Method for preparation of stabilized carboxylated cellulose

Abstract

The invention is directed to a method of making a heat and light stable carboxylated cellulose fiber 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 salt as a primary oxidant and a peracid and halide salt as a secondary oxidant in an aqueous environment. The oxammonium compounds may be formed in situ from their corresponding amine, hydroxylamine, and nitroxyl compounds. The oxidized cellulose is then stabilized against D.P. loss and color reversion by further treatment with an oxidant such as sodium chlorite, a chlorine dioxide/hydrogen peroxide mixture, or a peracid under acidic conditions. Alternatively it may be treated with a reducing agent such as sodium borohydride. 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.

1 . A method of making a fibrous carboxylated cellulose which comprises: 
 oxidizing cellulose fiber by reacting it in an aqueous suspension 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 oxammonium nitrogen lack α-hydrogen substitution, the corresponding amines, hydroxylamines, and nitroxides compounds of these oxammonium salts, and mixtures thereof, and a secondary oxidant selected from peracids and an alkali metal halide salt in a sufficient amount 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 and color deterioration by further treating them in aqueous suspension with a stabilizing agent selected from the group consisting of oxidizing agents and reducing agents in order to remove any cellulose substituents which tend to cause molecular chain breakage.    
     
     
         2 . The method of  claim 1  in which the peracid is selected from the group consisting of peroxymonosulfuric acid, peracetic acid, and mixtures thereof  
     
     
         3 . The method of  claim 1  in which the alkali metal halide in the secondary oxidant is present in an amount of about 0.1-10% by weight based on cellulose.  
     
     
         4 . The method of  claim 3  in which the alkali metal halide is sodium bromide.  
     
     
         5 . 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.  
     
     
         6 . The method of  claim 5  in which the primary oxidant is present in the range of about 0.02-0.25% based on weight of cellulose present  
     
     
         7 . The method of  claim 1  in which the peracid secondary oxidant is present in the range of about 0.1-10% based on weight of cellulose present.  
     
     
         8 . The method of  claim 6  in which the peracid secondary oxidant is present in the range of 0.5-5% based on weight of cellulose present.  
     
     
         9 . The method of  claim 1  in which the initial oxidizing reaction is carried out at a pH between about 5.0-8.5.  
     
     
         10 . The method of  claim 9  in which the initial oxidizing reaction is carried out at a pH between about 7.5-8.0.  
     
     
         11 . The method of  claim 1  in which the initial oxidation step is carried out for a time between 1 minute and about 10 hours at a temperature in the range of about 5°-95° C.  
     
     
         12 . The method of  claim 11  in which the initial oxidation step is carried out for a time between 0.2-2.5 hours at a temperature in the range of 20°-80° C.  
     
     
         13 . The method of  claim 1  which further comprises treating the carboxylated cellulose fibers with a tertiary oxidizing agent to stabilize the product by substantially converting any aldehyde substituents to additional carboxyl groups.  
     
     
         14 . The method of  claim 13  which comprises further stabilizing the carboxylated cellulose fibers after treatment with the tertiary oxidizing agent by treatment with a reducing agent.  
     
     
         15 . The method of  claim 13  in which the tertiary oxidant is selected from the group consisting of alkali metal chlorites, a chlorine dioxide/hydrogen peroxide mixture, and peracids.  
     
     
         16 . The method of  claim 15  in which the tertiary oxidant is a mixture of chlorine dioxide and hydrogen peroxide.  
     
     
         17 . The method of  claim 16  in which chlorine dioxide is present in an amount of about 0.1-20% by weight and hydrogen peroxide is present in an amount of about 0.01-10% by weight based on the cellulose present.  
     
     
         18 . The method of  claim 17  in which chlorine dioxide is present in an amount of about 0.3-1.0% by weight and hydrogen peroxide is present in an amount of about 0.05-1.0% by weight based on the cellulose present.  
     
     
         19 . The method of  claim 16  in which the oxidative stabilization treatment is carried out under acidic conditions in the range of about pH 0-5.  
     
     
         20 . The method of  claim 19  in which the oxidative stabilization treatment is carried out under acidic conditions in the range of about pH 2-3.  
     
     
         21 . The method of  claim 13  in which the tertiary oxidant is sodium chlorite.  
     
