Method for producing superabsorbent resin particles

Abstract

The present invention provides a method for producing superabsorbent resin particles, comprising the steps of: supplying a composition containing a water-soluble ethylenically unsaturated monomer having at least a partially neutralized acidic group, an internal crosslinking agent, and a polymerization initiator to a polymerization reactor and carrying out a polymerization reaction to obtain a hydrogel polymer; primarily drying the hydrogel polymer in a hot air dryer to obtain a primary dried body; secondarily drying the primary dried body in a heat conduction dryer to obtain a secondary dried body; and pulverizing the secondary dried body to obtain superabsorbent resin particles.

Description

Technical Field

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0121702 filed on September 26, 2022, and all the contents disclosed in the document of the Korean patent application are incorporated herein by reference in their entirety.

[0002] The present invention relates to a method for manufacturing superabsorbent resin particles, and more particularly, to a method for manufacturing superabsorbent resin particles with improved drying uniformity and physical properties using two types of dryers.

Background Art

[0003] A superabsorbent polymer (SAP) is a synthetic polymer material having a function of absorbing about 500 to 1,000 times its own weight of water, and different names such as SAM (Super Absorbency Material) and AGM (Absorbent Gel Material) are given for each development company. The superabsorbent resin as described above has begun to be put into practical use as a sanitary product, and currently, it is widely used as a soil moisture retainer for horticulture, a water stop material for civil engineering and construction, a seedling raising sheet, a freshness maintainer in the food distribution field, and a material for steaming, etc., and is mainly used in the field of sanitary materials such as diapers and sanitary napkins.

[0004] On the other hand, superabsorbent resin particles can generally be manufactured through polymerization, drying, pulverization, and surface crosslinking steps. However, when the drying uniformity decreases in the drying step, over-drying is performed, thereby reducing the physical properties and generating a large amount of fine powder in the pulverization step.

[0005] Therefore, there is a need for a drying method that can achieve high drying uniformity in the drying step, improve the physical properties of the finally manufactured superabsorbent resin particles, and reduce the content of fine powder.

Summary of the Invention

Problems to be Solved by the Invention

[0006] The problem to be solved by the present invention is to perform a drying step with a high drying uniformity in order to solve the problems mentioned in the background art of the above invention, reduce the amount of fine powder generated in the pulverization step, and ensure the physical properties of the superabsorbent resin particles as the final product. That is, to provide a method for producing superabsorbent resin particles.

Means for Solving the Problems

[0007] According to one embodiment of the present invention for solving the above problems, the present invention supplies a composition containing a water-soluble ethylenically unsaturated monomer having at least partially neutralized acidic groups, an internal cross-linking agent, and a polymerization initiator to a polymerization reactor, and performs a polymerization reaction to obtain a water-containing gel polymer; a step of primary drying the water-containing gel polymer with a hot air dryer to obtain a primary dried product; a step of secondary drying the primary dried product with a heat conduction dryer to obtain a secondary dried product; and a step of pulverizing the secondary dried product to obtain superabsorbent resin particles.

Effects of the Invention

[0008] According to the method for producing superabsorbent resin particles of the present invention, the drying step can be performed by primary drying and secondary drying. By performing the primary drying with a hot air dryer, the pores of the dried product can be maintained, and by performing the secondary drying with a heat conduction dryer, the drying uniformity can be improved. Thereby, the problem that an undried product is introduced into the next pulverization step can be solved, and the amount of fine powder generated during pulverization can be reduced. Also, the physical properties of the final superabsorbent resin particles can be improved.

Brief Description of the Drawings

[0009]

Figure 1

Modes for Carrying Out the Invention

[0010] The terms and words used in the description and claims of the present invention should not be construed as being limited to their ordinary or dictionary meanings. The inventors should interpret them in accordance with the meaning and concept consistent with the technical idea of the present invention, in accordance with the principle that they can appropriately define the concept of the terms in order to explain their invention in the best way.

[0011] Hereinafter, in order to facilitate the understanding of the present invention, the present invention will be described in more detail with reference to FIG. 1.

[0012] The method for producing superabsorbent resin particles according to an embodiment of the present invention includes supplying a composition containing a water-soluble ethylenically unsaturated monomer having at least partially neutralized acidic groups, an internal crosslinking agent, and a polymerization initiator to a polymerization reactor, and subjecting the composition to a polymerization reaction to obtain a hydrogel polymer; a step of primary drying the hydrogel polymer with a hot air dryer to obtain a primary dried product; a step of secondary drying the primary dried product with a heat conduction dryer to obtain a secondary dried product; and a step of pulverizing the secondary dried product to obtain superabsorbent resin particles.

