Hypochlorite free method for preparation of stable carboxylated carbohydrate products Number:7,135,557 from the United States Patent and Trademark Office (PTO) owispatent A method of making a carboxylated carbohydrate is disclosed, cellulose being a preferred carbohydrate material. Carboxylated cellulose fibers can be produced whose fiber strength and degree of polymerization is not significantly sacrificed. The method involves the use of a catalytic amount of a hindered cyclic oxammonium compounds as a primary oxidant and chlorine dioxide as a secondary oxidant in an aqueous environment. The oxammonium compounds may be formed in situ from their corresponding amine, hydroxylamine, or nitroxyl compounds. The oxidized cellulose may be stabilized against D.P. loss and color reversion by further treatment with an oxidant such as sodium chlorite or a chlorine dioxide/hydrogen peroxide mixture. Alternatively it may be treated with a reducing agent such as sodium borohydride. In the case of cellulose 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 .alpha.-cellulose fiber but is suitable for regular market pulps.<H carbohydrate, carboxylated, Hypochlorite, fr, 7135557.html, Patent, pto, 7135557

Title: Hypochlorite free method for preparation of stable carboxylated carbohydrate products

Abstract: A method of making a carboxylated carbohydrate is disclosed, cellulose being a preferred carbohydrate material. Carboxylated cellulose fibers can be produced whose fiber strength and degree of polymerization is not significantly sacrificed. The method involves the use of a catalytic amount of a hindered cyclic oxammonium compounds as a primary oxidant and chlorine dioxide as a secondary oxidant in an aqueous environment. The oxammonium compounds may be formed in situ from their corresponding amine, hydroxylamine, or nitroxyl compounds. The oxidized cellulose may be stabilized against D.P. loss and color reversion by further treatment with an oxidant such as sodium chlorite or a chlorine dioxide/hydrogen peroxide mixture. Alternatively it may be treated with a reducing agent such as sodium borohydride. In the case of cellulose 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 .alpha.-cellulose fiber but is suitable for regular market pulps.

Patent Number: 7,135,557 Issued on 11/14/2006 to Weerawarna, et al.

Inventors: Weerawarna; S. Ananda (Seattle, WA), Komen; Joseph Lincoln (Bothell, WA), Jewell; Richard A. (Bellevue, WA)
Assignee: Weyerhaeuser Company (Federal Way, WA)

Appl. No.: 10/856,198

Filed: September 3, 2004

Current U.S. Class: 536/1.11 ; 536/102; 536/105; 536/110; 536/119; 536/124; 536/56; 536/63

Current International Class: C07H 1/00 (20060101); C07H 13/02 (20060101); C08B 3/00 (20060101); C08B 31/00 (20060101); C08B 31/04 (20060101)

Field of Search: 536/1.11,56,63,102,105,110,124,119

References Cited [Referenced By]

U.S. Patent Documents


3575177	April 1971	Briskin et al.
4100341	July 1978	Brasey et al.
4401810	August 1983	Tang et al.
4480089	October 1984	Chen et al.
4505775	March 1985	Harding et al.
4679278	July 1987	Caumont
4889654	December 1989	Mason et al.
5667637	September 1997	Jewell et al.
5755828	May 1998	Westland
5824462	October 1998	Ashida et al.
6031101	February 2000	Devine et al.
6117833	September 2000	Racherla et al.
6127573	October 2000	Li et al.
6228126	May 2001	Cimecioglu et al.
6379494	April 2002	Jewell et al.
6409881	June 2002	Jaschinski
6447644	September 2002	Seltzer et al.
6524348	February 2003	Jewell et al.
2002/0072600	June 2002	Bragd et al.

Foreign Patent Documents


0 574 666	Apr., 1993	EP
1 077 221	Feb., 2001	EP
1 077 285	Feb., 2001	EP
1 077 286	Feb., 2001	EP
2674528	Oct., 1992	FR
2001/49591	Feb., 2001	JP
WO 95/07303	Mar., 1995	WO
WO 96/36621	Nov., 1996	WO
WO 96/38484	Dec., 1996	WO
WO 98/27117	Jun., 1998	WO
WO 99/23117	May., 1999	WO
WO 99/57158	Nov., 1999	WO
WO 00/50388	Aug., 2000	WO
WO 00/50462	Aug., 2000	WO
WO 00/50463	Aug., 2000	WO
WO 00/50621	Aug., 2000	WO
WO 00/56978	Sep., 2000	WO
WO 01/29309	Apr., 2001	WO
WO 01/23657	May., 2001	WO

Other References

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Chang, P.S. and J. F. Robyt. Oxidation of primary alcohol groups of naturally occurring polysaccharides with 2,2,6,6-tetramethylpiperidine oxoammonium ion. Journal of Carbohydrate Chemistry 15(7): 819-830 (1996). cited by other .
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Ganiev, I. M., Q. K. Timerghazin, A. F. Khalizov, V. V. Shereshovets, I. M. Grofor'ev, and G. A. Tolskitov. Complex of Chlorine dioxide with TEMPO and its conversion into oxoammonium salt. Journal of Physical Organic Chemistry 14: 38-42 (2001). cited by other .
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de Nooy, A. E. J., A. C. Besemer, and H. van Bekkum. Highly selective TEMPO mediated oxidation of primary alcohol groups in polysaccharides. Receuil des Traveau Chimiques des Oays-Bas 113(3): 165-166 (1994). cited by other .
de Nooy, A. E. J., A. C. Besemer, and H. van Bekkum. Highly selective nitroxyl radical-mediated oxidation of primary alcohol groups in water soluble glucans. Carbohydrate Research 269:89-98 (1995). cited by other .
de Nooy, A. E. J., A. C. Besemer, and H. van Beckum. On the use of stable or-ganic nitroxyl radicals for the oxidation of primary and secondary alcohols. Syn-thesis: Journal of Synthetic Organic Chemistry Oct. 1996 pp. 1153-1174. cited by other .
Shenai, V. A. and A. S. Narkhede. Hypochlorite oxidation of cellulose in the presence of cobalt sulfide. Textile Dyer and Printer 20: 17-22 (1987). cited by other .
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Primary Examiner: Jiang; Shaojia Anna
Assistant Examiner: White; Everett

Parent Case Text

This application is a continuation of U.S. patent application Ser. No. 09/875,177 filed Jun. 6, 2001.

