Acidic superabsorbent hydrogels

Abstract

The present invention relates to novel hydrophilic swellable addition polymers comprising improved odor control, a process for their preparation and their use for absorbing aqueous fluids and a process for screening superabsorbents.

1 . Hydrogel forming polymers capable of absorbing aqueous fluids and having a pH absorbency index PH AI , which is calculated as follows:  
         pH   AI   =ΔpH ·( AUL   0.7psi   +CRC ) where  ΔpH= 7 −pH  of product  AUL 0.7psi  is the absorbency under pressure at 0.7 psi    CRC is the centrifuge retention capacity    of at least 80, preferably at least 90 and especially at least 100, and having a pH of not more than 5.7:    
     
     
         2 . Hydrogel forming polymers as claimed in  claim 1 , having a pH of from 4 to 5.5 and especially from 4.4 to 4.6 or from 5.1 to 5.3.  
     
     
         3 . Hydrogel forming polymers as claimed in  claim 1  or  2 , having a Nessler value of less than 60%, preferably less than 40% and especially less than 30%.  
     
     
         4 . Hydrogel forming polymers as claimed in any of  claims 1  to  3 , having a CRC greater than 15 g/g and preferably greater than 26 g/g.  
     
     
         5 . Hydrogel forming polymers as claimed in any of  claims 1  to  4 , having an AUL 0.7 psi greater than 13 g/g and preferably greater than 15 g/g.  
     
     
         6 . Hydrogel forming polymers as claimed in any of  claims 1  to  5 , obtainable from a base polymer having a pH of not more than 5.7, preferably from 4 to 5.5 and especially from 4.4 to 4.6 or from 5.1 to 5.3 by surface postcrosslinking.  
     
     
         7 . Hydrogel forming polymers as claimed in any of  claims 1  to  6 , being an acrylic acid (co)polymer partially neutralized to an alkali metal, alkaline earth metal, ammonium or amine salt.  
     
     
         8 . Hydrogel forming polymers as claimed in any of  claims 1  to  7 , having antimicrobial properties.  
     
     
         9 . The use of hydrogel forming polymers as claimed in any of  claims 1  to  8  for preparing absorbent articles capable of permanently or temporarily binding aqueous fluids.  
     
     
         10 . A process for preparing hydrogel forming polymers as claimed in any of  claims 1  to  8 , which comprises surface postcrosslinking a base polymer having a pH of not more than 5.7, preferably from 4 to 5.5 and especially from 4.4 to 4.6 or from 5.1 to 5.3.  
     
     
         11 . Gel prepared by polymerization of partially neutralized acrylic acid with a crosslinking agent, wherein 
 (i) the partial neutralization can be effected using NaOH 50%: 20-30 mol % based on acrylic acid, preferably 22-29 mol %, more preferably 23 to 28 mol %, particularly preferably 24 to 27 mol %, especially about 25.2 mol % based on acrylic acid, a corresponding neutralization can also be achieved with other neutralizing agents, 
 the crosslinking can be effected using polyethylene glycol 400 diacrylate: 0.005-1.0% by weight based on acrylic acid, preferably 0.1-0.4% by weight, particularly preferably 0.15-0.25% by weight, especially about 0.2% by weight based on acrylic acid;  
 or some other crosslinker which produces the same degree of crosslinking,  
 or  
   (ii)the partial neutralization can be effected using NaHCO 3 : 
 23-28% by weight based on acrylic acid, preferably 27 to 35% by weight, particularly preferably 30 to 33% by weight, especially about 31.5% by weight based on acrylic acid, a corresponding neutralization can also be achieved with other neutralizing agents,  
 the crosslinking can be effected using allyl methacrylate: 0.005-1.0% by weight based on acrylic acid, preferably 0.1-0.5% by weight, particularly preferably 0.2-0.4% by weight, especially about 0.3% by weight based on acrylic acid;  
 or some other crosslinker which produces the same degree of crosslinking,  
 or (iii) the partial neutralization can be effected using LiOH×H 2 O: 15-30% by weight based on acrylic acid, preferably 17 to 25% by weight, particularly preferably 19 to 23% by weight, especially about 20.4% by weight based on acrylic acid, a corresponding neutralization can also be achieved with other neutralizing agents,  
 the crosslinking can be effected using allyl methacrylate: 0.005-1.0% by weight based on acrylic acid, preferably 0.1-0.6% by weight, particularly preferably 0.3-0.5% by weight, especially about 0.4% by weight based on acrylic acid;  
 or some other crosslinker which produces the same degree of crosslinking,  
 or  
   (iv)the partial neutralization can be effected using NaHCO 3 : 
 35-55% by weight based on acrylic acid, preferably 40 to 50% by weight, particularly preferably 43 to 46% by weight, especially about 44.5% by weight based on acrylic acid, a corresponding neutralization can also be achieved using other neutralizing agents when  
 2-acrylamido-2-methylpropanesulfonic acid is present at 30-55% by weight based on acrylic acid, preferably 35 to 50% by weight, particularly preferably 41 to 45% by weight, especially about 43% by weight based on acrylic acid additionally to acrylic acid,  
 the crosslinking can be-effected using allyl methacrylate: 0.005-1.0% by weight based on acrylic acid, preferably 0.1-0.6% by weight, particularly preferably 0.3-0.5% by weight, especially about 0.4% by weight based on acrylic acid,  
 or some other crosslinker which produces the same degree of crosslinking.  
   
     
     
         12 . Base polymer obtainable by drying a gel mentioned in  claim 11 .  
     
     
         13 . A process for screening superabsorbents, which comprises determining the pH, CRC and AUL 0.7 psi of a plurality of superabsorbent samples and determining the pH absorbency index therefrom.  
     
     
         14 . A process for optimizing superabsorbents, which comprises iterating a process as claimed in  claim 13  and further varying the production parameters of the samples having the highest pH absorbency indices.  
     
     
         15 . Hydrogel forming polymers as claimed in any of  claims 1  to  8 , obtainable by a process as claimed in  claim 14 .  
     
     
         16 . Hygiene articles comprising 
 (A) a liquid pervious topsheet    (B) a liquid impervious backsheet    (C) a core positioned between (A) and (B) and comprising 
 10-100% by weight of the hydrogel forming polymer 0-90% by weight of hydrophilic fiber material as set forth in any of  claims 1  to  8  or  15   
   (D) optionally a tissue layer positioned directly above and below said core (C) and    (E) optionally an acquisition layer positioned between (A) and (C).