[0013] First, the method for producing superabsorbent resin particles according to an embodiment of the present invention can perform a step of supplying a composition containing a water-soluble ethylenically unsaturated monomer having at least partially neutralized acidic groups, an internal crosslinking agent, and a polymerization initiator to a polymerization reactor, and subjecting the composition to a polymerization reaction to obtain a hydrogel polymer. As described above, the composition can include a water-soluble ethylenically unsaturated monomer, an internal crosslinking agent, and a polymerization initiator, and in addition, if necessary, it can further include additives such as a thickener, a plasticizer, a storage stabilizer, and an antioxidant.

[0014] Here, the water-soluble ethylenically unsaturated monomer can be (meth)acrylic acid or a salt thereof. For example, when an alkali metal salt of acrylic acid in which at least a part of acrylic acid and / or its sodium salt is neutralized is used as the water-soluble ethylenically unsaturated monomer, a superabsorbent resin with improved water absorption can be obtained.

[0015] In addition, examples of the water-soluble ethylenically unsaturated monomer include anionic monomers such as maleic anhydride, fumaric acid, crotonic acid, itaconic acid, 2-acryloylethanesulfonic acid, 2-methacryloylethanesulfonic acid, 2-(meth)acryloylpropanesulfonic acid, or 2-(meth)acrylamide-2-methylpropanesulfonic acid and salts thereof; nonionic hydrophilic-containing monomers such as (meth)acrylamide, N-substituted (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, methoxypolyethylene glycol (meth)acrylate, or polyethylene glycol (meth)acrylate; and amino group-containing unsaturated monomers such as (N,N)-dimethylaminoethyl (meth)acrylate and (N,N)-dimethylaminopropyl (meth)acrylamide and quaternized products thereof. The salt of the anionic monomer can be a metal salt, divalent metal salt, ammonium salt, or organic amine salt.

[0016] The water-soluble ethylenically unsaturated monomer can contain an acidic group that is at least partially neutralized. The water-soluble ethylenically unsaturated monomer containing an acidic group that is at least partially neutralized can be produced by subjecting the acidic group of the water-soluble ethylenically unsaturated monomer to a neutralization reaction using a neutralizing agent. Here, as the neutralizing agent, basic substances such as sodium hydroxide (caustic soda), potassium hydroxide, or ammonium hydroxide that can neutralize the acidic group can be used.

[0017] On the other hand, the internal crosslinking agent can play a role in crosslinking and polymerizing the unsaturated bonds of the above water-soluble ethylenically unsaturated monomer.

[0018] As the internal crosslinking agent, it can be a (meth)acrylate compound in which a crosslinking reaction is carried out by a Free-Radical Polymerization (FRP) reaction. Specifically, the internal crosslinking agent can be one or more compounds selected from the group consisting of ethylene glycol di(meth)acrylate, polyethylene glycol (meth)acrylate, polyethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, polypropylene glycol (meth)acrylate, butanediol di(meth)acrylate, butylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, hexanediol di(meth)acrylate, triethylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, and tetraethylene glycol di(meth)acrylate. More specifically, among these, it can be polyethylene glycol di(meth)acrylate.

[0019] On the other hand, in the composition, as the polymerization initiator, depending on the polymerization method, a thermal polymerization initiator or a photoinitiator by UV irradiation can be used. However, even in the photo-polymerization method, a certain amount of heat is generated by ultraviolet irradiation, and as the exothermic polymerization reaction proceeds, a certain degree of heat is generated, so a thermal polymerization initiator can also be further included.

[0020] As the photoinitiator, a compound capable of forming radicals by light such as ultraviolet rays can be used. The photoinitiator can be one or more selected from the group consisting of benzoin ether, dialkyl acetophenone, hydroxyl alkylketone, phenyl glyoxylate, Benzyl Dimethyl Ketal, acyl phosphine, and α-aminoketone. On the other hand, specific examples of acyl phosphine can be diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide, and ethyl(2,4,6-trimethylbenzoyl)phenylphosphinate.

[0021] Also, as the thermal initiator, one or more selected from the group of initiators consisting of persulfate-based initiators, azo-based initiators, hydrogen peroxide, and ascorbic acid can be used. Specifically, examples of persulfate-based initiators include Sodium persulfate (Na2S2O8), Potassium persulfate (K2S2O8), and Ammonium persulfate ((NH4)2S2O8).

[0022] Specifically, the polymerization method can be roughly classified into thermal polymerization and photo polymerization according to the polymerization energy source. When performing thermal polymerization, it can be carried out in a reactor having a stirring shaft such as a kneader, and when performing photo polymerization, it can be carried out in a reactor equipped with a movable conveyor belt.

