface modification of the base resin (step 3).

2. The method for preparing a superabsorbent polymer according to claim 1, wherein in step 2, the hydrophobic material is dry-mixed with the base resin, and then, the epoxy-based surface cross-linking agent is dissolved in water and mixed in a state of a surface cross-linking solution.

3. The method for preparing superabsorbent polymer of claim 1 wherein the hydrophobic material has a melting point lower than the elevated temperature of step 3.

4. The method for preparing superabsorbent polymer of claim 1 wherein the hydrophobic material comprises one or more selected from the group consisting of: glyceryl stearate, ethylene glycol stearate, magnesium stearate, glyceryl laurate, sorbitan stearate, sorbitan trioleate, and PEG-4 dilaurate.

5. The method for preparing a superabsorbent polymer of claim 1 wherein the hydrophobic material is mixed in an amount of 0.02 parts by weight to 0.5 parts by weight based on 100 parts by weight of the base resin.

6. The method for preparing a superabsorbent polymer of claim 1 wherein the epoxide-based surface cross-linking agent comprises one or more selected from the group consisting of: ethylene glycol diglycidyl ether, diethylene glycol diglycidyl ether, triethylene glycol diglycidyl ether, tetraethylene glycol diglycidyl ether, glycerol polyglycidyl ether, and sorbitol polyglycidyl ether.

7. The method for preparing a superabsorbent polymer of claim 1 wherein the epoxy-based surface cross-linking agent is mixed in an amount of 0.005 to 0.2 parts by weight based on 100 parts by weight of the base resin.

8. The method for preparing superabsorbent polymer of claim 1 wherein the step 3 is carried out at a temperature of 120 ℃ to 190 ℃.

9. The method for preparing a superabsorbent polymer of claim 1 wherein the step 1 comprises:

polymerizing a monomer composition comprising an acrylic acid-based monomer having at least partially neutralized acid groups, an internal crosslinking agent, and a polymerization initiator to form a hydrogel polymer;

drying the hydrogel polymer;

grinding the dried polymer; and

the milled polymer was sieved.

10. The method for preparing a superabsorbent polymer of claim 1 wherein the superabsorbent polymer has a vortex time of 40 seconds or less.

11. The method for preparing a superabsorbent polymer of claim 1 wherein the superabsorbent polymer has a permeability (in seconds) measured according to the following equation 1 of 35 seconds or less:

[ equation 1]

Permeability (second) ═ T1-B

In the case of the equation 1, the,

t1 is the time taken until the height of the liquid level drops from 40ml to 20ml after 0.2 ± 0.0005g of the sieved superabsorbent polymer sample (30# to 50#) was put into a chromatographic column and saline was added to a volume of 50ml and then, it was left to stand for 30 minutes; and B is the time taken in the column filled with brine until the height of the liquid level decreased from 40ml to 20 ml.

12. A superabsorbent polymer comprising:

a base resin comprising a crosslinked polymer crosslinked by an acrylic acid-based monomer having at least partially neutralized acid groups therein; and

a surface-modified layer formed on the surface of the base resin particle, in which the crosslinked polymer is additionally crosslinked by an epoxy-based surface crosslinking agent,

wherein the surface modification layer comprises a hydrophobic material with H L B being 0 or greater and 6 or less.

13. The superabsorbent polymer of claim 12 wherein the permeability (in seconds) measured according to equation 1 below is 35 seconds or less:

[ equation 1]

Permeability (second) ═ T1-B

In the case of the equation 1, the,

t1 is the time taken until the height of the liquid level drops from 40ml to 20ml after 0.2 ± 0.0005g of the sieved superabsorbent polymer sample (30# to 50#) was put into a chromatographic column and saline was added to a volume of 50ml and then, it was left to stand for 30 minutes; and B is the time taken in the column filled with brine until the height of the liquid level decreased from 40ml to 20 ml.

14. The superabsorbent polymer of claim 12 wherein the hydrophobic material comprises one or more selected from the group consisting of: glyceryl stearate, ethylene glycol stearate, magnesium stearate, glyceryl laurate, sorbitan stearate, sorbitan trioleate, and PEG-4 dilaurate.

15. The superabsorbent polymer of claim 12 wherein the epoxide-based surface cross-linker comprises one or more selected from the group consisting of: ethylene glycol diglycidyl ether, diethylene glycol diglycidyl ether, triethylene glycol diglycidyl ether, tetraethylene glycol diglycidyl ether, glycerol polyglycidyl ether, and sorbitol polyglycidyl ether.