Claims

The invention claimed is:

1. A method of making a carboxylated carbohydrate product which comprises: oxidizing a carbohydrate compound by reacting the carbohydrate compound in an aqueous system with a sufficient amount of a primary oxidant selected from the group consisting of hindered heterocyclic oxammonium salts in which the carbon atoms adjacent the oxammoniuin nitrogen lack alpha-hydrogen substitution, the corresponding amines, hydroxylamines, and nitroxides of these oxammonium salts, and mixtures thereof, and a secondary oxidant selected from chlorine dioxide and latent sources of chlorine dioxide in a sufficient amount to induce an increase in carboxyl substitution in the carbohydrate of at least 2 meq/100 g.

2. The method of claim 1 in which the carbohydrate is selected from the group consisting of cellulose and starch.

3. The method of claim 2 in which the carbohydrate product is a starch.

4. The method of claim 2 in which the carbohydrate product is cellulose.

5. The method of claim 4 in which the cellulose is fibrous and is selected from the group consisting of bleached and unbleached kraft wood puips, prehydrolyzed kraft wood pulps, sulfite wood puips and mixtures thereof.

6. The method of claim 5 in which the cellulose fiber is recycled secondary fiber.

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

8. The method of claim 7 in which the nitroxides are compositions having the structure ##STR00011## in which R.sub.1 R.sub.4 are one to four carbon alkyl groups but R.sub.1 with R.sub.2 and R.sub.3 with R.sub.4 may together be included in a five or six carbon alicyclic ring structure, X is sulfur or oxygen, and R.sub.5 is hydrogen, C.sub.1 C.sub.12 alkyl, benzyl, 2-dioxanyl, a dialkyl ether, an alkyl polyether, or a hydroxyalkyl, and X with R.sub.5 being absent may be hydrogen or a mirror image moiety to form a bipiperidinyl nitroxide.

9. The method of claim 8 in which the nitroxide is 2,2,6,6-tetramethylpiperidinyl-1-oxy free radical.

10. The method of claim 1 in which the aqueous system is buffered to a pH between about 4 12 during the oxidizing reaction.

11. The method of claim 10 in which the aqueous system is buffered to a pH between about 6 8 during the oxidizing reaction.

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

13. The method of claim 12 in which the primary oxidant is present in the range of 0.02 0.25% based on weight of carbohydrate present.

14. The method of claim 1 in which the secondary oxidant is present in the range of 0.2 35% based on weight of carbohydrate present.

15. The method of claim 14 in which the secondary oxidant is present in the range of 0.5 10% based on weight of carbohydrate present.

16. The method of claim 1 in which the initial oxidation step is carried out for a time between 0.5 minutes and about 50 hours.

17. The method of claim 16 in which the initial oxidation step is carried out for a time between about 10 minutes to 2 hours.

18. The method of claim 1 in which the carbohydrate is a polymeric compound and which further comprises protecting the carbohydrate against degree of polymerization (D.P.) loss by further treating it in aqueous suspension with a stabilizing agent selected from the group consisting of oxidizing agents and reducing agents m order to remove any substituents which tend to cause molecular chain breakage.

19. The method of claim 18 in which the carbohydrate compound is cellulose.

20. The method of claim 19 which further comprises treating the carboxylated cellulose product with a tertiary oxidizing agent to stabilize the product by substantially converting any aldehyde substituents to additional carboxyl groups.

21. The method of claim 20 which comprises further stabilizing the carboxylated cellulose product after treatment with the tertiary oxidizing agent by treatment with a reducing agent.

22. The method of claim 21 in which the tertiary oxidizing agent is sodium chlorite and the reducing agent is sodium borohydride.

23. The method of claim 20 in which the tertiary oxidant is selected from the group consisting of alkali metal clilorites, a chlorine dioxide/hydrogen peroxide mixture, and peracids.

24. The method of claim 23 in which the tertiary oxidant is a mixture of chlorine dioxide and hydrogen peroxide.

25. The method of claim 24 in which the chlorine dioxide is present in an amount of about 0.01 20% based on the weight of cellulose and the hydrogen peroxide is present in an amount of about 0.01 10% by weight of cellulose.

26. The method of claim 25 in which the chlorine dioxide is present in an amount of about 0.3 1.0% based on the weight of cellulose and the hydrogen peroxide is present in an amount of about 0.05 1.0% by weight of cellulose.

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

28. The method of claim 27 in which the sodium chlorite is present during the stabilization reaction in a concentration of 0.1 20% by weight of cellulose.

29. The method of claim 28 in which the sodium chlorite is present during the stabilization reaction in a concentration of 1 9% by weight of cellulose.

30. The method of claim 23 in which the tertiary oxidant is a peracid.

31. The method of claim 20 in which the tertiary oxidant is present in the aqueous suspension during the stabilization reaction in a molar ratio of 1.5 15 times the aldehyde substitution on the carboxylated cellulose.

32. The method of claim 31 in which the tertiary oxidant is present in the aqueous suspension during the stabilization reaction in a molar ratio of 5 10 times the aldehyde substitution on the carboxylated cellulose.

33. The method of claim 20 in which the oxidation during the stabilization reaction proceeds for a time between 5 minutes and 50 hours.

34. The method of claim 33 in which the oxidation during the stabilization reaction proceeds for a time between 10 minutes and 2 hours.

35. The method of claim 20 in which the oxidative stabilization treatment is carried out under acidic conditions at a pH between about 0 5.

36. The method of claim 35 in which the oxidative stabilization treatment is carried out under acidic conditions at a pH between about 2 4.

37. The method of claim 19 which further comprises treating the carboxylated cellulose product with a reducing agent to stabilize the product by substantially converting any aldehyde or ketone carbonyl substituents to hydroxyl groups.

38. The method of claim 37 in which the reducing agent in the aqueous suspension is a borohydride salt selected from the group consisting of alkali metal borohydrides, cyanoborohydrides, and mixtures thereof.

39. The method of claim 38 in which the reducing agent is present in an amount of about 0.1 4% by weight of oxidized cellulose.

40. The method of claim 38 in which the reducing agent is present in an amount of about 1 3% by weight of oxidized cellulose present.

