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US-12624179-B2 - Method for preparing super absorbent polymer

US12624179B2US 12624179 B2US12624179 B2US 12624179B2US-12624179-B2

Abstract

The present disclosure relates to a method for preparing a super absorbent polymer. More specifically, the above method performs a polymerization reaction of a monomer in the presence of an aqueous dispersion of hydrophobic particles, and thus a super absorbent polymer having an improved absorption rate can be prepared without deterioration in absorption properties.

Inventors

  • Kwangin Shin
  • Jin Hyuck JU
  • Hyungki Yoon
  • Taebin Ahn
  • Dong Hoon Park

Assignees

  • LG CHEM, LTD.

Dates

Publication Date
20260512
Application Date
20211217
Priority Date
20201218

Claims (20)

  1. 1 . A method for preparing a super absorbent polymer, comprising the steps of: step 1: preparing a monomer composition containing an acrylic acid-based monomer having at least partially neutralized acidic groups and an internal cross-linking agent; step 2: preparing a hydrogel polymer by cross-linking polymerization of the monomer composition in the presence of an aqueous dispersion of hydrophobic particles and an encapsulated foaming agent; step 3: forming a powder-type base resin by drying and pulverizing the hydrogel polymer; and step 4: forming a surface cross-linked layer by further cross-linking the surface of the powder-type base resin in the presence of a surface cross-linking agent, wherein the aqueous dispersion of hydrophobic particles is a colloidal solution in which hydrophobic particles are dispersed by a surfactant, the hydrophobic particles contain a metal salt of a C7 to C24 fatty acid, and the encapsulated foaming agent has a structure having a core comprising a hydrocarbon and a shell formed of a thermoplastic resin surrounding the core.
  2. 2 . The method for preparing a super absorbent polymer of claim 1 , wherein the super absorbent polymer satisfies the following 1) and 2): 1) a vortex time at 24.0° C. is 35 seconds or less; and 2) one-minute tap water absorbency, which is defined as a weight of water absorbed in the super absorbent polymer in 1 minute when 1 g of the super absorbent polymer is immersed in 2 L of tap water and swollen for 1 minute, is 115 g or more.
  3. 3 . The method for preparing a super absorbent polymer of claim 1 , wherein the hydrophobic particles are at least one metal salt of stearic acid selected from the group consisting of calcium stearate, magnesium stearate, sodium stearate, zinc stearate and potassium stearate.
  4. 4 . The method for preparing a super absorbent polymer of claim 1 , wherein the hydrophobic particles have an average particle diameter of 0.5 μm to 20 μm.
  5. 5 . The method for preparing a super absorbent polymer of claim 1 , wherein the hydrophobic particles are used in an amount of 0.005 to 0.4 parts by weight based on 100 parts by weight of the acrylic acid-based monomer.
  6. 6 . The method for preparing a super absorbent polymer of claim 1 , wherein the surfactant comprises a nonionic surfactant and an anionic surfactant.
  7. 7 . The method for preparing a super absorbent polymer of claim 1 , wherein the encapsulated foaming agent has an average diameter before expansion of 5 to 30 μm, and a maximum expansion ratio in air of 5 to 15 times.
  8. 8 . The method for preparing a super absorbent polymer of claim 1 , wherein the encapsulated foaming agent has a maximum expansion size in air of 20 to 190 μm.
  9. 9 . The method for preparing a super absorbent polymer of claim 1 , wherein the hydrocarbon is at least one selected from the group consisting of n-propane, n-butane, iso-butane, cyclobutane, n-pentane, iso-pentane, cyclopentane, n-hexane, iso-hexane, cyclohexane, n-heptane, iso-heptane, cycloheptane, n-octane, iso-octane and cyclooctane.
  10. 10 . The method for preparing a super absorbent polymer of claim 1 , wherein the thermoplastic resin is a polymer formed from at least one monomer selected from the group consisting of a (meth)acrylate-based compound, a (meth) acrylonitrile-based compound, an aromatic vinyl compound, a vinyl acetate compound and a halogenated vinyl compound.
  11. 11 . The method for preparing a super absorbent polymer of claim 1 , wherein the encapsulated foaming agent and the hydrophobic particles are used in a weight ratio of 1:0.1 to 1:3.5.
  12. 12 . The method for preparing a super absorbent polymer of claim 1 , wherein in step 2, the cross-linking polymerization is performed in the presence of the aqueous dispersion of hydrophobic particles, the encapsulated foaming agent, and further at least one foam stabilizer selected from the group consisting of an alkyl sulfate-based compound and a polyoxyethylene alkyl ether-based compound.
  13. 13 . The method for preparing a super absorbent polymer of claim 12 , wherein the encapsulated foaming agent and the foam stabilizer are used in a weight ratio of 1:0.01 to 1:0.5.
  14. 14 . The method for preparing a super absorbent polymer of claim 1 , wherein the surface cross-linking agent is the same as the internal cross-linking agent.
  15. 15 . The method for preparing a super absorbent polymer of claim 1 , wherein the super absorbent polymer has an effective capacity (EFFC) calculated by the following Equation 1 of 22 g/g or more: Effective capacity (EFFC)={CRC+0.7 psi AUP}/2 [Equation 1] in Equation 1, CRC is centrifuge retention capacity (CRC) of the super absorbent polymer measured according to EDANA WSP 241.3, and 0.7 psi AUP is absorbency under pressure (AUP) at 0.7 psi of the super absorbent polymer measured according to EDANA WSP 242.3.
  16. 16 . The method for preparing a super absorbent polymer of claim 1 , wherein the acrylic acid-based monomer is included in an amount of about 20 to 60 wt % based on a total weight of the monomer composition.