[0023] The water-containing gel polymer of the present invention can be produced from a polymerization reaction in which a composition containing a water-soluble ethylenically unsaturated monomer, an internal crosslinking agent, and a polymerization initiator is supplied to a polymerization reactor. Here, in the composition, the internal crosslinking agent can be 0.01 to 5 parts by weight with respect to 100 parts by weight of the water-soluble ethylenically unsaturated monomer. For example, the internal crosslinking agent can be 0.01 part by weight or more, 0.05 part by weight or more, 0.1 part by weight or 0.2 part by weight or more, and 5 parts by weight or less, 3 parts by weight or less, 2 parts by weight or less, 1 part by weight or less, or 0.5 part by weight or less with respect to 100 parts by weight of the water-soluble ethylenically unsaturated monomer. When the content of the internal crosslinking agent is too low, crosslinking may not be sufficiently carried out, and it may be difficult to achieve a strength above an appropriate level. When the content of the internal crosslinking agent in the upper part is too high, the internal crosslinking density may increase, and it may be difficult to achieve the desired water retention capacity.

[0024] The composition of the present invention can be prepared in the form of a solution dissolved in a solvent. Here, as the solvent that can be used, those capable of dissolving the above-mentioned raw material substances can be used. For example, as the solvent, water, ethanol, ethylene glycol, diethylene glycol, triethylene glycol, 1,4-butanediol, propylene glycol, ethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monomethyl ether acetate, methyl ethyl ketone, acetone, methyl amyl ketone, cyclohexanone, cyclopentanone, diethylene glycol monomethyl ether, diethylene glycol ethyl ether, toluene, xylene, butyrolactone, carbitol, methyl cellosolve acetate, N,N-dimethylacetamide, or a mixture thereof can be used.

[0025] Next, a step of drying and pulverizing the water-containing gel polymer obtained by the polymerization reaction to obtain superabsorbent resin particles can be performed. Here, when drying the water-containing gel polymer, in order to improve efficiency, a step of chopping (coarse pulverization) can be further performed. The "chopping" is distinguished from the pulverization performed in the pulverization step described later, and may mean coarse pulverization of the water-containing gel polymer before the drying step.

[0026] Specific examples of the pulverizer used in the chopping step include any one selected from the group of pulverizing devices consisting of a vertical cutter (Vertical pulverizer), a turbo cutter (Turbo cutter), a turbo grinder (Turbo grinder), a rotary cutter mill (Rotary cutter mill), a cutter mill (Cutter mill), a disc mill (Disc mill), a shred crusher (Shred crusher), a crusher (Crusher), a chopper (chopper), and a disc cutter (Disc cutter).

[0027] Next, a drying step can be performed on the water-containing gel polymer chopped as described above. The drying step can be performed including primary drying and secondary drying.

[0028] According to an embodiment of the present invention, the water-containing gel polymer can be primary dried with a hot air dryer to obtain a primary dried product. The hot air dryer can be a device that directly evaporates and discharges heat by applying heat through high-temperature air (hot air) to the moisture of the object to be dried until it reaches the target moisture content. In this specification, the hot air dryer can directly dry the object to be dried with hot air while stirring it with a rotary drum or the like. Here, as the hot air dryer, a kiln dryer, a tube bundle dryer, a rotary dryer, or the like can be used, and in the present invention, a rotary dryer (Rotary Dryer) can be used.

[0029] The rotary dryer can be configured to include a central pipe within a rotating drum, and one or more hot air supply pipes are attached to the central pipe. The water-containing gel polymer introduced into the interior of the rotary dryer can be agitated by the rotation of the rotating drum and heated by the hot air supplied from one or more hot air supply pipes. The hot air can directly contact the water-containing gel polymer introduced into the rotary dryer, enabling efficient drying and shortening the drying time.

[0030] Here, the temperature of the hot air from the hot air supply pipe in the rotary dryer can be 100°C or higher, 110°C or higher, or 120°C or higher, and 200°C or lower, 190°C or lower, or 180°C or lower.

[0031] In addition, the wind speed of the hot air can be 20 m / s or higher, 22 m / s or higher, or 24 m / s or higher, and 30 m / s or lower, 29 m / s or lower, or 27 m / s or lower. By operating the temperature and wind speed of the hot air in the rotary dryer within the above ranges to perform primary drying, the desired moisture content of the primary dried product can be obtained in the present invention.

[0032] According to an embodiment of the present invention, the moisture content of the primary dried product can be 20% by weight or higher, 23% by weight or higher, or 25% by weight or higher, and 35% by weight or lower, 32% by weight or lower, or 30% by weight or lower. By drying the moisture content of the primary dried product within the above range, the primary dried product can be introduced into a heat conduction dryer described later for secondary drying without being overly dried in a hot air dryer.

[0033] When the moisture content of the primary dried product is less than 20% by weight, the content of fine powder in the primary dried product can increase excessively. When the moisture content of the primary dried product exceeds 35% by weight, the adhesiveness of the primary dried product increases excessively, making it difficult to recover in a hot air dryer. Here, the moisture content can be measured by a moisture meter (AND MX_50).

[0034] According to an embodiment of the present invention, the primary dried product can be secondarily dried with a heat conduction dryer to obtain a secondary dried product.

