Method of making carboxylated cellulose fibers and products of the method

• US 6,524,348 B1
• Filed: 08/17/2000
• Issued: 02/25/2003
• Est. Priority Date: 03/19/1999
• Status: Expired due to Term

• Current Assignee: International Paper Company
• Original Assignee: Weyerhaeuser Company (Weyerhauser)
• Inventors: Li, Yong, Su, Bing, Weerawarna, S. Ananda, Komen, Joseph Lincoln, Jewell, Richard A.
• Primary Examiner(s): EINSMANN, MARGARET V
• Application Number:
  US09/641,276
• Time in Patent Office: 922 Days
• Field of Search: 818/1, 811/61, 162/9
• US Class Current: 8/116.1
• CPC Class Codes: C08B 15/04 Carboxycellulose, e.g. prepared by oxidation with nitrogen dioxide
• CPC Class Codes: D21C 9/004 inorganic compounds
• CPC Class Codes: D21C 9/005 organic compounds
• CPC Class Codes: D21H 11/20 Chemically or biochemically modified fibres

Abstract

The invention is directed to a method of making carboxylated cellulose fibers whose fiber strength and degree of polymerization is not significantly sacrificed. The method involves the use of cyclic nitroxide free radical compounds as a primary oxidant and a hypohalite salt as a secondary oxidant in an aqueous environment. Preferably the oxidized cellulose is then stabilized against D.P. loss in alkaline environments and color reversion with a reducing agent such as sodium borohydride. Alternatively it may be treated with an oxidant such as sodium chlorite. The method results in a high percentage of carboxyl groups located at the fiber surface. The product is especially useful as a papermaking fiber where it contributes strength and has a higher attraction for cationic additives. The product is also useful as an additive to recycled fiber to increase strength. The method can be used to improve properties of either virgin or recycled fiber. It does not require high α-cellulose fiber but is suitable for regular market pulps.

Claims

1. A method of making a stable fibrous carboxylated cellulose suitable as a papermaking fiber which comprises:

oxidizing cellulose fibers selected from the group consisting of bleached and unbleached kraft wood pulps, prehydrolyzed kraft wood pulps, sulfite wood pulps, and mixtures thereof in which the α

-cellulose content does not exceed about 90% by reacting them in an aqueous alkaline suspension with a sufficient amount of a primary oxidant selected from the group consisting of heterocyclic nitroxides stable under aqueous alkaline conditions in which the carbon atoms adjacent the nitroxide nitrogen lack hydrogen substitution, the corresponding oxammonium salts, amines, and hydroxylamines of these compounds, and mixtures thereof, and a sufficient amount of a secondary oxidant to induce an increase in carboxyl substitution in the cellulose of at least 2 meq/100 g; and

protecting the carboxylated fibers against degree of polymerization (D.P.) loss by further treating them in aqueous suspension with a stabilizing agent selected from the group consisting of reducing agents and tertiary oxidizing agents, the reducing agents being selected from the group consisting of alkali metal borohydrides, cyanoborohydrides, and mixtures thereof, and the tertiary oxidizing agents being selected from the group consisting of alkali metal chlorites, permanganates, chromic acid, bromine, silver oxide, chlorine dioxide, hydrogen peroxide, and mixtures thereof, in order to remove any cellulose substituents which tend to cause molecular chain breakage.

2. The method of claim 1 in which the nitroxides have a six membered ring structure with di-lower alkyl substitution on each carbon atom adjacent the nitroxide.

3. The method of claim 2 in which the nitroxides are compositions having the structure

in which R₁-R₄ are one to four carbon alkyl groups but R₁ with R₂ and R₃ with R₄ may together be included in a five or six carbon alicyclic ring structure, X is sulfur or oxygen, and R₅ is hydrogen, C₁-C₁₂ alkyl, benzyl, 2-dioxanyl, a dialkyl ether, an alkyl polyether, or a hydroxyalkyl, and X with R₅ being absent may be hydrogen or a mirror image moiety to form a bipiperidinyl nitroxide.

4. The method of claim 3 in which the nitroxide is 2,2,6,6-tetramethylpiperidinyl-1-oxy free radical.

5. The method of claim 3 in which the nitroxide is 2,2,2′

,2′

,6,6,6′

,6′

-octamethyl-4,4′

-bipiperidinyl-1,1′

-dioxy di-free radical.