41. The method of claim 38 in which the reduction reaction proceeds for a time between 10 minutes and 2 hours.

Description

The present invention is a process for preparation of carboxylated carbohydrates having available primary hydroxyl groups. It is particularly applicable for preparation of a heat and light stable fibrous carboxylated cellulose suitable for papermaking and related applications. The cellulose product of the invention is one in which fiber strength and degree of polymerization are not significantly sacrificed. The process is particularly environmentally advantageous since no chlorine or hypochlorite compounds are required.

BACKGROUND OF THE INVENTION

Carbohydrates are polyhydroxy aldehyde or ketone compounds or substances that yield these compounds on hydrolysis. They frequently occur in nature as long chain polymers of simple sugars. As the term is used in the present invention it is intended to be inclusive of any monomeric, oligomeric, and polymeric carbohydrate compound which has a primary hydroxyl group available for reaction.

Cellulose is a carbohydrate consisting of a long chain of glucose units, all .beta.-linked through the 1' 4 positions. Native plant cellulose molecules may have upwards of 2200 anhydroglucose units. The number of units is normally referred to as degree of polymerization or simply D.P. Some loss of D.P. inevitably occurs during purification. A D.P. approaching 2000 is usually found only in purified cotton linters. Wood derived celluloses rarely exceed a D.P. of about 1700. The structure of cellulose can be represented as follows:

##STR00001##

Chemical derivatives of cellulose have been commercially important for almost a century and a half Nitrocellulose plasticized with camphor was the first synthetic plastic and has been in use since 1868. A number of cellulose ether and ester derivatives are presently commercially available and find wide use in many fields of commerce. Virtually all cellulose derivatives take advantage of the reactivity of the three available hydroxyl groups. Substitution at these groups can vary from very low; e.g. about 0.01 to a maximum 3.0. Among important cellulose derivatives are cellulose acetate, used in fibers and transparent films; nitrocellulose, widely used in lacquers and gun powder; ethyl cellulose, widely used in impact resistant tool handles; methyl cellulose, hydroxyethyl, hydroxypropyl, and sodium carboxymethyl cellulose, water soluble ethers widely used in detergents, as thickeners in foodstuffs, and in papermaking.

Cellulose itself has been modified for various purposes. Cellulose fibers are naturally anionic in nature as are many papermaking additives. A cationic cellulose is described in Harding et al. U.S. Pat. No. 4,505,775. This has greater affinity for anionic papermaking additives such as fillers and pigments and is particularly receptive to acid and anionic dyes. Jewell et al., in U.S. Pat. No. 5,667,637, teach a low degree of substitution (D.S.) carboxyethyl cellulose which, along with a cationic resin, improves the wet to dry tensile and burst ratios when used as a papermaking additive. Westland, in U.S. Pat. No. 5,755,828 describes a method for increasing the strength of articles made from cross linked cellulose fibers having free carboxylic acid groups obtained by covalently coupling a polycarboxylic acid to the fibers.

For some purposes cellulose has been oxidized to make it more anionic; e.g., to improve compatibility with cationic papermaking additives and dyes. Various oxidation treatments have been used. U.S. Pat. No. 3,575,177 to Briskin et al. describes a cellulose oxidized with nitrogen dioxide useful as a tobacco substitute. The oxidized material may then be treated with a borohydride to reduce functional groups, such as aldehydes, causing off flavors. After this reduction the product may be further treated with an oxidizing agent such as hydrogen peroxide for further flavor improvement. Other oxidation treatments use nitrogen dioxide and periodate oxidation coupled with resin treatment of cotton fabrics for improvement in crease recovery as suggested by R. T. Shet and A. M. Yabani, Textile Research Journal November 1981: 740 744. Earlier work by K. V. Datye and G. M. Nabar, Textile Research Journal, July 1963: 500 510, describes oxidation by metaperiodates and dichromic acid followed by treatment with chlorous acid for 72 hours or 0.05 M sodium borohydride for 24 hours. Copper number was greatly reduced by borohydride treatment and less so by chlorous acid. Carboxyl content was slightly reduced by borohydride and significantly increased by chlorous acid. The products were subsequently reacted with formaldehyde. P. Luner et al., Tappi 50(3): 117 120 (1967) oxidized southern pine kraft spring wood and summer wood fibers with potassium dichromate in oxalic acid. Handsheets made with the fibers showed improved wet strength believed due to aldehyde groups. P. Luner et al., in Tappi 50(5): 227 230 (1967) expanded this earlier work and further oxidized some of the pulps with chlorite or reduced them with sodium borohydride. Handsheets from the pulps treated with the reducing agent showed improved sheet properties over those not so treated. R. A. Young, Wood and Fiber, 10(2): 112 119 (1978) describes oxidation primarily by dichromate in oxalic acid to introduce aldehyde groups in sulfite pulps for wet strength improvement in papers.

Brasey et al, in U.S. Pat. No. 4,100,341, describe oxidation of cellulose with nitric acid. They note that the reaction was specific at the C6 position and that secondary oxidation at the C2 and C3 positions was not detected. They further note that the product was " . . . stable without the need for subsequent reduction steps or the introduction of further reactants [e.g., aldehyde groups] from which the oxidized cellulose has to be purged".

V. A. Shenai and A. S. Narkhede, Textile Dyer and Printer May 20, 1987: 17 22 describe the accelerated reaction of hypochlorite oxidation of cotton yarns in the presence of physically deposited cobalt sulfide. The authors note that partial oxidation has been studied for the past hundred years in conjunction with efforts to prevent degradation during bleaching. They also discuss in some detail the use of 0.1 M sodium borohydride as a reducing agent following oxidation. The treatment was described as a useful method of characterizing the types of reducing groups as well as acidic groups formed during oxidation. The borohydride treatment noticeably reduced copper number of the oxidized cellulose. Copper number gives an estimate of the reducing groups such as aldehydes present on the cellulose. Borohydride treatment also reduced alkali solubility of the oxidized product but this may have been related to an approximate 40% reduction in carboxyl content of the samples.

R. Andersson et al. in Carbohydrate Research 206: 340 346 (1990) teach oxidation of cellulose with sodium nitrite in orthophosphoric acid and describe nuclear magnetic resonance elucidation of the reaction products.