16. The superabsorbent polymer of claim 12 wherein the superabsorbent polymer has a vortex time of 40 seconds or less.

17. The superabsorbent polymer of claim 12 wherein the superabsorbent polymer has a rewet characteristic (tap water short term rewet without pressure) of 2.0g or less, the rewet characteristic being defined as: the weight of water seeped from the superabsorbent polymer to the filter paper after 1g of the superabsorbent polymer was immersed in 100g of tap water to swell for 10 minutes, and then the swollen superabsorbent polymer was placed on the filter paper from 1 hour from the initial time when the superabsorbent polymer was immersed in the tap water.

18. The superabsorbent polymer of claim 12 wherein the superabsorbent polymer has a rewet characteristic (tap water long term rewet under pressure) of 1.2g or less, the rewet characteristic being defined as: the weight of water that seeped from the superabsorbent polymer to the filter paper after 4g of the superabsorbent polymer was immersed in 200g of tap water to swell for 6 hours, and then the swollen superabsorbent polymer was placed on the filter paper under a pressure of 0.75psi for 1 minute.

Technical Field

Cross Reference to Related Applications

This application claims the benefits of korean patent application No. 10-2017-.

The present invention relates to superabsorbent polymers and methods of making the same. More particularly, the present invention relates to superabsorbent polymers having improved rewetting characteristics and absorption rate and a method of preparing the same.

Background

Superabsorbent polymers (SAP) are synthetic polymer materials that can absorb moisture 500 to 1000 times their own weight, and are also variously called superabsorbent materials (SAM), Absorbent Gel Materials (AGM), etc. according to development companies. Superabsorbent polymers are beginning to be commercialized as sanitary articles, and are currently widely used as sanitary articles (e.g., disposable diapers and the like), water-retaining materials for soil, water-stopping materials for civil engineering and construction, sheets for growing seedlings, freshness-keeping agents in the field of food circulation, hot compress materials, and the like.

In most cases, such superabsorbent polymers are widely used in the field of sanitary products (e.g., diapers or sanitary napkins, etc.), and for such uses, it is required that the superabsorbent polymers exhibit high moisture absorption capacity, etc., and absorbed moisture should not escape even under external pressure, and further, that they should properly maintain their shape even when they absorb moisture and expand in volume (swell), thereby exhibiting excellent permeability.

However, it is known that it is difficult to simultaneously improve the Centrifuge Retention Capacity (CRC) indicating the basic absorption capacity and the water retention capacity of the superabsorbent polymer and the Absorbency Under Pressure (AUP) indicating the characteristic of retaining the absorbed water despite the external pressure. In the case where the overall crosslinking density of the superabsorbent polymer is controlled to be low, the centrifuge retention capacity may become relatively high, but the crosslinked structure may become loose and the gel strength may be reduced, thus deteriorating the absorption rate under pressure. Conversely, in the case where the crosslinking density is controlled to be high to improve the absorption rate under pressure, it may become difficult to absorb moisture between the dense crosslinked structures, thus deteriorating the centrifuge retention capacity. For these reasons, there is a limitation in providing superabsorbent polymers having both improved centrifuge retention capacity and improved absorption under pressure.

However, with recent thinning of sanitary articles such as diapers and sanitary napkins, higher absorption performance of superabsorbent polymers is required. Among them, an important problem is to improve the contradictory properties of both the centrifuge retention capacity and the absorption rate under pressure, and to improve the permeability and the like.

Also, in the case of sanitary articles such as diapers or sanitary napkins, pressure can be applied by the weight of the user. In particular, if a super absorbent polymer applied to a diaper or a sanitary napkin absorbs liquid and then pressure is applied by the weight of a user, a rewet phenomenon in which a portion of the liquid absorbed in the super absorbent polymer oozes out again and urine leaks out may occur.

Therefore, many attempts have been made to suppress such rewetting phenomenon. However, no specific means for effectively suppressing the rewetting phenomenon has been proposed.

Disclosure of Invention

Technical problem

In order to solve the problems of the prior art, it is an object of the present invention to provide a superabsorbent polymer in which rewetting and urine leakage are suppressed, and a method of preparing the same.

Technical scheme

To achieve the object, one aspect of the present invention provides a method for preparing a superabsorbent polymer, comprising the steps of:

preparing a base resin in which an acrylic acid-based monomer having at least partially neutralized acid groups is crosslinked with an internal crosslinking agent (step 1);

mixing a hydrophobic material H L B of 0 or more and 6 or less and an epoxide-based surface cross-linking agent with a base resin (step 2), and

the temperature of the mixture of step 2 is raised to perform surface modification of the base resin (step 3).

