Granted Patent B2

US 7,591,974 · App. 11/657,869 · Granted Sep 22, 2009

Abstract

Claims (27)

1. A method of producing a superabsorbent polymer product for use in agricultural applications, comprising:

graft polymerizing a monomer onto a starch in the presence of an initiator to form a starch graft copolymer, the starch graft copolymer forming a polymerization dough;

feeding the polymerization dough into a heated extruder comprising an extrusion screw at a pressure of between 25 psi and 40 psi, the polymerization dough having a viscosity of between 2.5 million centipoise and 0.5 million centipoise when introduced to the heated extruder;

operating the extrusion screw at a speed of between about 20 rpm to about 150 rpm;

reducing the moisture content of the polymerization dough by drying the polymerization dough in the heated extruder, wherein the heated portions of the extruder heat the polymerization dough at a temperature of between 50° C. and 160° C.;

cooling the polymerization dough in such a manner to allow moisture to be drawn to the outer surface of the polymerization dough after exiting the heated extruder; and

drying the polymerization dough a second time after cooling the polymerization dough.

2. The method of claim 1 , wherein reducing the moisture content comprises reducing the moisture content of the polymerization dough to less than 65% by weight by drying the polymerization dough in the heated extruder.

3. The method of claim 1 , wherein reducing the moisture content comprises reducing the moisture content of the polymerization dough to about 40% by weight or less by drying the polymerization dough in the heated extruder.

4. The method of claim 3 , wherein reducing the moisture content of the polymerization dough comprises passing the polymerization dough through the heated extruder multiple times until the moisture content is about 40% by weight or less.

5. The method of claim 1 , further comprising:

saponifying the starch graft copolymer before drying the polymerization dough in the heated extruder;

wherein the monomer is at least acrylonitrile.

6. The method of claim 1 , further comprising:

adjusting a pH of the starch graft copolymer to between about 6.0 and about 8.0 before drying the polymerization dough in the heated extruder.

7. The method of claim 6 , further comprising:

cross-linking the starch graft copolymer;

wherein the monomer is other than acrylonitrile and the monomer is chosen from: acrylic acid, acrylamide, methacrylamide, 2-acrylamido-2-methyl-propanesulfonic acid, methacrylic acid, vinyl sulfonic acid, ethyl acrylate, potassium acrylate, and derivatives and mixtures thereof.

8. The method of claim 1 , further comprising:

supplying an additive to the polymerization dough, wherein the additive is chosen from at least one of the following: fertilizers, herbicides, pesticides, fungicides, and growth regulators.

9. The method of claim 1 , wherein reducing the moisture content comprises heating the polymerization dough in the heated extruder at a temperature between about 125° C. and about 150° C.

10. The method of claim 1 , wherein the extrusion screw is operated at a speed between about 50 rpm and about 80 rpm.

11. The method of claim 1 , wherein the heated extruder comprises at least one vent, the method further comprising:

applying a negative pressure to the at least one vent to facilitate the removal of steam from the heated extruder.

12. The method of claim 1 , wherein cooling the polymerization dough further comprises cooling the polymerization dough at an interval within the heated extruder.

13. The method of claim 1 , further comprising:

granulating the polymerization dough after the moisture content of the polymerization dough is reduced in the heated extruder.

Assignments (9)