     
         22 . The method of  claim 21  in which the sodium chlorite is present during the stabilization reaction in a concentration of about 0.1-20% by weight of cellulose  
     
     
         23 . The method of  claim 22  in which the sodium chlorite is present during the stabilization reaction in a concentration of about 1-9% by weight of cellulose.  
     
     
         24 . The method of  claim 21  in which the oxidative stabilization treatment is carried out under acidic conditions at a pH between about 1.5-5.  
     
     
         25 . The method of  claim 24  in which the oxidative stabilization treatment is carried out under acidic conditions at a pH between about 2-4.  
     
     
         26 . The method of  claim 13  in which the tertiary oxidant is a peracid.  
     
     
         27 . The method of  claim 26  in which the peracid is peroxymonosulfuric acid.  
     
     
         28 . The method of  claim 26  in which the peracid is present in a concentration of about 0.1-10% by weight of cellulose.  
     
     
         29 . The method of  claim 13  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.  
     
     
         30 . The method of  claim 13  in which the oxidation during the stabilization reaction proceeds for a time between 5 minutes and 50 hours.  
     
     
         31 . The method of  claim 30  in which the oxidation during the stabilization reaction proceeds for a time between 10 minutes and 2 hours.  
     
     
         32 . The method of  claim 1  which further comprises treating the carboxylated cellulose fibers with a reducing agent to stabilize the product by substantially converting any aldehyde or ketone carbonyl substituents to hydroxyl groups.  
     
     
         33 . The method of  claim 32  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.  
     
     
         34 . The method of  claim 33  in which the reducing agent is present in an amount of about 0.1-4.0% by weight of oxidized cellulose.  
     
     
         35 . The method of  claim 34  in which the reducing agent is present in an amount of about 1.0-3.0% by weight of oxidized cellulose.  
     
     
         36 . The method of  claim 32  in which the reduction reaction proceeds for a time between 10 minutes and 2 hours.  
     
     
         37 . The method of  claim 1  in which the cellulose fiber is selected from the group consisting of bleached and unbleached kraft wood pulps, prehydrolyzed kraft wood pulps, sulfite wood pulps and mixtures thereof  
     
     
         38 . The method of  claim 36  in which the cellulose fiber is recycled secondary fiber.  
     
     
         39 . A method of making a fibrous carboxylated cellulose which comprises: 
 oxidizing cellulose fibers by reacting them in an aqueous suspension with a sufficient amount of a primary oxidant selected from the group consisting of nitroxides having the composition                          wherein R 1 -R 4  are one to four carbon alkyl groups but R 1  with R 2  and R 3  with R 4  may together be included in a five or six carbon alicyclic ring structure, X is methylene, oxygen, sulfur, or alkylamino, and R 8  and R 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 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 comprising a peracid with an alkali metal halide salt 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 in order to remove any cellulose substituents which tend to cause molecular chain breakage.    
     
     
         40 . The method of  claim 39  in which each X is oxygen, the oxygen atoms being linked by a two to three carbon alkyl chain to form a cyclic ketal substituent.  
     
     
         41 . The method of  claim 40  in which the nitroxide composition is selected from the group consisting of the 1,2-ethanediol, 1,3-propanediol, 2,2-dimethyl-1,3-propanediol, and the glyceryl ketals of 2,2,6,6-tetramethyl-4-piperidone-1-oxy free radical.  
     
     
         42 . The method of  claim 41  in which the nitroxide composition is the 1,2-ethanediol ketal of 2,2,6,6-tetramethyl-4-piperidone-1-oxy free radical.  
     
     
         43 . The method of  claim 41  in which the nitroxide composition is the 1,3-propanediol ketal of 2,2,6,6-tetramethyl-4-piperidone-1-oxy free radical.  
     
     
         44 . The method of  claim 41  in which the nitroxide composition is the 2,2-dimethyl-1,3-propanediol ketal of 2,2,6,6-tetramethyl-4-piperidone-1-oxy free radical.  
     
     
         45 . The method of  claim 41  in which the nitroxide composition is the glyceryl ketal of 2,2,6,6-tetramethyl-4-piperidone-1-oxy free radical.  
     
     
         46 . The method of  claim 39  in which the peracid is selected from the group consisting of peroxymonosulfuric acid and peracetic acid.  
     
     
         46 . The method of  claim 39  in which the peracid is peroxymonosulfuric acid.  
     
     
         47 . The method of  claim 39  in which the peracid is peracetic acid.