An article by P. L. Anelli et al. in Journal of Organic Chemistry 54: 2970 2972 (1989) appears to be one of the earlier papers describing oxidation of hydroxyl compounds by oxammonium salts. They employed a system of 2,2,6,6-tetramethyl-piperidinyloxy free radical (TEMPO) with sodium hypochlorite and sodium bromide in a two phase system to oxidize 1,4-butanediol and 1,5-pentanediol.

R. V. Casciani et al, in French Patent 2,674,528 (1992) describe the use of sterically hindered N-oxides for oxidation of polymeric substances, among them alkyl polyglucosides having primary hydroxyl groups. A preferred oxidant was TEMPO although many related nitroxides were suggested. Calcium hypochlorite was present as a secondary oxidant.

N. J. Davis and S. L. Flitsch, Tetrahedron Letters 34(7): 1181 1184 (1993) describe the use and reaction mechanism of (TEMPO) with sodium hypochlorite to achieve selective oxidation of primary hydroxyl groups of monosaccharides. Following the Davis et al. paper this route to carboxylation then began to be very actively explored, particularly in the Netherlands and later in the United States. A. E. J. de Nooy et al., in a short paper in Receuil des Travaux Chimiques des Pays-Bas 113: 165 166 (1994), report similar results using TEMPO and hypobromite for oxidation of primary alcohol groups in potato starch and inulin. The following year, these same authors in Carbohydrate Research 269: 89 98 (1995) report highly selective oxidation of primary alcohol groups in water soluble glucans using TEMPO and a hypochlorite/bromide oxidant.

European Patent Application 574,666 to Kaufhold et al. describes a group of nitroxyl compounds based on TEMPO substituted at the 4-position. These are useful as oxidation catalysts using a two phase system. Formation of carboxylated cellulose did not appear to be contemplated.

PCT published patent application WO 95/07303 (Besemer et al.) describes a method of oxidizing water soluble carbohydrates having a primary alcohol group, using TEMPO, or a related di-tertiary-alkyl nitroxide, with sodium hypochlorite and sodium bromide. Cellulose is mentioned in passing in the background although the examples are principally limited to starches. The method is said to selectively oxidize the primary alcohol at C-6 to carboxyl. None of the products studied were fibrous in nature.

A year following the above noted Besemer PCT publication, the same authors, in Cellulose Derivatives, T. J. Heinze and W. G. Glasser, eds., Ch. 5, pp 73 82 (1996), describe methods for selective oxidation of cellulose to 2,3-dicarboxy cellulose and 6-carboxy cellulose using various oxidants. Among the oxidants used were a periodate/chlorite/hydrogen peroxide system, oxidation in phosphoric acid with sodium nitrate/nitrite, and with TEMPO and a hypochlorite/bromide primary oxidant. Results with the TEMPO system were poorly reproduced and equivocal. The statement that " . . . some of the material remains undissolved" was puzzling. In the case of TEMPO oxidation of cellulose, little or none would have been expected to go into water solution unless the cellulose was either badly degraded and/or the carboxyl substitution was very high. The homogeneous solution of cellulose in phosphoric acid used for the sodium nitrate/sodium nitrite oxidation was later treated with sodium borohydride to remove any carbonyl function present.

De Nooy et al. have published a very extensive review, both of the literature and the chemistry of nitroxyls as oxidizers of primary and secondary alcohols, in Synthesis: Journal of Synthetic Organic Chemistry (10): 1153 1174 (1996).

Heeres et al., in PCT application WO 96/38484, discuss oxidation of carbohydrate ethers useful as sequestering agents. They use the TEMPO oxidation system described by the authors just noted above to produce relatively highly substituted products, including cellulose.

P.-S. Chang and J. F. Robyt, Journal of Carbohydrate Chemistry 15(7): 819 830 (1996), describe oxidation of ten polysaccharides including .alpha.-cellulose at 0.degree. C. and 25.degree. C. using TEMPO with sodium hypochlorite and sodium bromide. Ethanol addition was used to quench the oxidation reaction. The resulting oxidized .alpha.-cellulose had a water solubility of 9.4%. The authors did not further describe the nature of the .alpha.-cellulose. It is presumed to have been a so-called dissolving pulp or cotton linter cellulose.

Heeres et al., in WO 96/36621, describe a method of recovering TEMPO and its related compounds following their use as an oxidation catalyst. An example is given of the oxidation of starch followed by TEMPO recovery using azeotropic distillation.

D. Barzyk et al., in Journal of pulp and paper Science 23(2): J59 J61 (1997) and in Transactions of the 11.sup.th Fundamental Research Symposium, Vol. 2, 893 907 (1997), note that carboxyl groups on cellulose fibers increase swelling and impact flexibility, bonded area and strength. They designed experiments to increase surface carboxylation of fibers. However, they ruled out oxidation to avoid fiber degradation and chose to form carboxymethyl cellulose in an isopropanol/methanol system.

Isogai, A. and Y. Kato, in Cellulose 5: 153 164 (1998) describe treatment of several native, mercerized, and regenerated celluloses with TEMPO to obtain water soluble and insoluble polyglucuronic acids. They note that the water soluble products had almost 100% carboxyl substitution at the C-6 site. They further note that oxidation proceeds heterogeneously at the more accessible regions on solid cellulose.

Isogai, in Cellulose Communications 5(3): 136 141 (1998) describes preparation of water soluble oxidized cellulose products using mercerized or regenerated celluloses as starting materials in a TEMPO oxidation system. Using native celluloses or bleached wood pulp he was unable to obtain a water soluble material since he achieved only low amounts of conversion. He further notes the beneficial properties of the latter materials as papermaking additives.

Kitaoka et al., in a preprint of a short 1998 paper for Sen'i Gakukai (Society of Studies of Fiber) speak of their work in the surface modification of fibers using a TEMPO mediated oxidation system. They were concerned with the receptivity of alumbased sizing compounds.

PCT application WO 99/23117 (Viikari et al.) teaches oxidation using TEMPO in combination with the enzyme laccase or other enzymes along with air or oxygen as the effective oxidizing agents of cellulose fibers, including kraft pine pulps.