6. The method of claim 3 in which the nitroxide is 2,2,6,6-tetramethyl-4-hydroxypiperidinyl-1-oxy free radical.

7. The method of claim 3 in which the nitroxide is 2,2,6,6-tetramethyl-4-methoxypiperidinyl-1-oxy free radical.

8. The method of claim 3 in which the nitroxide is 2,2,6,6-tetramethyl-4-benzyloxypiperidinyl-1-oxy free radical.

9. The method of claim 2 in which the nitroxides are compositions having the structure

in which R₁-R₄ are one to four carbon alkyl groups but R₁ with R₂ and R₃ with R₄ may together be included in a five or six carbon alicyclic ring structure, and R₆ is hydrogen or C₁-C₅ alkyl, and R₇ is hydrogen, C₁-C₈ alkyl, phenyl, carbamoyl, alkyl carbamoyl, phenyl carbamoyl, or C₁-C₈ acyl.

10. The method of claim 9 in which the nitroxide is 2,2,6,6-tetramethyl-4-aminopiperidinyl-1-oxy free radical.

11. The method of claim 9 in which the nitroxide is 2,2,6,6-tetramethyl-4-acetylaminopiperidinyl-1-oxy free radical.

12. The method of claim 2 in which the nitroxides are compositions having the structure

In which R₁-R₄ are one to four carbon alkyl groups but R₁ with R₂ and R₃ with R₄ may together be included in a five or six carbon alicyclic ring structure, and X is oxygen, sulfur, NH, N-alkyl, NOH, or NO₈ where R₈ is lower alkyl.

13. The method of claim 3 in which the nitroxide is 2,2,6,6-tetramethyl-4-piperidone-1-oxy free radical.

14. The method of claim 2 in which the nitroxides are compositions having the structure

wherein R₁-R₄ are one to four carbon alkyl groups but R₁ with R₂ and R₃ with R₄ may be linked into a five or six carbon alicyclic ring structure, X is oxygen, sulfur, -alkyl amino, or acyl amino.

15. The method of claim 14 in which the nitroxide is 3,3,5,5-tetramethylmorpholine-1-oxy free radical.

16. The method of claim 2 in which the nitroxides are compositions having the structure

wherein R₁-R₄ are one to four carbon alkyl groups but R₁ with R₂ and R₃ with R₄ may be linked into a five or six carbon alicyclic ring structure.

17. The method of claim 16 in which the nitroxide is 3,4-dehydro-2,2,6,6,-tetramethylpiperidinyl-1-oxy free radical.

18. The method of claim 2 in which the nitroxides are compositions having the structure

wherein R₁-R₄ are one to four carbon alkyl groups but R₁ with R₂ and R₃ with R₄ may together be included in a five or six carbon alicyclic ring structure, X is methylene, oxygen, sulfur, or alkylamino, and R₉ and R₁₀ are one to five carbon alkyl groups and may together be included in a five or six member ring structure, which, in turn, may contain one to four substituent groups.

19. The method of claim 18 in which each X is oxygen, the oxygen atoms being linked by a two to three carbon alkyl chain to form a cyclic acetal substituent.

20. The method of claim 19 in which the nitroxide composition is the ethylene cyclic acetal of 2,2,6,6-tetramethyl-4-piperidone-1-oxy free radical.

21. The method of claim 19 in which the nitroxide composition is the propylene cyclic acetal of 2,2,6,6-tetramethyl-4-piperidone-1-oxy free radical.

22. The method of claim 19 in which the nitroxide composition is the neopentyl cyclic acetal of 2,2,6,6-tetramethyl-4-piperidone-1-oxy free radical.

23. The method of claim 1 in which the nitroxides have a five atom ring structure with 2,2,5,5 lower alkyl substitution.

24. The method of claim 23 in which the nitroxides are compositions having the structure

in which R₁-R₄ are one to four carbon alkyl groups but R₁ with R₂ and R₃ with R₄ may together be included in a five or six carbon alicyclic ring structure, and X may be methylene, sulfur, oxygen, —

NH, or NR₁₁, in which R₁₁ is a lower alkyl.