Also, another aspect of the present invention provides a superabsorbent polymer comprising:

a base resin comprising a crosslinked polymer crosslinked by an acrylic acid-based monomer having at least partially neutralized acid groups therein; and

a surface-modified layer formed on the surface of the base resin particle, in which surface-modified layer the crosslinked polymer is additionally crosslinked by an epoxide-based surface crosslinking agent,

wherein the surface modification layer comprises a hydrophobic material with H L B being 0 or greater and 6 or less.

Advantageous effects

According to the superabsorbent polymer and the preparation method of the present invention, a superabsorbent polymer exhibiting excellent absorption characteristics in which rewetting and urine leakage are suppressed can be provided.

Detailed Description

While the present invention is susceptible to various modifications and alternative forms, specific examples will be shown and described in detail below. It should be understood, however, that these are not intended to limit the invention to the particular disclosure, and that the invention includes all modifications, equivalents, and alternatives thereof without departing from the spirit and technical scope of the invention.

Hereinafter, a method for preparing a superabsorbent polymer composition according to an embodiment of the present invention will be explained in more detail.

The method for preparing a superabsorbent polymer according to one embodiment of the present invention includes the steps of:

preparing a base resin in which an acrylic acid-based monomer having at least partially neutralized acid groups is crosslinked with an internal crosslinking agent (step 1);

mixing a hydrophobic material H L B of 0 or more and 6 or less and an epoxide-based surface cross-linking agent with a base resin (step 2), and

the temperature of the mixture of step 2 is raised to perform surface modification of the base resin (step 3).

Throughout the specification, "base resin" or "base resin powder" means a polymer of a water-soluble ethylenically unsaturated monomer which is made into a granular or powder form by drying and grinding, and is not subjected to the surface modification or surface crosslinking step described below.

The hydrogel polymer obtained by polymerization of the acrylic acid-based monomer may be subjected to drying, grinding, sieving, surface crosslinking, etc., and commercialized as a powdered superabsorbent polymer product.

Recently, how long the surface can be kept dry when the diaper is actually used and the absorption characteristics of the super absorbent polymer such as centrifuge retention capacity, permeability, etc. have become important measures for evaluating the diaper characteristics.

It was confirmed that the superabsorbent polymer obtained by the preparation method of one embodiment has excellent centrifuge retention capacity, absorption rate under pressure, permeability, and the like, thus exhibiting excellent absorption properties, remains dry even after being swollen by saline, and can effectively prevent rewetting and urine leakage.

In the method of preparing the superabsorbent polymer of the present invention, raw materials of the superabsorbent polymer, i.e., a monomer composition including an acrylic-based monomer having an acid group at least partially neutralized, an internal crosslinking agent, and a polymerization initiator, are polymerized to obtain a hydrogel polymer, and the hydrogel polymer is dried, ground, and sieved to prepare a base resin (step 1).

Hereinafter, it will be explained in more detail.

The monomer composition which is the starting material for the superabsorbent polymer comprises an acrylic acid-based monomer having at least partially neutralized acid groups and a polymerization initiator.

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

[ chemical formula 1]

R¹-COOM¹

In the chemical formula 1, the first and second,

R¹is a C2-5 hydrocarbon group containing an unsaturated bond,

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

Preferably, the acrylic-based 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.

Here, the acrylic acid-based monomer may have an acid group, and at least a part of the acid group may be neutralized. Preferably, monomers partially neutralized with an alkaline substance such as sodium hydroxide, potassium hydroxide, ammonium hydroxide, etc. may be used. Here, the neutralization degree of the acrylic acid-based monomer may be 40 to 95 mol%, or 40 to 80 mol%, or 45 to 75 mol%. Although the range of the neutralization degree may vary depending on the final characteristics, if the neutralization degree is too high, the neutralized monomer may precipitate, thus rendering smooth polymerization difficult, and conversely, if the neutralization degree is too low, the absorption capacity of the polymer may be significantly reduced, and the polymer may exhibit rubber-like characteristics, which are difficult to handle.

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 of the superabsorbent polymer and the solvent, and may be controlled to an appropriate concentration in consideration of polymerization time, reaction conditions, and the like. However, if the concentration of the monomer becomes too low, the yield of the superabsorbent polymer may be reduced and economic efficiency may be lowered, and if it becomes too high, process problems such as partial precipitation of the monomer or low grinding efficiency during grinding of the polymerized hydrogel polymer may occur and the properties of the superabsorbent polymer may be deteriorated.