Kitaoka, T., A., A. Isogai, and F. Onabe, in Nordic Pulp and Paper Research Journal, 14(4): 279 284 (1999), describe the treatment of bleached hardwood kraft pulp using TEMPO oxidation. Increasing amounts of carboxyl content gave some improvement in dry tensile index, Young's modulus and brightness, with decreases in elongation at breaking point and opacity. Other strength properties were unaffected. Retention of PAE-type wet strength resins was somewhat increased. The products described did not have any stabilization treatment after the TEMPO oxidation.

Van der Lugt et al., in WO 99/57158, describe the use of peracids in the presence of TEMPO or another di-tertiary alkyl nitroxyl for oxidation of primary alcohols in carbohydrates. They claim their process to be useful for producing uronic acids and for introducing aldehyde groups which are suitable for crosslinking and derivitization. Among their examples are a series of oxidations of starch at pH ranges from 5 10 using a system including TEMPO, sodium bromide, EDTA, and peracetic acid. Carboxyl substitution was relatively high in all cases, ranging from 26 91% depending on reaction pH.

Besemer et al. in PCT published application WO 00/50388 teach oxidation of various carbohydrate materials in which the primary hydroxyls are converted to aldehyde groups. The system uses TEMPO or related nitroxyl compounds in the presence of a transition metal using oxygen or hydrogen peroxide.

Jaschinski et al. In PCT published application WO 00/50462 teach oxidation of TEMPO oxidized bleached wood pulps to introduce carboxyl and aldehyde groups at the C6 position. The pulp is preferably refined before oxidation. One process variation uses low pH reaction conditions without a halogen compound present. The TEMPO is regenerated by ozone or another oxidizer, preferably in a separate step. In particular, the outer surface of the fibers are said to be modified. The products were found to be useful for papermaking applications.

Jetten et al. in related PCT applications WO 00/50463 and WO 00/50621 teach TEMPO oxidation of cellulose along with an enzyme or complexes of a transition metal. A preferred complexing agent is a polyamine with at least three amino groups separated by two or more carbon atoms. Manganese, iron, cobalt, and copper are preferred transition metals. Although aldehyde substitution at C6 seems to be preferred, the primary products can be further oxidized to carboxyl groups by oxidizers such as chlorites or hydrogen peroxide.

TEMPO catalyzed oxidation of primary alcohols of various organic compounds is reported in U.S. Pat. Nos. 6,031,101 to Devine et al. and 6,127,573 to Li et al. The oxidation system is a buffered two phase system employing TEMPO, sodium chlorite, and sodium hypochlorite. The above investigators are joined by others in a corresponding paper to Zhao et al. Journal of Organic Chemistry 64: 2564 2566 (1999). Similarly, Einhorn et al., Journal of Organic Chemistry 61: 7452 7454 (1996) describe TEMPO used with N-chlorosuccinimide in a two phase system for oxidation of primary alcohols to aldehydes.

I. M. Ganiev et al in Journal of Physical Organic Chemistry 14: 38 42 (2001) describe a complex of chlorine dioxide with TEMPO and its conversion into oxammonium salt. Specific applications of the synthesis product were not noted.

Isogai, in Japanese Kokai 2001-49591A, describes treating cellulose fiber using a TEMPO/hypochlorite oxidation system to achieve low levels of surface carboxyl substitution. The treated fiber has good additive retention properties without Loss of strength when used in papermaking applications.

Published European Patent Applications 1,077,221; 1,077,285; and 1,077,286 to Cimecloglu et al. respectively describe a polysaccharide paper strength additive, a paper product, and a modified cellulose pulp in which aldehyde substitution has been introduced using a TEMPO/hypochlorite system.

Published PCT application WO 01/29309 to Jewell et al. describes a cellulose fiber carboxylated using TEMPO or its related compounds which is stabilized against color or D.P. degradation by the use of a reducing or additional oxidizing step to eliminate aldehyde or ketone substitution introduced during the primary oxidation.

None of the previous workers have described a stable fibrous carboxylated cellulose or related carbohydrate material that can be made and used in conventional papermill equipment, using environmentally friendly chemicals, with no requirement for hypochlorites.

SUMMARY OF THE INVENTION

The present invention is directed to a method for preparation of a carboxylated carbohydrate product using a catalytic amount of a hindered cyclic oxammonium salt as the effective primary oxidant. This may be generated in situ by the use of a corresponding amine, hydroxylamine, or nitroxide. The catalyst is not consumed and may be recycled for reuse. The method does not require an alkali metal or alkaline earth hypohalite compound as a secondary oxidant to regenerate the oxammonium salt. Instead, chlorine dioxide has proved to be very satisfactory for this function. If maximum stability of the product is desired, the initially oxidized product may be treated, preferably with a tertiary oxidant or, alternatively, a reducing agent, to convert any unstable substituent groups into carboxyl or hydroxyl groups.

In the discussion and claims that follow, the terms nitroxide, oxammonium salt, amine, or hydroxylamine of a corresponding hindered heterocyclic amine compound should be considered as full equivalents. The oxammonium salt is the catalytically active form but this is an intermediate compound that is formed from a nitroxide, continuously used to become a hydroxylamine, and then regenerated, presumably back to the nitroxide. The secondary oxidant will convert the amine form to the free radical nitroxide compound. Unless otherwise specified, the term "nitroxide" will normally be used hereafter in accordance with the most common usage in the related literature.

The method is broadly applicable to many carbohydrate compounds having available primary hydroxyl groups, of which only one is cellulose. The terms "cellulose" and "carbohydrate" should thus be considered equivalents when used hereafter.

The method is suitable for carboxylation of many carbohydrate products such as simple sugars, relatively low molecular weight oligomers of sugars, starches, chitin, chitosan and many others that have an accessible primary hydroxyl group. Cellulose is preferred carbohydrate material and a chemically purified fibrous cellulose market pulp is a particularly preferred raw material for the process. This may be, but is not limited to, bleached or unbleached sulfite, kraft, or prehydrolyzed kraft hardwood or softwood pulps or mixtures of hardwood and softwood pulps. While included within the broad scope of the invention, so-called high alpha cellulose or chemical pulps; i.e., those with an .alpha.-cellulose content greater than about 92%, are not generally preferred as raw materials.