25. The method of claim 24 in which the nitroxide composition is 2,2,5,5-tetramethylpyrrolidinyl-1-oxy free radical.

26. The method of claim 1 in which the secondary oxidant is a hypohalite salt.

27. The method of claim 26 in which the hypohalite salt is sodium hypochlorite.

28. The method of claim 26 in which the hypohalite salt is formed by an admixture of sodium hypochlorite and sodium bromide.

29. The method of claim 26 in which the primary oxidant and hypohalite salt are premixed in the presence of water, whereupon the mixture is added to a buffered suspension of cellulose fibers in water.

30. The method of claim 29 in which the fiber suspension is buffered to a pH between about 8-11 during the oxidizing reaction.

31. The method of claim 30 in which the fiber suspension is buffered to a pH between about 9-10 during the oxidizing reaction.

32. The method of claim 1 in which the primary oxidant is present in a range of 0.005-1.0% based on weight of cellulose present.

33. The method of claim 32 in which the primary oxidant is present in the range of 0.02-0.25% based on weight of cellulose present.

34. The method of claim 1 in which the secondary oxidant is present in the range of 0.5-35% based on weight of cellulose present.

35. The method of claim 34 in which the secondary oxidant is present in the range of 1.3-10.5% based on weight of cellulose present.

36. The method of claim 1 in which the initial oxidation step is carried out for a time between 1 minute and about 10 hours.

37. The method of claim 1 in which the reducing agent stabilizes the product by substantially converting any aldehyde or ketone carbonyl substituents to hydroxyl groups.

38. The method of claim 1 in which the carboxyl substitution is raised between about 2-150 meq/100 g over the carboxyl content of the starting cellulose.

39. The method of claim 1 in which the reducing agent is present in the aqueous suspension in a concentration of about 0.1-100 g/L.

40. The method of claim 39 in which the reducing agent is present in the aqueous suspension in a concentration of about 0.25-5 g/L.

41. The method of claim 37 in which the reduction reaction proceeds for a time of at least 10 minutes.

42. The method of claim 41 in which the reduction reaction proceeds for a time between 10 minutes and 2 hours.

43. The method of claim 1 by substantially converting any aldehyde substituents to additional carboxyl groups.

44. The method of claim 43 which comprises further stabilizing the carboxylated cellulose fibers after treatment with the tertiary oxidizing agent by treatment with a reducing agent.

45. The method of claim 43 in which the tertiary oxidant is selected from the group consisting of alkali metal chlorites, a chlorine dioxide/hydrogen peroxide mixture, and mixtures thereof.

46. The method of claim 43 in which the tertiary oxidant is a mixture of chlorine dioxide and hydrogen peroxide.

47. The method of claim 23 in which the tertiary oxidant is sodium chlorite.

48. The method of claim 47 in which the sodium chlorite is present during the stabilization reaction in a concentration of 0.001-0.20 grams sodium chlorite per gram of fiber.

49. The method of claim 48 in which the sodium chlorite is present during the stabilization reaction in a concentration of 0.01-0.09 grams of sodium chlorite per gram of fiber.

50. The method of claim 43 in which the tertiary oxidant is present in the aqueous suspension during the stabilization reaction in a molar ratio of 5-10 times the presumed aldehyde substitution on the carboxylated cellulose.

51. The method of claim 43 in which the oxidation during the stabilization reaction proceeds for a time between 5 minutes and 50 hours.

52. The method of claim 51 in which the oxidation during the stabilization reaction proceeds for a time between 10 minutes and 2 hours.

53. The method of claim 44 in which the tertiary oxidizing agent is sodium chlorite and the reducing agent is sodium borohydride.

54. The method of claim 43 in which the oxidative stabilization treatment is carried out under acidic conditions at a pH between about 1.5-5.

55. The method of claim 54 in which the oxidative stabilization treatment is carried out under acidic conditions at a pH between about 2-4.