The polymerization initiator used in the preparation method of the superabsorbent polymer is not particularly limited as long as it is generally used for preparing the superabsorbent polymer.

Specifically, as the polymerization initiator, a thermal polymerization initiator or a photopolymerization initiator by UV irradiation may be used according to the polymerization method. However, even in the case of photopolymerization, since a certain amount of heat is generated by UV irradiation or the like and some degree of heat is generated according to the progress of the exothermic polymerization reaction, a thermal polymerization initiator may be additionally included.

The photopolymerization initiator is not limited in its constitution as long as it is a compound capable of forming radicals by light such as UV.

As the photopolymerization initiator, one or more selected from the group consisting of benzoin ethers, dialkacetophenone, hydroxyalkyl ketones, phenyl glyoxylate, benzyl dimethyl ketal, acyl phosphine and α -aminoketone may be used, wherein as the acyl phosphine, commercially available lucirin TPO may be included, i.e., 2,4, 6-trimethyl-benzoyl-trimethylphosphine oxide may be used.

The photopolymerization initiator may be included at a concentration of about 0.01 wt% to about 1.0 wt%, based on the monomer composition. If the concentration of the photopolymerization initiator is too low, the polymerization speed may become slow, and if it is too high, the molecular weight of the superabsorbent polymer may be small and the characteristics may become irregular.

Further, as the thermal polymerization initiator, a compound selected from persulfate initiators, azo initiators and per initiators can be usedAt least one of hydrogen oxide and ascorbic acid. Specific examples of the persulfate initiator may include sodium persulfate (Na)₂S₂O₈) Potassium persulfate (K)₂S₂O₈) Ammonium persulfate ((NH)₄)₂S₂O₈) Etc., and specific examples of the azo initiator may include 2, 2-azobis (2-amidinopropane) dihydrochloride, 2-azobis- (N, N-dimethylene) isobutyramidine dihydrochloride, 2- (carbamoylazo) isobutyronitrile, 2-azobis [2- (2-imidazolin-2-yl) propane ] dihydrochloride]Dihydrochloride, 4-azobis- (4-cyanovaleric acid), and the like. Further different thermal initiators are described in Odian's book "principles of Polymerization (Wiley, 1981)" page 203, but are not limited to the examples mentioned above.

According to an embodiment of the present invention, the monomer composition may further include an internal crosslinking agent as a raw material of the superabsorbent polymer. As the internal crosslinking agent, a crosslinking agent having one or more functional groups capable of reacting with the acrylic acid-based monomer and having one or more ethylenically unsaturated groups; or a crosslinking agent having two or more functional groups capable of reacting with a substituent of the acrylic acid-based monomer and/or a substituent formed by hydrolysis of the monomer.

The internal crosslinking agent is used for internal crosslinking of a polymer in which an acrylic acid-based monomer is polymerized, and is different from a surface crosslinking agent used for surface crosslinking of the polymer.

Specific examples of the internal crosslinking agent may include one or more selected from the group consisting of: n, N' -methylenebisacrylamide, trimethylolpropane tri (meth) acrylate, ethylene glycol di (meth) acrylate, polyethylene glycol di (meth) acrylate, propylene glycol di (meth) acrylate, polypropylene glycol di (meth) acrylate, butane glycol di (meth) acrylate, butanediol di (meth) acrylate, diethylene glycol di (meth) acrylate, hexanediol di (meth) acrylate, triethylene glycol di (meth) acrylate, tripropylene glycol di (meth) acrylate, tetraethylene glycol di (meth) acrylate, dipentaerythritol pentaacrylate, glycerol tri (meth) acrylate, pentaerythritol tetraacrylate, triarylamine, ethylene glycol diglycidyl ether, propylene glycol, glycerol, and ethylene carbonate.

Such internal crosslinking agents may be included at a concentration of about 0.01 wt% to about 1.0 wt% based on the monomer composition to crosslink the polymerized polymer.

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

The above-mentioned raw materials such as the acrylic acid-based monomer having an acid group at least partially neutralized, the photopolymerization initiator, the thermal polymerization initiator, the internal crosslinking agent, and the additive may be prepared in the form of a solution dissolved in a solvent.

Here, the solvent that can be used is not limited in its constitution as long as it can dissolve or disperse the above-mentioned raw materials, and for example, one or more selected from the following may be used alone or in combination: water, ethanol, ethylene glycol, diethylene glycol, triethylene glycol, 1, 4-butane diol, propylene glycol, ethylene glycol monobutyl ether, propylene glycol monomethyl ether acetate, methyl ethyl ketone, acetone, methyl amyl ketone, cyclohexanone, cyclopentanone, diethylene glycol monomethyl ether, diethylene glycol diethyl ether, toluene, xylene, butyrolactone, carbitol, methyl cellosolve acetate, N-dimethylacetamide, and the like.