[0035] The heat conduction dryer has a jacket structure in which a heat medium (heat medium oil or steam) flows inside, and the object to be dried receives heat energy supply by conduction from the outside of the jacket and is indirectly heated and dried. In this specification, the heat conduction dryer can perform indirect drying while stirring the object to be dried with a paddle or screw of a jacket structure. Here, as the heat conduction dryer, depending on the structure and type in which heat transfer occurs, a disk dryer, a screw dryer, a paddle dryer, a drum dryer, a stirrer dryer, a thin film dryer, etc. can be used. In the present invention, a paddle dryer can be used.

[0036] The paddle dryer includes an agitator arranged in the horizontal direction, and the agitator can include one or more paddles. The agitator in the paddle dryer rotates, and a heat source via heat medium oil or steam flows inside the paddle of the jacket structure, and the object to be dried can be indirectly dried by contact with the paddle and the inner wall surface. Therefore, the heat efficiency of the paddle dryer can be maximized by heat conduction due to friction.

[0037] On the other hand, the temperature of the heat medium (heat medium oil or steam) flowing inside the agitator provided in the paddle dryer and the paddle attached to the agitator can be 100°C or higher, 110°C or higher, 120°C or higher, or 130°C or higher, and 200°C or lower, 190°C or lower, 180°C or lower, or 170°C or lower. By raising the temperature of the heat medium within the above range, the temperature of the secondary drying performed with the paddle dryer can be within the above range.

[0038] The stirrer within the paddle dryer can rotate at 10 rpm or more, 15 rpm or more, 20 rpm or more, or 25 rpm or more, and 50 rpm or less, 45 rpm or less, 40 rpm or less, or 35 rpm or less. Here, the rpm (rotations per minute) can indicate the unit of the number of rotations per minute of the stirrer. By rotating the stirrer of the paddle dryer within the above range, secondary drying can be smoothly performed, and the problem of introducing undried material into the subsequent grinding step can be prevented.

[0039] According to an embodiment of the present invention, the moisture content of the secondary dried material can be 8% by weight or more, 10% by weight or more, or 12% by weight or more, and 20% by weight or less, 18% by weight or less, or 16% by weight or less. When the moisture content of the secondary dried material is dried within the above range, it is possible to achieve the desired level of water absorption performance of the finally produced superabsorbent resin particles.

[0040] On the other hand, for example, when the primary drying or secondary drying is performed using a band dryer, the drying uniformity of the material to be dried may be low. The band dryer can be, for example, a conveyor belt. Such a dryer can dry by passing hot air over the material to be dried while loading and moving it on a conveyor (such as a wire mesh or a perforated plate). However, since the material to be dried cannot be stirred and is dried in the form of being loaded on the conveyor (in a block-like form), a part of the material to be dried (inside the block form) may remain undried, so over-drying can be performed. However, by performing over-drying, the absorption rate (vortex) and water absorption performance of the superabsorbent resin particles may decrease, and excessive fine powder may be generated in the grinding step. Therefore, in order to improve this, a hot air dryer and a heat conduction dryer that stir and dry the material to be dried according to an embodiment of the present invention can be used.

[0041] The hot air dryer can easily improve the absorption rate of the object to be dried by maintaining the pores of the object to be dried through rotation by means of a rotating drum or the like. However, since the filling level of the object to be dried in the rotating drum of the hot air dryer must be charged within 15% of the total capacity, the drying capacity is low with respect to the equipment size. Furthermore, the drying uniformity of the object to be dried dried by the hot air dryer may be relatively low.

[0042] On the other hand, the heat conduction dryer has a high drying capacity and can dry the object to be dried with a high drying uniformity. However, for that reason, the pores of the object to be dried can be reduced by the high shear force applied to the object to be dried. Therefore, the absorption rate (vortex) of the object to be dried can be reduced.

[0043] In order to complement the disadvantages of the above-mentioned hot air dryer and heat conduction dryer with each other, in the present invention, the hot air dryer and the heat conduction dryer can be connected in series. As described above, by performing primary drying through a hot air dryer, the pores of the water-containing gel polymer are first maintained, and by performing secondary drying through a heat conduction dryer, the drying uniformity of the secondary dried product can be further improved. In this way, by connecting the hot air dryer and the heat conduction dryer in series to perform primary and secondary drying, it is possible to prevent the undried material from flowing into the subsequent grinding step, and finally, it becomes easy to ensure the physical properties of the completed superabsorbent resin particles. Here, the physical properties of the superabsorbent resin particles may mean the absorption properties that the superabsorbent resin particles are initially intended for, and specifically, may be properties related to absorption such as water absorption performance and absorption rate.

[0044] Next, the step of pulverizing the secondary dried product obtained through the drying steps of primary and secondary drying in this way to obtain superabsorbent resin particles can be performed. The pulverizing step can be carried out first, but as an example, it can be carried out in one or more passes. According to the present invention, after the pulverizing step, a classification step can be further performed. Furthermore, the classification can be carried out after each pass of pulverization.

