Catalytic carboxylation of cellulose fibers in a continuous process with multiple additions of catalyst, secondary oxidant and base to a moving slurry of cellulose fibers

1. A method of making a stable fibrous carboxylated cellulose, the method comprising:

carboxylating cellulose wood pulp fibers in an aqueous alkaline suspension by catalytically carboxylating the fibers in at least two catalytic carboxylation stages in which the catalytic carboxylation stages are in series and each later catalytic carboxylation stage further catalytically carboxylates the carboxylated fibers from the previous catalytic carboxylation stage;

wherein the first catalytic carboxylation stage is provided with a precursor of primary oxidant, the active oxammonium salt catalyst, selected from the group consisting of heterocyclic nitroxides, their corresponding hydroxyl amines or their corresponding amines, stable under aqueous alkaline conditions in which the carbon atoms adjacent the nitroxide nitrogen lack hydrogen substitution, and mixtures thereof; a secondary oxidant to regenerate the active catalyst; and a sufficient amount of alkaline material to cause the fibers to be at an alkaline pH of 8-11 at the beginning of the catalytic carboxylation stage to carboxylate the cellulose fibers;

wherein each subsequent catalytic carboxylation stage is provided additional amounts of the active catalyst precursor, the secondary oxidant, and alkaline material to adjust the pH to 8-11 to further carboxylate the cellulose fibers from the previous catalytic carboxylation stage; and

wherein the catalytically carboxylated cellulose fibers have a carboxylation level of 18 to 100 meq. per 100 g of oven dry pulp.

2. The method of claim 1 in which the secondary oxidant is chlorine dioxide.

3. The method of claim 1 in which there are 2 to 10 catalytic carboxylation stages.

4. The method of claim 1 in which the precursor of the active oxammonium salt catalyst selected from the group consisting of heterocyclic nitroxides, their corresponding hydroxylamines or their corresponding amines; secondary oxidant to regenerate the active catalyst; and the alkaline material are added at the start of each catalytic carboxylation stage.

5. The method of claim 1 in which the time in each catalytic carboxylation stage is from 10 seconds to 5 minutes.

6. The method of claim 1 in which the temperature at the beginning of each catalytic carboxylation stage is from 25 to 65° C.

7. The method of claim 1 in which the pH at the beginning of each catalytic carboxylation stage is 9-11.

8. The method of claim 1 in which the alkaline material is a combination of sodium hydroxide and sodium bicarbonate or sodium carbonate.

9. The method of claim 1 in which after the last catalytic carboxylation stage the carboxylated fibers are stabilized by further treating them in aqueous suspension with a peroxide stabilizing agent, chlorite stabilization agent, and an acid stabilizing agent.

10. The method of claim 9 in which aldehyde groups on the carboxylated fibers are converted to carboxyl groups.

11. The method of claim 1 in which the carboxylated fibers are fibrillated.