The solvent may be included in the remaining amount other than the above components based on the total content of the monomer composition.

Meanwhile, a method of forming a hydrogel polymer by thermally or photopolymerizing a monomer composition is not particularly limited in its constitution as long as it is a commonly used polymerization method.

Specifically, polymerization methods are largely classified into thermal polymerization and photopolymerization according to energy sources. In general, thermal polymerization may be performed in a reactor equipped with a stirring shaft, such as a kneader, and photopolymerization may be performed in a reactor equipped with a movable conveyor belt, but the above polymerization method is merely an example, and the present invention is not limited thereto.

For example, the hydrogel polymer can be obtained by: the above monomer composition is introduced into a reactor equipped with a stirring shaft (for example, a kneader), and hot air is supplied or the reactor is heated to perform thermal polymerization. Here, the size of the hydrogel polymer discharged to the outlet of the reactor may be several centimeters to several millimeters, depending on the shape of the stirring shaft provided in the reactor. Specifically, the size of the obtained hydrogel polymer may vary depending on the concentration of the introduced monomer composition, the introduction speed, and the like, and in general, a hydrogel polymer having a (weight average) particle diameter of 2mm to 50mm may be obtained.

Also, in the case where photopolymerization of the monomer composition is carried out in a reactor equipped with a movable conveyor belt as described above, a hydrogel polymer in the form of a sheet having the width of the belt can be obtained. Here, the thickness of the sheet may vary depending on the concentration of the introduced monomer composition and the introduction speed, but it is preferable to feed the monomer composition so as to obtain the polymer in the form of a sheet having a thickness of about 0.5cm to about 5 cm. If the monomer composition is fed such that the thickness of the sheet polymer may become too thin, production efficiency may be reduced, and if the thickness of the sheet polymer is greater than 5cm, polymerization may not uniformly occur throughout the thickness due to an excessively thick thickness.

Here, the hydrogel polymer obtained by such a method may have a water content of about 40% by weight to about 80% by weight. Throughout the specification, "water content" is a content of moisture based on the total weight of the hydrogel polymer, and it means a value obtained by subtracting the weight of the polymer in a dry state from the weight of the hydrogel polymer. Specifically, it is defined as a value calculated by measuring the weight loss according to the evaporation of moisture in the polymer while raising the temperature of the polymer by infrared heating to dry. At this time, the drying conditions were set such that the temperature was raised from room temperature to about 180 ℃, then maintained at 180 ℃, and the total drying time was 20 minutes, 5 minutes including the temperature raising step.

Subsequently, the obtained hydrogel polymer was dried.

Wherein, a coarse grinding step may be further performed before drying the hydrogel polymer to improve drying efficiency.

Here, the grinder that can be used for the rough grinding is not limited in terms of composition, but specifically, one selected from the following may be used: vertical mills, turbo cutters, turbo grinders, rotary shredders (rotary mill), shredders (cutter mill), disc mills, chip breakers, crushers, shredders (choppers), disc cutters, but the mills are not limited thereto.

The particle size of the hydrogel polymer may be controlled to be about 2mm to about 10mm by the coarse grinding step.

Due to the high water content of the hydrogel polymer, grinding to a particle size of less than 2mm is technically not easy and leads to agglomeration between the ground particles. Meanwhile, if ground to a particle size of more than 10mm, the effect of improving efficiency in a subsequent drying step may not be significant.

The hydrogel polymer that has been coarsely ground as described above or the hydrogel polymer that has not been subjected to the coarse grinding step immediately after polymerization is dried. Here, the drying temperature may be about 150 ℃ to about 250 ℃. If the drying temperature is less than 150 ℃, the drying time may become excessively long and the characteristics of the finally formed superabsorbent polymer may be deteriorated, whereas if the drying temperature is more than 250 ℃, only the surface of the polymer may be dried to generate fine powder in a subsequent grinding process and the characteristics of the finally formed superabsorbent polymer may be deteriorated. Thus, drying is preferably carried out at a temperature of from about 150 ℃ to about 200 ℃, more preferably from about 160 ℃ to about 180 ℃.

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

Also, the drying method is not limited in composition as long as it is generally used as a drying process of the hydrogel polymer. Specifically, the drying step may be performed by hot air supply, infrared irradiation, ultrahigh frequency irradiation, UV irradiation, or the like. The polymer dried by such a process may exhibit a moisture content of about 0.1 wt% to about 10 wt%.