56. The method of claim 1 in which the cellulose fiber is recycled secondary fiber.

57. A method of making a stable fibrous carboxylated cellulose suitable as a papermaking fiber which comprises:

oxidizing cellulose fibers selected from the group consisting of bleached and unbleached kraft wood pulps, prehydrolyzed kraft wood pulps, sulfite wood pulps, and mixtures thereof in which the α

-cellulose content does not exceed about 90% by reacting them in an aqueous alkaline suspension with a sufficient amount of a primary oxidant selected from the group consisting of nitroxides stable under aqueous alkaline conditions and having the composition

 

wherein R₁-R₄ are one to four carbon alkyl groups but R₁ with R₂ and R₃ with R₄ may together be included in a five or six carbon alicyclic ring structure, X is methylene, oxygen, sulfur, or alkylamino, and R₉ and R₁₀ are one to five carbon alkyl groups and may together be included in a five or six member ring structure, which, in turn, may contain one to four substituent groups, the corresponding oxammonium compounds, amines, and hydroxylamines of these nitroxides, and mixtures thereof, and a sufficient amount of a secondary oxidant to induce an increase in carboxyl substitution in the cellulose of at least 2 meq/100 g; and

protecting the carboxylated fibers against degree of polymerization (D.P.) loss by further treating them in aqueous suspension with a stabilizing agent selected from the group consisting of reducing agents and tertiary oxidizing agents, the reducing agents being selected from the group consisting of alkali metal borohydrides, cyanoborohydrides, and mixtures thereof, and the tertiary oxidizing agents being selected from the group consisting of alkali metal chlorites, permanganates, chromic acid, bromine, silver oxide, chlorine dioxide, hydrogen peroxide, and mixtures thereof, in order to remove any cellulose substituents which tend to cause molecular chain breakage.

58. The method of claim 57 in which each X is oxygen, the oxygen atoms being linked by a two to three carbon alkyl chain to form a cyclic acetal substituent.

59. The method of claim 58 in which the cyclic acetal is selected from the group consisting of the ethylene, propylene, and neopentyl acetals of 2,2,6,6-tetramethyl-4-piperidone-1-oxy free radicals and mixtures thereof.

60. The method of claim 59 in which the nitroxide composition is the ethylene cyclic acetal of 2,2,6,6-tetramethyl-4-piperidone-1-oxy free radical.

61. The method of claim 59 in which the nitroxide composition is the propylene cyclic acetal of 2,2,6,6-tetramethyl-4-piperidone-1-oxy free radical.

62. The method of claim 59 in which the nitroxide composition is the neopentyl cyclic acetal of 2,2,6,6-tetramethyl-4-piperidone-1-oxy free radical.

63. A method of making a stable fibrous carboxylated cellulose suitable as a papermaking fiber which comprises:

oxidizing cellulose fibers selected from the group consisting of bleached and unbleached kraft wood pulps, prehydrolyzed kraft wood pulps, sulfite wood pulps, and mixtures thereof in which the α

-cellulose content does not exceed about 90% by reacting them in an aqueous alkaline suspension with a sufficient amount of a primary oxidant selected from the group consisting of heterocyclic nitroxides stable under aqueous alkaline conditions in which the carbon atoms adjacent the nitroxide nitrogen lack hydrogen substitution, the corresponding oxammonium salts, amines, and hydroxylamines of these compounds, and mixtures thereof, and a sufficient amount of a secondary oxidant to induce an increase in carboxyl substitution in the cellulose of at least 2 meq/100 g; and

protecting the carboxylated fibers against degree of polymerization (D.P.) loss by further treating them in aqueous suspension with a tertiary oxidizing stabilizing agent selected from the group consisting of alkali metal chlorites, permanganates, chromic acid, bromine, silver oxide, chlorine dioxide, hydrogen peroxide, and mixtures thereof in order to remove any cellulose substituents which tend to cause molecular chain breakage.

64. The method of claim 63 in which the heterocyclic nitroxide has the composition

wherein R₁-R₄ are one to four carbon alkyl groups but R₁ with R₂ and R₃ with R₄ may together be included in a five or six carbon alicyclic ring structure, X is methylene, oxygen, sulfur, or alkylamino, and R₉ and R₁₀ are one to five carbon alkyl groups and may together be included in a five or six member ring structure, which, in turn, may contain one to four substituent groups, the corresponding oxammonium compounds, amines, and hydroxylamines of these nitroxides, and mixtures thereof.