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EP-3873974-B1 - PROCESS FOR PRODUCING LONG-TERM COLOR STABLE SUPERABSORBENT POLYMER PARTICLES

EP3873974B1EP 3873974 B1EP3873974 B1EP 3873974B1EP-3873974-B1

Inventors

  • BAUMANN, KATRIN
  • BAUER, STEPHAN
  • DANIEL, THOMAS
  • HERFERT, NORBERT
  • BAUDUIN, CHRISTOPHE

Dates

Publication Date
20260506
Application Date
20191023

Claims (13)

  1. A process for producing long-term color stable superabsorbent polymer particles, comprising polymerization of a monomer solution, comprising a) partly neutralized acrylic acid, b) 0.05 to 1.5% by weight, based on acrylic acid prior to neutralization, of at least one crosslinker, and c) at least one initiator, drying the resulting polymer gel, optionally grinding and classifying the resulting dried polymer gel and thermally post-crosslinking and cooling the resulting polymer particles, wherein 0.005 to 5% by weight of a thermal post-crosslinker and 0.016 to 0.080% by weight of hydrogen peroxide, based in each case on the polymer particles, are added to the polymer particles prior to the thermal post-crosslinking.
  2. The process according to claim 1, wherein from 0.02 to 0.05% by weight of the peroxide, based on the polymer particles, is added to the polymer particles prior to the post-crosslinking.
  3. The process according to claims 1 or 2, wherein the temperature of the post-crosslinking is from 130 to 190°C.
  4. The process according to any of claims 1 to 3, wherein the residence time of the post-crosslinking is from 20 to 60 minutes.
  5. The process according to any of claims 1 to 4, wherein the monomer solution comprises a polymerization inhibitor.
  6. The process according to any of claims 1 to 5, wherein the monomer solution comprises from 0.004 to 0.010% by weight of a polymerization inhibitor, based on acrylic acid.
  7. The process according to claims 5 or 6, wherein hydroquinone monomethyl ether is used as polymerization inhibitor.
  8. The process according to any of claims 1 to 7, wherein at least one chelating agent is added to is added to the polymer particles during or after the cooling.
  9. The process according to any of claims 1 to 8, wherein from 0.005 to 1% by weight of at least one chelating agent, based on the polymer particles, is added to the polymer particles during or after the cooling.
  10. The process according to claims 8 or 9, wherein 1-hydroxyethane-1,1-diphosphonic acid and/or salts thereof are used as chelating agent.
  11. The process according to any of claims 1 to 10, wherein the post-crosslinking is without any irradiation with active energy rays.
  12. Superabsorbent polymer particles obtainable according to a process of any of claims 1 to 11, wherein the superabsorbent polymer particles have a centrifuge retention capacity of at least 40 g/g, an absorption under high load of at least 20 g/g and a yellowness index after 14 days of aging at 70°C and a relative humidity of 80% of not more than 54, wherein the centrifuge retention capacity of superabsorbent polymer particles is determined by the EDANA recommended test method No. WSP 241.2 (05) "Gravimetric Determination of Fluid Retention Capacity in Saline Solution After Centrifugation", wherein for higher values of the centrifuge retention capacity lager tea bags have to be used, the absorption under high load of the water-absorbent polymer particles is determined analogously to the EDANA recommended test method No. WSP 242.2 (05) "Gravimetric Determination of Absorption Under Pressure", except using a weight of 49.2 g/cm 2 instead of a weight of 21.0 g/cm 2 and the yellowness index of YI D1925 (2/C) is measured per ASTM D-1925, 2 deg./III. °C.
  13. The superabsorbent polymer particles according to claim 12, wherein the superabsorbent polymer particles have a yellowness index after 14 days of aging at 70°C and a relative humidity of 80% of not more than 50, wherein the yellowness index of YI D1925 (2/C) is measured per ASTM D-1925, 2 deg./III.°C.