Next, the dried polymer obtained by the drying step is ground.

The particle size of the polymer powder obtained after the milling step may be 150 μm to 850 μm. As a grinder for grinding to such a particle size, specifically, a pin mill, a hammer mill, a screw mill, a roll mill, a disc mill, a jog mill, or the like can be used, but the grinder is not limited thereto.

Also, in order to control the characteristics of the superabsorbent polymer powder finally classified by product after the grinding step, the polymer powder obtained after grinding may be subjected to a separate sieving process according to particle size.

Next, a hydrophobic material in which H L B is 0 or more and 6 or less and an epoxy-based surface cross-linking agent are mixed with the base resin (step 2).

In a common method of preparing superabsorbent polymers, a dried and ground polymer (i.e., a base resin) is mixed with a surface crosslinking solution containing a surface crosslinking agent, and then the mixture is heated to increase the temperature, thereby performing a surface crosslinking reaction of the ground polymer.

The surface crosslinking step is a step of causing a crosslinking reaction on the surface of the ground polymer in the presence of a surface crosslinking agent, thereby forming a superabsorbent polymer having more improved characteristics. By surface crosslinking, a surface crosslinked layer (surface modified layer) is formed on the surface of the ground polymer particles.

Generally, since the surface cross-linking agent is coated on the surface of the superabsorbent polymer particles, a surface cross-linking reaction occurs on the surface of the superabsorbent polymer particles, and it improves the cross-linking property on the surface of the particles without substantially affecting the inside of the particles. Thus, surface crosslinked superabsorbent polymer particles have a higher degree of crosslinking around the surface than inside.

Meanwhile, as the surface crosslinking agent, a compound capable of reacting with a functional group of the polymer may be used, and for example, a polyol compound, an epoxy compound, a polyamine compound, a halogenated epoxy compound, a condensation product of a halogenated epoxy compound, a polyamine compound, a halogenated epoxy compound, a polymer,an oxazoline compound, a polyvalent metal salt, or an alkylene carbonate compound.

Meanwhile, according to the preparation method of the present invention, an epoxy-based surface cross-linking agent is used, and it is determined that the absorption capacity can be further improved without deteriorating the rewetting characteristics of the superabsorbent polymer by using a specific epoxy-based surface cross-linking agent.

Examples of the epoxy-based surface cross-linking agent satisfying such requirements may include ethylene glycol diglycidyl ether, diethylene glycol diglycidyl ether, triethylene glycol diglycidyl ether, tetraethylene glycol diglycidyl ether, glycerol polyglycidyl ether, sorbitol polyglycidyl ether, and the like.

The content of the epoxy-based surface cross-linking agent added may be about 0.005 parts by weight or more, or about 0.01 parts by weight or more, or about 0.02 parts by weight or more, and about 0.2 parts by weight or less, or about 0.1 parts by weight or less, or 0.05 parts by weight or less, based on 100 parts by weight of the base resin.

If the content of the epoxy-based surface cross-linking agent is too low, absorption characteristics (e.g., absorption rate under load, permeability, etc.) may be deteriorated due to too low cross-linking density of the surface cross-linked layer, and if it is too high, rewetting characteristics may be deteriorated due to the progress of an excessive surface cross-linking reaction.

When the epoxide-based surface cross-linking agent is added, it may be added in the form of a surface cross-linking solution by additionally mixing together with water. The addition of water has the advantage that the surface cross-linking agent can be homogeneously dispersed in the polymer. Among them, the content of the added water may preferably be about 1 part by weight to about 10 parts by weight based on 100 parts by weight of the polymer, in order to cause uniform dispersion of the surface cross-linking agent, prevent agglomeration of the polymer powder, and optimize the surface penetration depth of the surface cross-linking agent.

Meanwhile, in addition to the above surface crosslinking agent, a polyvalent metal salt, for example, one or more selected from the group consisting of: aluminum salts, more specifically, sulfates of aluminum; a potassium salt; an ammonium salt; sodium salt and hydrochloride salt.

By additionally using such a polyvalent metal salt, the permeability of the superabsorbent polymer prepared by the method of one embodiment can be further improved. Such a polyvalent metal salt may be added to the surface crosslinking solution together with the surface crosslinking agent, and may be used in an amount of about 0.01 to 4 parts by weight, based on 100 parts by weight of the base resin.

