Super-absorbent polymer having improved absorbency properties

• US 5,145,906 A
• Filed: 04/15/1991
• Issued: 09/08/1992
• Est. Priority Date: 09/28/1989
• Status: Expired due to Fees

• Current Assignee: Hoechst Celanese Corporation (Sanofi )
• Original Assignee: Hoechst Celanese Corporation (Sanofi )
• Inventors: Fujiura, Yoji, Chambers, Douglas R., Fowler, Hubert H. Jr., Masuda, Fusayoshi
• Primary Examiner(s): Schofer, Joseph L.
• Assistant Examiner(s): DELMENDO, ROMULO H
• Application Number:
  US07/685,974
• Time in Patent Office: 512 Days
• Field of Search: 524/732, 524/734, 525/119, 525/327.8, 527/314
• US Class Current: 524/732
• CPC Class Codes: C08F 251/00 Macromolecular compounds obtained by polymerising monomers on to polysaccharides or derivatives thereof
• CPC Class Codes: C08F 8/00 Chemical modification by after-treatment graft polymers, block polymers, crosslinking with unsaturated monomers or with polymers C08F251/00 - C08F299/00; of conjugated diene rubbers C08C

Abstract

Super-absorbent polymers having superior dryness properties when incorporated into absorbent articles are made from acrylic acid and crosslinking agent polymerized under controlled conditions.

Claims

1. A process of preparing a super-absorbent polymer which comprises:

(a) forming an aqueous solution of acrylic acid, a water soluble polysaccharide, and a multifunctional monomer, wherein said multifunctional monomer has at least two polymerizable ethylenically unsaturated double bonds per molecule, wherein said acrylic acid and said polysaccharide are present in the amount of about 90 to about 100 weight percent acrylic acid and about 0 to about 10 weight percent polysaccharide wherein said weight percents are based on the weight of the acrylic acid and the polysaccharide,

wherein said multifunctional monomer is present in the amount of about 0.075 to about 1 mole percent based on the moles of acrylic acid, and wherein the acrylic acid and the polysaccharide are present in the aqueous solution in the amount of about 5 to about 30 weight percent wherein the weight percent is based on the weight of acrylic acid, polysaccharide and water;

(b) adding to the aqueous solution at a temperature of about 5°

to about 20°

C., a redox catalyst system and a thermal free radical initiator wherein the reducing agent part of the redox catalyst is present in the amount of about 6×

10^-5 to about 2.5×

10^-3 mole percent based the moles of acrylic acid,

wherein the oxidizing agent part of the redox catalyst is present in the amount of about 3.4×

10^-3 to about 0.42 mole percent based on the moles of acrylic acid and wherein the thermal free radical initiator is present in the amount of about 0.1 to about 0.4 weight percent based on the weight of acrylic acid,

(c) allowing the exothermic temperature to rise under adiabatic conditions to a peak temperature not to exceed 90°

C., thereby forming a polymer gel

(d) holding the temperature within 10°

C. of the peak temperature for a time sufficient to reduce residual monomer content below 1000 ppm;

(e) neutralizing about 50 to about 100 percent of the carboxylic acid groups in the polymer with an aqueous base;

(f) adding a multifunctional compound having at least two groups capable of forming ionic or covalent bonds with carboxylic acid groups wherein said multifunctional compound is added in the amount of about 0.05 to about 15 mole percent based on the moles of acrylic acid;

(g) heating at a temperature of about 20°

C. to about 200°

C. to dry the polymer to a moisture content below about 10 percent by weight;

(h) and grinding the polymer to a particle size of about 20 to 400 mesh (U.S. Standard Sieve).

2. The process of claim 1 wherein the polysaccharide is a natural starch.

3. The process of claim 1 wherein the multifunctional monomer is present in the amount of about 0.1 to about 0.3 mole percent, the reducing agent part of the redox catalyst system is present in the amount of about 6×

10^-4 to about 2.5×

10^-3 mole percent, the oxidizing agent part of the redox catalyst is present in the amount of about 0.15 to about 0.25 mole percent, the thermal free radical initiator is present in the amount of about 0.25 to about 0.35 weight percent and the multifunctional compound is added in the amount of about 0.05 to about 0.15 mole percent.

4. The process of claim 1 wherein the exothermic peak temperature is about 60°

C. to about 75°

C. and the temperature is held within 5°

C. of the peak temperature.

5. The process of claim 1 wherein the polymer gel after the addition of the multifunctional compound is heated at about 100°

C. to about 200°

C. to effect the post-crosslinking reaction and to dry the polymer.

6. The process of claim 1 wherein the multifunctional monomer is tetraallyloxyethane and the post-crosslinking compound is the diglycidyl ether of ethylene glycol.

7. The process of claim 1 wherein the reducing agent is ascorbic acid, the oxidizing agent is hydrogen peroxide an the thermal free radical initiator is 2,2-azobisamidino propane dihydrochloride.