The suitability of lower cost market pulps is a significant advantage of the process. Market pulps are used for many products such as fine papers, diaper fluff, paper towels and tissues, etc. These pulps generally have about 86 88% .alpha.-cellulose and 12 14% hemicellulose whereas the high .alpha.-cellulose chemical or dissolving pulps have about 92 98% .alpha.-cellulose. By stable is meant minimum D.P. loss in alkaline environments, and very low self cross linking and color reversion. The method of the invention is particularly advantageous for treating secondary (or recycled) fibers. Bond strength of the sheeted carboxylated fibers is significantly improved over untreated recycled fiber.

The "cellulose" used with the present invention is preferably a wood based cellulose market pulp below 90% .alpha.-cellulose, generally having about 86 88% .alpha.-cellulose and a hemicellulose content of about 12%.

The process of the invention will lead to a product having an increase in carboxyl substitution over the starting material of at least about 2 meq/100 g, preferably at least about 5 meq/100 g. Carboxylation occurs predominantly at the hydroxyl group on C-6 of the anhydroglucose units to yield uronic acids.

The cellulose fiber in an aqueous slurry or suspension is first oxidized by addition of a primary oxidizer comprising a cyclic oxammonium salt. This may conveniently be formed in situ from a corresponding amine, hydroxylamine or nitroxyl compound which lacks any a-hydrogen substitution on either of the carbon atoms adjacent the nitroxyl nitrogen atom. Substitution on these carbon atoms is preferably a one or two carbon alkyl group. For sake of convenience in description it will be assumed, unless otherwise noted, that a nitroxide is used as the primary oxidant and that term should be understood to include all of the precursors of the corresponding nitroxide or its oxammonium salt.

Nitroxides having both five and six membered rings have been found to be satisfactory. Both five and six membered rings may have either a methylene group or a heterocyclic atom selected atom nitrogen, sulfur or oxygen at the four position in the ring, and both rings may have one or two substituent groups at this location.

A large group of nitroxide compounds have been found to be suitable. 2,2,6,6-tetramethylpiperidinyl-1-oxy free radical (TEMPO) is among the exemplary nitroxides found useful. Another suitable product linked in a mirror image relationship to TEMPO is 2,2,2'2',6,6,6',6'-octamethyl-4,4'-bipiperidinyl-1,1'-dioxy di-free radical (BI-TEMPO). Similarly, 2,2,6,6-tetramethyl-4-hydroxypiperidinyl-1-oxy free radical; 2,2,6,6-tetramethyl-4-methoxypiperidinyl-1-oxy free radical; and 2,2,6,6-tetramethyl-4-benzyloxypiperidinyl-1-oxy free radical; 2,2,6,6-tetramethyl-4-aminopiperidinyl-1-oxy free radical; 2,2,6,6-tetramethyl-4-acetylaminopiperidinyl-1-oxy free radical; 2,2,6,6-tetramethyl-4-piperidone-1-oxy free radical and ketals of this compound are examples of compounds with substitution at the 4 position of TEMPO that have been found to be very satisfactory oxidants. Among the nitroxides with a second hetero atom in the ring at the four position (relative to the nitrogen atom), 3,3,5,5-tetramethylmorpholine-1-oxy free radical (TEMMO) is useful.

The nitroxides are not limited to those with saturated rings. One compound anticipated to be a very effective oxidant is 3,4-dehydro-2,2,6,6-tetramethyl-piperidinyl-1-oxy free radical.

Six membered ring compounds with double substitution at the four position have been especially useful because of their relative ease of synthesis and lower cost. Exemplary among these are the 1,2-ethanediol, 1,3-propanediol, 2,2-dimethyl-1-3-propanediol (1,3-neopentyldiol) and glyceryl cyclic ketals of 2,2,6,6-tetramethyl-4-piperidone-1-oxy free radical.

Among the five membered ring products, 2,2,5,5-tetramethyl-pyrrolidinyl-1-oxy free radical is anticipated to be very effective.

The above named compounds should only be considered as exemplary among the many representatives of the nitroxides suitable for use with the invention and those named are not intended to be limiting in any way.

During the oxidation reaction the nitroxide is consumed and converted to an oxammonium salt then to a hydroxylamine. Evidence indicates that the nitroxide is continuously regenerated by the presence of a secondary oxidant. Chlorine dioxide, or a latent source of chlorine dioxide, is a preferred secondary oxidant. Since the nitroxide is not irreversibly consumed in the oxidation reaction only a catalytic amount of it is required. During the course of the reaction it is the secondary oxidant which will be depleted.

The amount of nitroxide required is in the range of about 0.005% to 1.0% by weight based on carbohydrate present, preferably about 0.02 0.25%. The nitroxide is known to preferentially oxidize the primary hydroxyl which is located on C-6 of the anhydroglucose moiety in the case of cellulose or starches. It can be assumed that a similar oxidation will occur at primary alcohol groups on hemicellulose or other carbohydrates having primary alcohol groups.

The chlorine dioxide secondary oxidant is present in an amount of 0.2 35% by weight of the carbohydrate being oxidized, preferably about 0.5 10% by weight.

As was noted earlier, it is considered to be within the scope of the invention to form nitroxides or their oxammonium salts in situ by oxidation of the corresponding amines or hydroxylamines of any of the nitroxide free radical products. While the free radical form of the selected nitroxide may be used, it is often preferable to begin with the corresponding amine. Among the many possible amino compounds useful as starting materials can be mentioned 2,2,6,6-tetramethylpiperidine, 2,2,6,6-tetramethyl-4-piperidone (triacetone amine) and its 1,2-ethanediol, 1,3-propanediol, 2,2-dimethyl-1,3-propanediol and glyceryl cyclic ketals.

When cellulose is the carbohydrate being treated, the usual procedure is to slurry the cellulose fiber in water with a small amount of sodium bicarbonate or another buffering material for pH control. The pH of the present process is not highly critical and may be within the range of about 4 12, preferably about 6 8. The nitroxide may be added in aqueous solution and chlorine dioxide added separately or premixed with the nitroxide. If the corresponding amine is used, they are preferably first reacted inaqueous solution with chlorine dioxide at somewhat chlorine dioxide is added to the cellulose slurry and the catalytic solution is then added and allowed to react, preferably at elevated temperature for about 30 seconds to 10 hours at temperatures from about 5.degree. 110.degree. C., preferably about 20.degree. 95.degree. C.