[0045] For example, first, primary pulverization of pulverizing the secondary dried product can be carried out. Specifically, as the pulverizer used at this time, a ball mill, a pin mill, a hammer mill, a screw mill, a roll mill, a disc mill, or a jog mill can be used.

[0046] Next, the primary pulverized product after the primary pulverization can be primarily classified using a standard mesh body of ASTM standard based on a specific particle size. Here, the specific particle size can be the reference particle size of the particles classified for performing secondary pulverization described later. By such classification, the particles corresponding to less than the specific particle size and the particles corresponding to the specific particle size or more can be classified. As an example, the primary pulverized product can be put into a mesh (mesh body) set, and classified into particles corresponding to #40 or more (425 μm or more) and particles corresponding to #40 or less (less than 425 μm) based on the #40 mesh.

[0047] Then, secondary pulverization of separately pulverizing the particles corresponding to the specific particle size or more classified by the primary classification to obtain a secondary pulverized product can be carried out. Here, the secondary pulverization can be carried out using the same pulverizer as that used in the primary pulverization.

[0048] After such secondary pulverization, in order to control the physical properties of the superabsorbent resin particles to be finally commercialized, secondary classification can be performed. The secondary classification can use the standard mesh of the ASTM standard used in the primary classification, and finally classify a mixture of particles corresponding to less than a specific particle size classified in the primary classification and the secondary pulverized product pulverized in the secondary pulverization according to an appropriate particle size. Here, the appropriate particle size (D50) can be a particle size of 150 μm to 850 μm, and particles having such a particle size can be classified as normal particles. On the other hand, particles having a particle size of less than 150 μm can be classified as fine particles. Such a particle size can be measured according to the method of European Disposables and Nonwovens Association (EDANA) standard EDANA WSP 220.3.

[0049] Through such a series of pulverization steps, normal particles, that is, superabsorbent resin particles, can be produced.

[0050] Next, if necessary, the step of surface-crosslinking the produced superabsorbent resin particles in the presence of a surface crosslinking agent to produce superabsorbent resin particles having a surface crosslinking layer formed at least partially can be further included. Here, the surface crosslinking layer can be generated from a surface crosslinking agent, the surface crosslinking agent contains a polyvalent epoxy compound, and the polyvalent epoxy compound can be a glycidyl ether compound of a polyhydric alcohol.

[0051] Specifically, the surface crosslinking agent can contain one or more polyvalent epoxy compounds selected from the group consisting of ethylene glycol diglycidyl ether, diethylene glycol diglycidyl ether, triethylene glycol diglycidyl ether, tetraethylene glycol diglycidyl ether, glycerin polyglycidyl ether, and sorbitol polyglycidyl ether.

[0052] The surface crosslinking agent can be mixed with the superabsorbent resin particles in a solution state, specifically, in a surface crosslinking solution state dissolved in a solvent. In addition to the surface crosslinking agent, the surface crosslinking solution can contain water and methanol. Generally, the surface crosslinking solution is applied to the surface of the superabsorbent resin particles. Therefore, the surface crosslinking reaction occurs on the surface of the superabsorbent resin particles, which improves the crosslinking property on the surface of the particles without substantially affecting the inside of the particles. Therefore, the surface-crosslinked superabsorbent resin particles have a higher degree of crosslinking near the surface than inside. By heating the polymer to which the surface crosslinking agent is added, the surface crosslinking reaction and drying can be carried out simultaneously.

[0053] Hereinafter, the present invention will be described in more detail by way of examples. However, it is obvious to those skilled in the art that the following examples are for illustrative purposes of the present invention, and various changes and modifications are possible within the scope of the present invention and the scope of the technical idea, and the scope of the present invention is not limited only to this.

[0054] Examples Example 1 (1) Production of water-containing gel polymer In a 3L glass container equipped with a stirrer and a thermometer, 1000 g of acrylic acid, 1.6 g of polyethylene glycol diacrylate (PEGDA) as an internal crosslinking agent, 0.08 g of diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide as a photoinitiator which is a polymerization initiator, 1.2 g of sodium persulfate as a thermal polymerization initiator, and 1235 g of a caustic soda (NaOH) solution as a neutralizing agent were mixed at room temperature so that the solid content was 45.0% by weight to produce a composition.

[0055] Next, the composition was supplied onto a conveyor belt with a 10 cm-wide and 2 m-long belt moving at a speed of 10 cm / min at a rate of 500 - 2000 mL / min. Here, simultaneously with the supply of the monomer composition, ultraviolet light having an intensity of 10 mW / cm 2 was irradiated to carry out a polymerization reaction for 60 seconds. After the polymerization reaction was completed, it was pulverized with a chopper to obtain a water-containing gel polymer having a water content of 65% by weight.