Meanwhile, although the absorption rate and permeability under load can be improved by the surface crosslinking reaction, further supplement of the rewetting characteristics is required.

According to the preparation method of the present invention, before the surface cross-linking agent is mixed with the base resin and the temperature is increased to perform the surface cross-linking reaction, the hydrophobic material may be mixed with the base resin, thereby further improving the rewetting characteristics. Also, the surface crosslinking efficiency can be improved, and thus the absorption speed and permeability can be further improved, as compared with a resin not using a hydrophobic material.

As the hydrophobic material, a material satisfying that the lower limit of H L B is 0 or more, or 1 or more, or 2 or more, and the upper limit is 6 or less, or 5 or less, or 5.5 or less may be used, and since the hydrophobic material should be melted during the surface crosslinking reaction and be located in the surface modification layer of the base resin, a material having a melting point lower than the surface crosslinking reaction temperature may be used.

Examples of the hydrophobic material that may be used may include glyceryl stearate, ethylene glycol stearate, magnesium stearate, glyceryl laurate, sorbitan stearate, sorbitan trioleate, PEG-4 dilaurate, and the like, and preferably, glyceryl stearate or glyceryl laurate may be used, but the present invention is not limited thereto.

The hydrophobic material is distributed in the surface modification layer of the surface of the base resin, and it can prevent the swollen polymer particles from aggregating or agglomerating due to increased pressure when the superabsorbent polymer absorbs liquid and swells, and can provide hydrophobicity to the surface, thereby promoting penetration and diffusion of the liquid. Thus, it may help to improve the rewet characteristics of the superabsorbent polymer.

The hydrophobic material may be mixed in the following amounts based on 100 parts by weight of the base resin: about 0.02 parts by weight or greater, or about 0.025 parts by weight or greater, or about 0.05 parts by weight or greater, and about 0.5 parts by weight or less, or about 0.3 parts by weight or less, or about 0.1 parts by weight or less. If the content of the hydrophobic material is less than 0.02 parts by weight, it may be insufficient for improving the rewetting characteristics, whereas if the hydrophobic material is included excessively in excess of 0.5 parts by weight, the base resin and the hydrophobic material may be separated from each other, and thus, a rewetting characteristics improving effect may not be obtained or the hydrophobic material may act as impurities. Therefore, the above range is preferable.

The method of mixing the hydrophobic material is not particularly limited as long as it can be uniformly mixed with the base resin.

For example, the hydrophobic material may be dry blended with the base resin prior to mixing the surface crosslinking solution comprising the epoxide-based surface crosslinking agent, or it may be dispersed in the surface crosslinking solution together with the surface crosslinking agent and mixed with the base resin. Alternatively, the hydrophobic material may be heated above the melting point and mixed in a solution state separately from the surface crosslinking solution.

Next, the mixture of the base resin and the epoxy-based surface cross-linking agent is heated to raise the temperature, thereby performing a surface modification step of the base resin (step 3).

The surface modification step may be carried out by heating at a temperature of from about 120 ℃ to about 190 ℃, preferably from about 130 ℃ to about 180 ℃ for from about 10 minutes to about 90 minutes, preferably from about 20 minutes to about 70 minutes. If the crosslinking reaction temperature is less than 120 ℃ or the reaction time is too short, the surface crosslinking reaction may not properly occur, and thus, the permeability may be decreased, and if the temperature is more than 190 ℃ or the reaction time is too long, the centrifuge retention capacity may be deteriorated.

The temperature raising means for the surface modification reaction is not particularly limited. The heating medium may be supplied, or the heat source may be directly supplied to heat. Here, the kind of the heating medium that may be used may include a fluid whose temperature is increased such as steam, hot air, hot oil, etc., but is not limited thereto, and the temperature of the supplied heating medium may be appropriately selected in consideration of the manner of the heating medium, the temperature increase speed, and the temperature to be increased. Meanwhile, the heat source directly supplied may include electric heating, gas heating, etc., but is not limited thereto.

Through the surface modification step, on the surface of the base resin, a surface cross-linked structure formed by the reaction of the epoxy-based surface cross-linking agent with the functional group of the base resin may be formed, and a surface modification layer in which the above-described hydrophobic material is uniformly distributed in the surface cross-linked structure may be formed.

Therefore, the superabsorbent polymer prepared by the preparation method of the present invention can have improved rewetting characteristics and initial absorption rate without deteriorating characteristics such as centrifuge retention capacity and absorption under load due to such a surface modification layer.