To achieve maximum stability and D.P. retention the oxidized product may be treated with a stabilizing agent to convert any substituent groups, such as aldehydes or ketones, to hydroxyl or carboxyl groups. The stabilizing agent may either be another oxidizing agent or a reducing agent. Unstabilized oxidized cellulose pulps have objectionable color reversion and may self crosslink upon drying, thereby reducing their ability to redisperse and form strong bonds when used in sheeted products. If sufficient unreacted ClO.sub.2 remains after the initial oxidation, it is only necessary to acidify the initial reaction mixture without even draining or washing the product. Otherwise one of the following oxidation treatments may be used

Alkali metal chlorites are one class of oxidizing agents used as stabilizers, sodium chlorite being preferred because of the cost factor. Other compounds that may serve equally well as oxidizers are permanganates, chromic acid, bromine, silver oxide, and peracids. A combination of chlorine dioxide and hydrogen peroxide is also a suitable oxidizer when used at the pH range designated for sodium chlorite. Oxidation using sodium chlorite may be carried out at a pH in the range of about 0 5, preferably 2 4, at temperatures between about 10.degree. 110.degree. C., preferably about 20.degree. 95.degree. C., for times from about 0.5 minutes to 50 hours, preferably about 10 minutes to 2 hours. One factor that favors oxidants as opposed to reducing agents is that aldehyde groups on the oxidized carbohydrate are converted to additional carboxyl groups, thus resulting in a more highly carboxylated product. These stabilizing oxidizers are referred to as "tertiary oxidizers" to distinguish them from the nitroxide/chlorine dioxide primary/secondary oxidizers. The tertiary oxidizer is used in a molar ratio of about 1.0 15 times the presumed aldehyde content of the oxidized carbohydrate, preferably about 5 10 times. In a more convenient way of measuring the needed tertiary oxidizer, the preferred sodium chlorite usage should fall within about 0.01 20% based on carbohydrate, preferably about 1 9% by weight based on carbohydrate, the chlorite being calculated on a 100% active material basis.

When stabilizing with a ClO.sub.2 and H.sub.2O.sub.2 mixture, the concentration of ClO.sub.2 present should be in a range of about 0.01 20% by weight of carbohydrate, preferably about 0.3 1.0%, and concentration of H.sub.2O.sub.2 should fall within the range of about 0.01 10% by weight of carbohydrate, preferably 0.05 1.0%. Time will generally fall within the range of 0.5 minutes to 50 hours, preferably about 10 minutes to 2 hours and temperature within the range of about 10.degree. 110.degree. C., preferably about 30.degree. 95.degree. C. The pH of the system is preferably about 3 but may be in the range of 0 5.

A preferred reducing agent is an alkali metal borohydride. Sodium borohydride (NaBH.sub.4) is preferred from the standpoint of cost and availability. However, other borohydrides such as LiBH.sub.4, or alkali metal cyanoborohydrides such as NaBH.sub.3CN are also suitable. NaBH4 may be mixed with LiCl to form a very useful reducing agent. When NaBH.sub.4 is used for reduction, it should be present in an amount between about 0.1 and 10.0 g/L. A more preferred amount would be about 0.25 5 g/L and a most preferred amount from about 0.5 2.0 g/L. Based on carbohydrate the amount of reducing agent should be in the range of about 0.1% to 4% by weight, preferably about 1 3%. Reduction may be carried out at room or higher temperature for a time between 10 minutes and 10 hours, preferably about 30 minutes to 2 hours.

After stabilization is completed, the carbohydrate is washed and may be dried if desired. Alternatively, the carboxyl substituents may be converted to other cationic forms beside hydrogen or sodium; e.g., calcium, magnesium, or ammonium.

One particular advantage of the process is that all reactions are carried out in an aqueous medium. A further advantage when the process is used with cellulose fiber is that the carboxylation is primarily located on the fiber surface. This conveys highly advantageous properties for papermaking. The product of the invention will have at least about 20% of the total carboxyl content on the fiber surface. Untreated fiber will typically have no more than a few milliequivalents of total carboxyl substitution and, of this, no more than about 10% will be located on the fiber surface.

Carboxylated cellulose made using the process of the invention is highly advantageous as a papermaking furnish, either by itself or in conjunction with conventional fiber. It may be used in amounts from 0.5 100% of the papermaking furnish. The carboxylated fiber is especially useful in admixture with recycled fiber to add strength. The method can be used to improve properties of either virgin or recycled fiber. The increased number of anionic sites on the fiber should serve to ionically hold significantly larger amounts of cationic papermaking additives than untreated fiber. These additives may be wet strength resins, sizing chemical emulsions, filler and pigment retention aids, charged filler particles, dyes and the like. Carboxylated pulps do not hornify (or irreversibly collapse) as much on drying and are a superior material when recycled. They swell more on rewetting, take less energy to refine, and give higher sheet strength.

It is a primary object of the invention to provide a convenient method whereby carboxyl substitution may be introduced into carbohydrate materials having primary hydroxyl groups.

It is an important object of the invention to provide a method of making a cellulose fiber having enhanced carboxyl content using an aqueous reaction medium.

It is also an object to provide a method for making a carboxylated cellulose fiber that does not employ chlorine or hypohalite compounds.

It is another object to provide a process for making a carboxylated cellulose fiber that can be carried out in equipment and with many chemicals commonly found in pulp or paper mills.

It is a further object to provide a cellulose fiber having an enhanced carboxyl content at the fiber surface.

It is yet an object to provide a carboxylated cellulose fiber that is stable against D.P. loss in alkaline environments.

It is an object to provide a stable cellulose fiber of enhanced carboxyl content with a D.P. of at least 850 measured as a sodium salt or 700 when measured in the free acid form.

It is still an object to provide a cellulose fiber having a high ionic attraction to cationic papermaking additives.

It is an additional object to provide cellulose pulp and paper products containing the carboxyl enhanced fiber.