[0056] (2) Production of superabsorbent resin particles Next, 1 kg of the water-containing gel polymer was supplied to a rotary dryer and primary drying was carried out until the water content of the primary dried product was 24.9% by weight. Here, the hot air supplied from the hot air supply pipe of the rotary dryer dried the water-containing gel polymer at a temperature of 100°C and a wind speed of 26 m / s to obtain a primary dried product. Here, the water content was measured by a moisture meter (AND MX_50; heating temperature: 180°C, heating time: 40 minutes).

[0057] Then, the primary dried product was put into a paddle dryer and secondary drying was carried out until the water content of the secondary dried product was 12.0% by weight. Here, the secondary drying temperature was also 105°C through a heat medium at 105°C flowing inside the stirrer (agitator) provided in the paddle dryer and the paddles attached to the stirrer, and the stirrer in the paddle dryer was rotated at 10 rpm.

[0058] Next, the secondary dried product was primarily pulverized using a roll mill to obtain a primarily pulverized product. After the primarily pulverized product was put into a mesh set (a mesh set consisting of ASTM standard meshes #6, #10, #20, #25, #30, #40, #50, #70, and #100), primary classification was performed to classify it into particles of #40 or more (425 μm or more) and particles of less than #40 (less than 425 μm). The particles of #40 or more (425 μm or more) classified by the primary classification were secondarily pulverized using the roll mill to obtain a secondarily pulverized product. Finally, the mixture of the particles of less than #40 (less than 425 μm) classified by the primary classification and the secondarily pulverized product was secondarily classified to obtain superabsorbent resin particles having a particle size of 150 μm to 850 μm. Here, the secondary classification was performed using the same mesh set as the primary classification.

[0059] Example 2 Superabsorbent resin particles were produced in the same manner as in Example 1, except that in Example 1, primary drying was performed until the water content of the primary dried product reached 25.5% by weight, and secondary drying (agitator rotation speed: 30 rpm) was performed until the water content of the secondary dried product reached 12.9% by weight.

[0060] Example 3 Superabsorbent resin particles were produced in the same manner as in Example 1, except that in Example 1, primary drying was performed until the water content of the primary dried product reached 21.9% by weight, and secondary drying (agitator rotation speed: 50 rpm) was performed until the water content of the secondary dried product reached 10.8% by weight.

[0061] Example 4 Superabsorbent resin particles were produced in the same manner as in Example 1, except that in Example 1, primary drying was performed until the water content of the primary dried product reached 33.6% by weight, and secondary drying was performed until the water content of the secondary dried product reached 11.4% by weight.

[0062] Example 5 Superabsorbent resin particles were produced in the same manner as in Example 1, except that in Example 1, primary drying was performed until the water content of the primary dried product reached 29.1% by weight, and secondary drying (temperature: 200 °C) was performed until the water content of the secondary dried product reached 11.8% by weight.

[0063] Example 6 Superabsorbent resin particles were produced in the same manner as in Example 1, except that in Example 1, primary drying was performed until the water content of the primary dried product reached 29.3% by weight, and secondary drying was performed until the water content of the secondary dried product reached 7.5% by weight (temperature: 200 °C, stirrer rotation speed: 50 rpm).

[0064] Example 7 Superabsorbent resin particles were produced in the same manner as in Example 1, except that in Example 1, primary drying was performed until the water content of the primary dried product reached 31.4% by weight, and secondary drying was performed until the water content of the secondary dried product reached 21.3% by weight.

[0065] Example 8 Superabsorbent resin particles were produced in the same manner as in Example 1, except that in Example 1, primary drying was performed until the water content of the primary dried product reached 16.5% by weight, and secondary drying was performed until the water content of the secondary dried product reached 10.3% by weight.

[0066] Example 9 Superabsorbent resin particles were produced in the same manner as in Example 1, except that in Example 1, primary drying was performed until the water content of the primary dried product reached 37.5% by weight, and secondary drying was performed until the water content of the secondary dried product reached 12.9% by weight.

[0067] Comparative Example Comparative Example 1 Superabsorbent resin particles were produced in the same manner as in Example 1, except that in Example 1, primary drying was performed using a rotary dryer (hot air temperature: 100 °C, wind speed: 26 m / s) until the water content of the primary dried product reached 10.2% by weight, and secondary drying was not performed.

[0068] Comparative Example 2 Superabsorbent resin particles were produced in the same manner as in Example 1, except that in Example 1, primary drying was performed using a rotary dryer (hot air temperature: 250 °C, wind speed: 8 m / s) until the water content of the primary dried product reached 11.0% by weight, and secondary drying was not performed.

[0069] Comparative Example 3 Superabsorbent resin particles were produced in the same manner as in Example 1, except that primary drying was performed using a paddle dryer (temperature: 105°C, stirrer rotation speed: 10 rpm) until the water content of the primary dried product reached 12.6% by weight in Example 1, and secondary drying was not performed.

[0070] Comparative Example 4 Superabsorbent resin particles were produced in the same manner as in Example 1, except that primary drying was performed using a paddle dryer (temperature: 200°C, stirrer rotation speed: 50 rpm) until the water content of the primary dried product reached 11.0% by weight in Example 1, and secondary drying was not performed.