  17. 17 . The method for preparing a super absorbent polymer of claim 1 , wherein the hydrophobic particles are included in an amount of 10 to 70 wt % based on a total weight of the aqueous dispersion.
  18. 18 . The method for preparing a super absorbent polymer of claim 1 , wherein the aqueous dispersion of hydrophobic particles has a pH of 7 or more.
  19. 19 . The method for preparing a super absorbent polymer of claim 1 , wherein the encapsulated foaming agent is included in an amount of 0.005 to 1 part by weight based on 100 parts by weight of the acrylic acid-based monomer.
  20. 20 . The method for preparing a super absorbent polymer of claim 1 , wherein the super absorbent polymer has a centrifuge retention capacity (CRC) measured according to EDANA WSP 241.3 of 28 to 35 g/g, and an absorbency under pressure (AUP) at 0.7 psi measured according to EDANA WSP 242.3 of 16 to 22 g/g.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS This present application is a national stage entry under 35 U.S.C. § 371 of International Application No. PCT/KR2021/019244 filed on Dec. 17, 2021, which claims priority from Korean Patent Applications No. 10-2020-0178433 filed on Dec. 18, 2020 and No. 10-2021-0180293 filed on Dec. 16, 2021, all the disclosures of which are incorporated herein by reference. TECHNICAL FIELD The present disclosure relates to a method for preparing a super absorbent polymer. More specifically, it relates to a method for preparing a super absorbent polymer having a high surface tension and an improved absorption rate without deterioration in absorption properties, because the generated carbon dioxide bubbles can be effectively captured by performing a polymerization reaction of a monomer in the presence of an aqueous dispersion of hydrophobic particles. BACKGROUND OF ART A super absorbent polymer (SAP) is a type of synthetic polymeric material capable of absorbing 500 to 1000 times its own weight of moisture. Various manufacturers have denominated it with different names, such as SAM (Super Absorbency Material), AGM (Absorbent Gel Material), and the like. Such super absorbent polymers started to be practically applied in sanitary products, and they are now being widely used not only for hygiene products, but also for water retaining soil products for gardening, water stop materials for the civil engineering and construction, sheets for raising seedling, fresh-keeping agents for food distribution fields, materials for poultices, or the like. These super absorbent polymers have been widely used in the field of hygienic materials such as diapers or sanitary napkins. In such hygienic materials, the super absorbent polymer is generally contained in a state of being spread in the pulp. In recent years, however, continuous efforts have been made to provide hygienic materials such as diapers having a thinner thickness. As a part of such efforts, the development of so-called pulpless diapers and the like in which the pulp content is reduced or pulp is not used at all is being actively advanced. As described above, in the case of hygienic materials in which the pulp content is reduced or the pulp is not used, a super absorbent polymer is contained at a relatively high ratio and these super absorbent polymer particles are inevitably contained in multiple layers in the hygienic materials. In order for the whole super absorbent polymer particles contained in the multiple layers to more efficiently absorb a large amount of liquid such as urine, it is necessary for the super absorbent polymer to exhibit high absorption performance as well as fast absorption rate. In order to prepare the super absorbent polymer with improved absorption rate, a method of increasing a specific surface area by using a foaming agent in the polymerization step to form pores is mainly used. In particular, an encapsulated foaming agent is commonly used in terms of price and availability, and when the polymerization step is performed in the presence of such an encapsulated foaming agent, carbon dioxide bubbles are generated and the specific surface area in the cross-linked polymer increases. In addition, a bubble stabilizer is used to minimize the generated carbon dioxide bubbles from escaping out of the cross-linked polymer network. However, the use of the bubble stabilizer causes a problem in that general properties of the super absorbent polymer are deteriorated. Accordingly, there is a continuous demand for the development of a super absorbent polymer having a fast absorption rate while maintaining centrifuge retention capacity (CRC), which is the property indicating basic absorbency and water retention capacity of the super absorbent polymer, and absorbency under pressure (AUP), which is the property of well retaining the absorbed liquid even under external pressure. DETAILED DESCRIPTION OF THE INVENTION Technical Problem Accordingly, the present disclosure relates to a method for preparing a super absorbent polymer capable of preparing a super absorbent polymer having an improved absorption rate by effectively capturing bubbles generated by a foaming agent after adding an aqueous dispersion of hydrophobic particles in the polymerization step of a monomer. Technical Solution In order to solve the above problems, there is provided a method for preparing a super absorbent polymer including the steps of: preparing a monomer composition containing an acrylic acid-based monomer having at least partially neutralized acidic groups and an internal cross-linking agent (step 1);preparing a hydrogel polymer by cross-linking polymerization of the monomer composition in the presence of an aqueous dispersion of hydrophobic particles and an encapsulated foaming agent (step 2);forming a powder-type base resin by drying and pulverizing the hydrogel polymer (step 3); andforming a surface cross-linked layer by further cross-linking the surfac