Suspension polymerized aqueous absorbent polymer particles

Abstract

A process for making clusters of individual polymer beads bonded together to minimize attritition in use or processing but characterized by an open structure for fast absorption of aqueous solutions comprises suspending a monomer mixture in a continuous phase at high shear agitation such that fine droplets of monomer are formed in an inert organic phase. A suspending agent which prevents the clusters from coalescing is provided. A first polyethylenically unsaturated crosslinking monomer, having substantial solubility in the aqueous phase, and an organic phase soluble initiator system are provided such that polymerization at the surface of the polymer particles occurs to form the porous clusters of the invention, A second substantially oil soluble crosslinker is optionally provided to further crosslink the clusters such that they do not coalesce.

Description

Claims

1. A process for preparing a polymer material, consisting essentially of:

(a) suspending an aqueous monomer mixture comprising a water soluble ethylenically unsaturated monomer in an inert organic liquid to form a suspension having a discontinuous phase of monomer droplets, said monomer mixture including a first polyethylenically unsaturated crosslinker having a solubility in water of at least about 0.1 grams crosslinker to 100 grams water, said suspension further containing a polymerization initiator which will partition such as to provide at least about 10 ppm polymerization initiator in said inert organic liquid, said suspension further containing a suspending agent to partially stabilize said suspension;

b) initiating the polymerization of said monomer and said first crosslinker such that a crosslinking polymerization occurs at the interface between the monomer droplets and organic liquid, wherein individual particles of polymer are bonded together to form clusters and wherein during said polymerization, a second polyethylenically unsaturated crosslinker having a solubility in the organic liquid continuous phase greater than about 0.1 grams per 100 grams organic liquid continuous phase is added; and

c) further polymerizing said clusters and second crosslinker such that further polymerization occurs at the cluster/organic phase interface.

2. The process of claim 1 wherein said ethylenically unsaturated monomer is an amide, carboxylic acid or its esters, vinyl amine, or their salts or mixture thereof.

3. The process of claim 1 wherein said polymer is crosslinked polyacrylic acid, crosslinked sodium polyacrylate or a crosslinked copolymer of acrylic acid and sodium acrylate.

4. The process of claim 1 wherein said monomer mixture also includes a component which graft polymerizes onto one or more other monomer components of the mixture.

5. The process of claim 4, wherein said component is polyvinyl alcohol.

6. The process of claim 5, wherein said clusters are greater than 75 micrometers in diameter.

7. The process of claim 6, wherein 2 grams of said clusters have a vortex rate of absorption of 50 grams of a 0.9 percent aqueous sodium chloride solution of less than 60 seconds.

8. The process of claim 1 further comprising:

(e) applying a wetting agent to said clusters.

9. The process of claim 8, wherein said wetting agent is a polyol.

10. The process of claim 1 wherein said suspending of said monomer mixture to form fine monomer droplets comprises agitating said monomer mixture at high shear rates.

11. The process of claim 1 wherein said suspending agent comprises a cellulose ether, provided in an amount of 0.5 weight percent, based on the weight of said monomer mixture.

12. The process of claim 1 wherein said suspending agent comprises a nonionic surface active composition having an HLB value greater than 7.

13. The process of claim 1 wherein said suspending agent comprises a mixture of hydrophobic silica and a copolymer of lauryl methacrylate and acrylic acid.

14. The process of claim 1 wherein said first crosslinker is trimethylol propane triacrylate (TMPTA), the diacrylate or dimethacrylate of ethylene glycol, the diacrylate or dimethacrylate of diethylene glycol, methylenebisacrylamide, triallyamine, tetraallyloxyethane, or combinations thereof.

15. The process of claim 14, wherein said first crosslinker comprises TMPTA in an amount of at least about 2000 ppm to less than 8000 ppm, based on the weight of said monomer.

16. The process of claim 14, wherein said second crosslinker is trimethylolpropane triacrylate (TMPTA) or tetraallyloxyethane.

17. The process of claim 16, wherein the second crosslinker comprises TMPTA in an amount from about 5000 ppm to about 20,000 ppm, based on the weight of said monomer.

18. The process of claim 1 wherein said polymerization initiator is t-butyl hydroperoxide, cumene hydroperoxide, or 2,5-dihydroperoxy-2,5-dimethylhexane.

19. The process of claim 18, wherein said initiating of step (b) is accomplished by introducing sulfur dioxide into said suspension.