[0071] Comparative Example 5 Superabsorbent resin particles were produced in the same manner as in Example 1, except that primary drying was performed using a paddle dryer (temperature: 105°C, stirrer rotation speed: 10 rpm) until the water content of the primary dried product reached 31.1% by weight in Example 1, and secondary drying was performed using a rotary dryer (temperature: 100°C, wind speed: 26 m / s) until the water content of the secondary dried product reached 12.6% by weight.

[0072] Experimental Example The results of measuring the superabsorbent resin particles produced in the above Examples or Comparative Examples by the following methods are shown in Tables 1 and 2 below.

[0073] 1) Water content The water content was measured from the change in weight generated by heating the primary dried product dried primarily or the secondary dried product dried secondarily during the production process of the superabsorbent resin particles in the Examples or Comparative Examples with a halogen lamp (400 W) in a moisture meter (AND MX_50) to evaporate the moisture. Specifically, the water content was measured according to the following order.

[0074] (i) Set the measurement conditions of the moisture meter (heating temperature: 180°C, heating time: 40 minutes). (ii) Stabilize the weight to zero (0) with no sample on the sample pan. (iii) Sample 5 g (error: 0.02%) of the primary or secondary dried product to be measured, uniformly load it onto the sample pan, and start the measurement. (iv) After the measurement is completed, check the moisture content.

[0075] 2) Drying uniformity The drying uniformity may mean the uniformity of the moisture content by particle size of the superabsorbent resin particles. Specifically, the drying uniformity of the superabsorbent resin particles produced in the examples and comparative examples was measured according to Mathematical Formula 1.

[0076]

Equation

[0077] Here, the drying uniformity was measured by sampling 200 g of the finally produced superabsorbent resin particles. The 200 g of the secondary dried product was put into a mesh set consisting of meshes #6, #10, #20, #25, #30, #40, #50, #70, and #100 and attached to a sieve shaker. The sieve shaker was operated for 10 minutes at an amplitude of 1.5 to classify the superabsorbent resin particles.

[0078] After separating the particles by particle size for each mesh, the moisture content of the particles separated as described above was measured using a moisture meter (AND MX_50; heating temperature: 180 °C, heating time: 40 minutes). The standard deviation of the moisture content of the particles for each particle size was indicated as "Std" in Mathematical Formula 1.

[0079] 3) Average particle size after drying and average particle size after pulverization The method for measuring the average particle size after drying was performed as follows.

[0080] First, the secondary dried product was classified in the same manner as the method for measuring the above-mentioned "drying uniformity". Specifically, the secondary dried product (200 g) after the completion of secondary drying in the examples and comparative examples was put into a mesh set consisting of meshes #6, #10, #20, #25, #30, #40, #50, #70, and #100, and then classified by attaching it to a vibrating sieve machine (time: 10 minutes, amplitude: 1.5). In this way, the weight fraction of the secondary dried product classified by mesh (#6, #10, #20, #25, #30, #40, #50, #70, and #100) was analyzed, and the result value obtained by calculating the arithmetic mean of the particle sizes for each mesh based on the weight fraction for each mesh is the "average particle size after drying" in Tables 1 and 2.

[0081] On the other hand, the "average particle size after grinding" was carried out in the same manner as the method for measuring the above-mentioned "average particle size after drying". However, unlike the "average particle size after drying" for which the secondary dried product was the measurement target, the "average particle size after grinding" was measured by classifying a mixture (particles less than #40 classified by primary classification and secondary ground secondary ground product) that had further undergone a grinding step.

[0082] 4) Content of fine powder Among the methods for producing superabsorbent resin particles in the above examples and comparative examples, before classifying the mixture (particles less than #40 classified by primary classification and secondary ground secondary ground product) for secondary classification, the weight of the mixture was measured. Next, after performing the secondary classification, the weight of fine powder particles having a particle size of 150 μm or less was measured. The percentage of the weight of the fine powder particles with respect to the weight of the mixture was indicated as the "content of fine powder". That is, the content of fine powder can mean the content of fine powder particles contained in the mixture.

[0083] 5) Absorption rate (Vortex) The absorption rates of the superabsorbent resin particles in the above examples and comparative examples were measured by the following method.

[0084] Using a graduated cylinder, 50 ml of 0.9% saline maintained at a temperature of 24°C was poured into a 100 ml beaker on a magnetic stirrer. During stirring, the magnetic stirrer was operated at a speed such that the lowest surface (central space) of the vortex generated in the 100 ml beaker contacted the magnetic bar. Among the superabsorbent resin particles of the above Examples and Comparative Examples, superabsorbent resin particles (1.99 g to 2.01 g) corresponding to a particle size of #40 to #50 (300 μm to 425 μm) were put into the vortex, and at the same time, the time was measured. By putting the superabsorbent resin particles into the saline as described above, viscosity occurred, and the time until the vortex disappeared and the liquid level became horizontal, measured in seconds, was defined as the "absorption rate" of the present invention.