Thus, according to another embodiment of the present invention, there is provided a superabsorbent polymer comprising a base resin comprising a crosslinked polymer crosslinked by an acrylic acid-based monomer having an acid group therein at least partially neutralized, and a surface-modified layer formed on the surface of the base resin particles, in which the crosslinked polymer is additionally crosslinked by an epoxide-based surface crosslinking agent, wherein the surface-modified layer comprises a hydrophobic material having H L B of 0 or more and 6 or less.

The specific method of preparation and the characteristics of the superabsorbent polymer are as described above.

The Centrifuge Retention Capacity (CRC) of the superabsorbent polymer, measured according to EDANA method WSP 241.3, may be in the following range: about 28g/g or more, or about 29g/g or more, or about 30g/g or more, and about 40g/g or less, or about 38g/g or less, or about 35g/g or less.

Also, the superabsorbent polymer's Absorbency Under Pressure (AUP) at 0.3psi as measured according to EDANA method WSP 242.3 may be in the following range: about 20g/g or more, or about 23g/g or more, or about 25g/g or more, and about 37g/g or less, or about 35g/g or less, or about 32g/g or less.

Also, the superabsorbent polymer may have a vortex time of 40 seconds or less, or 35 seconds or less, or about 30 seconds or less, or about 28 seconds or less. The vortex time is more excellent as it is smaller, and therefore, the lower limit of the vortex time is theoretically 0 second, but it may be, for example, about 5 seconds or longer, or about 10 seconds or longer, or about 12 seconds or longer.

The vortex time means the time (unit: seconds) taken until the liquid vortex disappears due to rapid absorption when the superabsorbent polymer is added to the salt solution and stirred, and it is considered that the superabsorbent polymer has a faster initial absorption speed as the time is shorter.

Also, the permeability (unit: second) of the superabsorbent polymer, measured according to equation 1 below, may be about 35 seconds or less, or about 30 seconds or less, or about 27 seconds or less. As the value is smaller, the permeability is more excellent, and therefore, the theoretical upper and lower limits are 0 second, but for example, it may be about 5 seconds or longer, or about 10 seconds or longer, or about 12 seconds or longer.

[ equation 1]

Permeability (second) ═ T1-B

In the case of the equation 1, the,

t1 is the time taken until the height of the liquid level drops from 40ml to 20ml after 0.2 ± 0.0005g of a sieved superabsorbent polymer sample (30# to 50#) was placed in a chromatographic column and saline was added to a volume of 50ml, which was then left to stand for 30 minutes; b is the time taken in the column filled with brine until the height of the liquid level decreased from 40ml to 20 ml.

Also, the superabsorbent polymer may exhibit more improved rewetting characteristics while exhibiting excellent absorption characteristics.

More specifically, the rewet characteristics (tap water short-term rewet without pressure) may be 2.0g or less, 1.5g or less, or 1.4g or less, or 1.3g or less, the rewet characteristics being defined as follows: after 1g of the superabsorbent polymer was immersed in 100g of tap water to swell for 10 minutes, and then the swollen superabsorbent polymer was placed on filter paper for 1 hour from the initial time when the superabsorbent polymer was immersed in the tap water, the weight of water that exudes from the superabsorbent polymer to the filter paper. The rewetting characteristics are more excellent as the weight of water is smaller, and therefore, the theoretical upper and lower limits are 0g, but it may be 0.1g or more, or 0.3g or more, or 0.5g or more, for example.

Also, the rewet characteristics (tap water long-term rewet under pressure) may be 1.2g or less, or 1.1g or less, or 1.0g or less, as defined below: the weight of water that seeped from the superabsorbent polymer to the filter paper after 4g of the superabsorbent polymer was immersed in 200g of tap water to swell for 6 hours and then the swollen superabsorbent polymer was placed on the filter paper under a pressure of 0.75psi for 1 minute. The rewetting characteristics are more excellent as the weight of water is smaller, and therefore, the theoretical upper and lower limits are 0g, but it may be 0.1g or more, or 0.3g or more, or 0.5g or more, for example.

In the rewetting characteristics evaluation, the conductivity of the tap water used may be 170. mu.S/cm to 180. mu.S/cm. Since the conductivity of tap water significantly affects the measured properties, it is necessary to use tap water having an equivalent level of conductivity to measure properties such as rewet properties.

As described above, the superabsorbent polymer of the present invention has excellent absorption capacity, and rewet and urine leakage can be suppressed even if it absorbs a large amount of urine.

The present invention will be explained in more detail in the following examples. However, these examples are presented only as illustrations of the present invention, and the scope of the present invention is not limited thereto.

< example >

Preparation of superabsorbent polymersPrepare for