These and many other objects will become readily apparent upon reading the following detailed description taken in conjunction with the drawings

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Abundant laboratory data indicates that a nitroxide catalyzed cellulose oxidation predominantly occurs at the primary hydroxyl group on C-6 of the anhydro-glucose moeity. In contrast to some of the other routes to oxidized cellulose, only very minor reaction has been observed to occur at the secondary hydroxyl groups at the C-2 and C-3 locations. Using TEMPO as an example, the mechanism to formation of a carboxyl group at the C-6 location proceeds through an intermediate aldehyde stage.

##STR00002##

The TEMPO is not irreversibly consumed in the reaction but is continuously regenerated. It is converted by the secondary oxidant into the oxammonium (or nitrosonium) ion which is the actual oxidant. During oxidation the oxammonium ion is reduced to the hydroxylamine from which TEMPO is again formed. Thus, it is the secondary oxidant which is actually consumed. TEMPO may be reclaimed or recycled from the aqueous system. The reaction is postulated to be as follows:

##STR00003##

As was noted earlier, formation of the oxammonium salt in situ by oxidation of the hydroxylamine or the amine is considered to be within the scope of the invention.

The resulting oxidized cellulose product will have a mixture of carboxyl and aldehyde substitution. Aldehyde substituents on cellulose are known to cause degeneration over time and under certain environmental conditions. In addition, minor quantities of ketone carbonyls may be formed at the C-2 and C-3 positions of the anhydroglucose units and these will also lead to degradation. Marked D.P., fiber strength loss, crosslinking, and yellowing are among the problems encountered. For these reasons, we have found it very desirable to oxidize aldehyde substituents to carboxyl groups, or to reduce aldehyde and ketone groups to hydroxyl groups, to ensure stability of the product.

The following groups of nitroxyl compounds and their corresponding amines or hydroxylamines are known to be effective primary oxidants:

##STR00004## in which R.sub.1 R.sub.4 are one to four carbon alkyl groups but R.sub.1 with R.sub.2 and R.sub.3 with R.sub.4 may together be included in a five or six carbon alicyclic ring structure; X is sulfur or oxygen; and R.sub.5 is hydrogen, C.sub.1 C.sub.12 alkyl, benzyl, 2-dioxanyl, a dialkyl ether, an alkyl polyether, or a hydroxyalkyl, and X with R.sub.5 being absent may be hydrogen or a mirror image moiety to form a bipiperidinyl nitroxide. Specific compounds in this group known to be very effective are 2,2,6,6-tetramethylpiperidinyl-1-oxy free radical (TEMPO); 2,2,2',2',6,6,6',6'-octamethyl-4,4'-bipiperidinyl-1,1'-dioxy di-free radical (BI-TEMPO); 2,2,6,6-tetramethyl-4-hydroxypiperidinyl-1-oxy free radical (4-hydroxy-TEMPO); 2,2,6,6-tetramethyl-4-methoxypiperidinyl-1-oxy free radical (4-methoxy-TEMPO); and 2,2,6,6-tetramethyl-4-benzyloxypiperidinyl-1-oxy free radical (4-benzyloxy-TEMPO).

##STR00005## in which R.sub.1 R.sub.4 are one to four carbon alkyl groups but R.sub.1 with R.sub.2 and R.sub.3 with R.sub.4 may together be included in a five or six carbon alicyclic ring structure; R.sub.6 is hydrogen or C.sub.1 C.sub.5 alkyl; R.sub.7 is hydrogen, C.sub.1 C.sub.8 alkyl, phenyl, carbamoyl, alkyl carbamoyl, phenyl carbamoyl, or C.sub.1 C.sub.8 acyl. Exemplary of this group is 2,2,6,6-tetramethyl-4-aminopiperidinyl-1-oxy free radical (4-aminoTEMPO); and 2,2,6,6-tetramethyl-4-acetylaminopiperidinyl-1-oxy free radical (4-acetylamino-TEMPO).

##STR00006## in which R.sub.1 R.sub.4 are one to four carbon alkyl groups but R.sub.1 with R.sub.2 and R.sub.3 with R.sub.4 may together be included in a five or six carbon alicyclic ring structure; and X is oxygen, sulfur, NH, N-alkyl, NOH, or NOR.sub.8 where R.sub.8 is lower alkyl. An example might be 2,2,6,6-tetramethyl-4-oxopiperidinyl-1-oxy free radical (2,2,6,6-tetramethyl-4-piperidone-1-oxy free radical).

##STR00007## wherein R.sub.1 R.sub.4 are one to four carbon alkyl groups but R.sub.1 with R.sub.2 and R.sub.3 with R.sub.4 may be linked into a five or six carbon alicyclic ring structure; X is oxygen, sulfur, -alkyl amino, or acyl amino. An example is 3,3,5,5-tetramethylmorpholine-4-oxy free radical. In this case the oxygen atom takes precedence for numbering but the dimethyl substituted carbons remain adjacent the nitroxide moiety.

##STR00008## wherein R.sub.1 R.sub.4 are one to four carbon alkyl groups but R.sub.1 with R.sub.2 and R.sub.3 with R.sub.4 may be linked into a five or six carbon alicyclic ring structure. An example of a suitable compound is 3,4dehydro-2,2,6,6,-tetramethylpiperidinyl-1-oxy free radical.

##STR00009## wherein R.sub.1 R.sub.4 are one to four carbon alkyl groups but R.sub.1 with R.sub.2 and R.sub.3 with R.sub.4 may together be included in a five or six carbon alicyclic ring structure; X is methylene, oxygen, sulfur, or alkylamino; and R.sub.9 and R.sub.10 are one to five carbon alkyl groups and may together be included in a five or six member ring structure, which, in turn may have one to four lower alkyl or hydroxy alkyl substitutients. Examples include the 1,2-ethanediol, 1,3-propanediol, 2,2-dimethyl-1,3-propanediol, and glyceryl cyclic ketals of 2,2,6,6-tetramethyl-4-piperidone-1-oxy free radical. These compounds are especially preferred primary oxidants because of their effectiveness, lower cost, ease of synthesis, and suitable water solubility.

##STR00010## in which R.sub.1 R.sub.4 are one to four carbon alkyl groups but R.sub.1 with R.sub.2 and R.sub.3 with R.sub.4 may together be included in a five or six carbon alicyclic ring structure; and X may be methylene, sulfur, oxygen, --NH, or NR.sub.11, in which R.sub.