[0085]

Table 1

[0086]

Table 2

[0087] Referring to Table 1 and Table 2 above, in the case of the Examples in which superabsorbent resin particles were produced by performing primary drying using a rotary dryer and secondary drying using a paddle dryer in the drying step, it was confirmed that the absorption rate was improved by maintaining pores by the rotary dryer, and the content of fine powder of the superabsorbent resin particles was reduced by improving the drying uniformity by the paddle dryer.

[0088] In contrast, Comparative Example 1 uses only a rotary dryer to dry the superabsorbent resin particles until the water content of the primary dried product, i.e., the final water content, reaches 10.2% by weight. Not only is secondary drying not performed, but the drying step is carried out with only one type of dryer. Although Comparative Example 1 performs primary drying at a level similar to the water content of the secondary dried product of the Example, it was confirmed that the drying uniformity decreased and the content of fine powder increased due to the rotary dryer. Comparative Example 2 performs a drying step using only a rotary dryer as in Comparative Example 1, but the drying is carried out with the operating conditions of the rotary dryer changed. By using only the rotary dryer, Comparative Example 2 was also able to confirm results similar to those of Comparative Example 1.

[0089] On the other hand, Comparative Example 3 uses only a paddle dryer to dry the superabsorbent resin particles until the water content of the primary dried product reaches 12.6% by weight. Not only is secondary drying not performed, but the drying step is carried out with only one type of dryer. It was confirmed that the absorption rate decreased compared to the Example in which secondary drying was carried out through two types of dryers. That is, it was confirmed that stress was generated in the object to be dried by the high shear force applied by the paddle of the paddle dryer, and the stress reduced the pores and thus the absorption rate decreased. Comparative Example 4 performs a drying step using only a paddle dryer as in Comparative Example 3, but the drying is carried out with the operating conditions of the paddle dryer changed. By using only the paddle dryer, Comparative Example 4 was also able to confirm results similar to those of Comparative Example 3.

[0090] On the other hand, Comparative Example 5 is a superabsorbent resin particle produced by performing a drying step with primary drying using a paddle dryer and secondary drying using a rotary dryer, and the types of the primary and secondary dryers were changed compared to the examples. As described above, when using a paddle dryer, the pores of the dried body may decrease and the absorption rate may decrease. In Comparative Example 5, by using the paddle dryer as the primary dryer first, the primary dried body in a state where the pores had decreased could be introduced into the rotary dryer which is the secondary dryer. However, it could be confirmed from the decreased absorption rate that the pores of the primary dried body whose pores had already decreased in the paddle dryer were maintained by the rotating drum of the rotary dryer.

Claims

1. Supplying a composition containing a water-soluble ethylenically unsaturated monomer having at least partially neutralized acidic groups, an internal crosslinking agent, and a polymerization initiator to a polymerization reactor and subjecting it to a polymerization reaction to obtain a hydrogel polymer; Primarily drying the hydrogel polymer with a hot air dryer to obtain a primarily dried product; Secondarily drying the primarily dried product with a heat conduction dryer to obtain a secondarily dried product; And pulverizing the secondarily dried product to obtain superabsorbent resin particles. The method for producing superabsorbent resin particles, wherein the hot air dryer is a rotary dryer, and the heat conduction dryer is a paddle dryer.

2. Before primarily drying the hydrogel polymer, The method for producing superabsorbent resin particles according to claim 1, further comprising the step of chopping the hydrogel polymer.

3. After the step of pulverizing the secondarily dried product to obtain superabsorbent resin particles, surface crosslinking is performed in the presence of a surface crosslinking agent, The method for producing superabsorbent resin particles according to claim 1, further comprising the step of forming a surface crosslinked layer on at least a part of the surface.

4. The method for producing superabsorbent resin particles according to any one of claims 1 to 3, wherein the hot air dryer and the heat conduction dryer are connected in series.

5. The method for producing superabsorbent resin particles according to any one of claims 1 to 3, wherein the water content of the primarily dried product is 20% to 35% by weight.

6. The method for producing superabsorbent resin particles according to any one of claims 1 to 3, wherein the water content of the secondarily dried product is 8% to 20% by weight.

7. The rotary dryer includes, inside, a central pipe and one or more hot air supply pipes attached to the central pipe, The temperature of the hot air supplied from the above-mentioned hot air supply pipe(s) of 1 or more is 100°C to 200°C, and the wind speed of the hot air is 20 m / s to 30 m / s. The method for producing superabsorbent resin particles according to claim 1.

8. The paddle dryer is equipped with a stirrer, The stirrer rotates at 10 rpm to 50 rpm. The method for producing superabsorbent resin particles according to claim 1.

9. The water-soluble ethylenically unsaturated monomer is (meth)acrylic acid or its salt. The method for producing superabsorbent resin particles according to claim 1.

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