Description

The invention relates to a process for producing long-term color stable superabsorbent polymer particles, comprising polymerization of a monomer solution, drying the resulting polymer gel, optionally grinding and classifying the resulting dried polymer gel and thermally surface post-crosslinking and cooling the resulting polymer particles, wherein a thermal surface post-crosslinker and hydrogen peroxide are added to the polymer particles prior to the thermal surface post-crosslinking. Superabsorbent polymer particles are used to produce diapers, tampons, sanitary napkins and other hygiene articles, but also as water-retaining agents in market gardening. The superab-sorbent polymer particles are often also referred to as "absorbent resins", "superabsorbents", "superabsorbent polymers", "absorbent polymers", "absorbent gelling materials", "hydrophilic polymers" or "hydrogels". The production of superabsorbent polymer particles is described in the monograph "Modern Superabsorbent Polymer Technology", F.L. Buchholz and A.T. Graham, Wiley-VCH, 1998, pages 71 to 103. The properties of superabsorbent polymer particles can be adjusted, for example, via the amount of crosslinker used. With increasing amount of crosslinker, the centrifuge retention capacity (CRC) falls and the absorption under a pressure of 21.0 g/cm2 (AUL) passes through a maximum. To improve the application properties, for example permeability of the swollen gel bed (SFC) in the diaper and absorption under a pressure of 49.2 g/cm2 (AUHL), superabsorbent polymer particles are generally surface post-crosslinked. This increases the degree of crosslinking of the particle surface, which allows the absorption under a pressure of 49.2 g/cm2 (AUHL) and the centrifuge retention capacity (CRC) to be at least partly de-coupled. This surface post-crosslinking can be performed in the aqueous gel phase. Preferably, however, dried, ground and screened-off polymer particles (base polymer) are surface coated with a surface post-crosslinker, thermally surface post-crosslinked and dried. Crosslinkers suitable for this purpose are compounds which can form covalent bonds with at least two carboxylate groups of the superabsorbing polymer particles. A problem which often occurs in superabsorbent polymer particles is that of discoloration, which occurs in the course of storage at elevated temperature or elevated air humidity. Such conditions often occur in the course of storage in tropical or subtropical countries. Under such conditions, superabsorbent polymer particles tend to yellow; they may even take on a brown or even almost black color. This discoloration of the actually colorless superabsorbent polymer particles is unsightly and undesired, since it is visible especially in the desired thin hygiene products, and consumers reject unsightly hygiene products. The cause of the discoloration has not been entirely clarified, but reactive compounds such as residual monomers from the polymerization, the use of some initiators, impurities in the monomers or in the neutralizing agent, surface post-crosslinkers or stabilizers in the monomers used appear to play a role. EP 1 770 113 A1 discloses surface post-crosslinking in the presence of peroxides for reduction of residual monomers. WO 2006/062258 A2 discloses a process for surface post-crosslinking by adding a watersoluble polymerization initiator and irradiating the mixture with active energy rays. The process does not require any heating. WO 2010/096595 A2 discloses superabsorbent polymers having antimicrobial properties. The superabsorbent polymers are coated with 0.5 to 20% by weight of hydrogen peroxide and dried. EP 2 915 548 A1 discloses a process for production of superabsorbent particles including the addition of peroxides after polymerization. CN 101 045 789 A, CN 102 311 599 A, WO 2016/134905 A1, CN 104 744 711 A, US 2007/078231 A1, EP 0 248 437 A2 and US 2010/298513 A1 disclose a process for production of superabsorbent particles. It was an object of the present invention to provide a process for producing superabsorbent polymer particles having an improved long-term color stability. The object was achieved by a process for producing long-term color stable superabsorbent polymer particles, comprising polymerization of a monomer solution, comprising a) partly neutralized acrylic acid,b) 0.05 to 1.5% by weight, based on acrylic acid prior to neutralization, of at least one crosslinker, andc) at least one initiator, drying the resulting polymer gel, optionally grinding and classifying the resulting dried polymer gel and thermally surface post-crosslinking and cooling the resulting polymer particles, wherein 0.005 to 5% by weight of a thermal surface post-crosslinker and 0.016 to 0.080% by weight of hydrogen peroxide, based in each case on the polymer particles, are added to the polymer particles prior to the thermal surface post-crosslinking. The thermal surface post-crosslinker is added to the polymer particles pri