JP4981735B2 - Method for producing cellulose nanofiber - Google Patents

Description

  The present invention relates to a method capable of producing cellulose nanofibers that are superior in transparency by oxidizing a previously acid-treated pulp with an N-oxyl compound.

When cellulose material is treated in the presence of a catalytic amount of 2,2,6,6-tetramethyl-1-piperidine-N-oxy radical (hereinafter referred to as TEMPO) and sodium hypochlorite which is an inexpensive oxidizing agent, cellulose Carboxyl groups can be efficiently introduced on the microfibril surface, and a uniform and transparent cellulose nanofiber aqueous solution can be produced with a little fibrillation energy (Non-Patent Document 1 Saito, T., et al., Cellulose Commun., 14 (2 ), 62 (2007)). This nanocellulose production technology is superior in environmental harmony as a reaction process, such as the use of water as a solvent and the fact that the reaction byproduct is only sodium chloride. Since bleached sulfite pulp has a higher cellulose purity than kraft pulp, it is easily converted into nanofibers by TEMPO oxidation treatment. In general, pulp derived from conifers tends to be nano-sized more than hardwoods. However, even unbleached pulp that is not partly nano-sized remains even in bleached sulfite pulp derived from softwood, which is considered to be most easily converted into nanofibers by TEMPO oxidation treatment, and there is a problem that transparency of the nanodispersion is lowered.
Saito, T., et al., Cellulose Commun., 14 (2), 62 (2007)

  An object of this invention is to provide the method of manufacturing efficiently the cellulose nanofiber dispersion liquid excellent in transparency than before by carrying out TEMPO oxidation of the acid-treated pulp.

  As a result of intensive studies to solve the problems of the prior art, the present inventors have efficiently obtained a highly transparent nanofiber dispersion from wood cellulose by warming the pulp with a certain acid concentration. The inventors have found that it can be prepared, and have reached the present invention based on the knowledge.

  By using the acid-treated pulp of the present invention as a raw material for cellulose nanofibers, cellulose having a high degree of transparency is obtained with extremely little residue of undissolved pulp compared to a nanofiber dispersion prepared from a conventional non-acid-treated pulp. Nanofibers can be produced.

  The acid used for the acid treatment of the present invention may be an inorganic acid or an organic acid. As the inorganic acid, mineral acids such as sulfuric acid, hydrochloric acid, nitric acid, sulfurous acid, nitrous acid, phosphoric acid, and residual acid of chlorine dioxide generator can be used. Sulfuric acid is preferred. As the organic acid, acetic acid, lactic acid, succinic acid, citric acid, formic acid and the like can be used. The pH during the acid treatment is in the range of 2-6, preferably 3-5. When the pH is less than 2, the removal of the polyvalent cation species present in the pulp is sufficient, but since the acid is excessive, the degree of polymerization of the cellulose is significantly reduced, and the viscosity of the nanofiber dispersion is reduced accordingly. growing. On the other hand, when the pH exceeds 6, the acid concentration is low, and the removal of the polyvalent cation species becomes insufficient. When the pH during acid treatment is 3 to 5, the temperature of the acid treatment can be lowered, and the effect that the acid treatment cost can be reduced occurs.

  The acid treatment can be carried out under atmospheric pressure or under pressure, and the treatment temperature is 30 ° C to 120 ° C, preferably 50 ° C to 100 ° C. If the temperature is less than 30 ° C., the metal removal effect is insufficient, and a transparent nanocellulose dispersion cannot be obtained. At temperatures higher than 120 ° C., there is an effect in terms of metal removal, but the cellulose nanofibers are partly extremely short and difficult to defibrate, so the transparency of the nanofiber dispersion decreases. Also, the dispersion viscosity is significantly reduced. In addition, if it is less than 100 degreeC, since a pressure-resistant reaction container is not required, it is advantageous at an equipment cost.

The pulp concentration during the acid treatment is in the range of 0.1 to 50% by weight, preferably 1 to 30% by weight, and more preferably 2 to 30% by weight.
The effect of removing the polyvalent metal ion is determined by the pH during the acid treatment, the reaction temperature, and the reaction time. Thus, the reaction time is appropriately set according to the other two conditions. From this, the reaction time is appropriately set according to the other two conditions, but the reaction time at a reaction temperature of 80 ° C. is 1.5 to 6 hours, the reaction time at a reaction temperature of 90 ° C. is 50 minutes to 5 hours, the reaction temperature A reaction time of 30 minutes to 4.5 hours at 100 ° C. and a reaction time of 5 to 50 minutes at a reaction temperature higher than 100 ° C. are suitable.

  In addition, in acid treatment, by using a chelating agent such as EDTA or DPTA in combination, mild acid treatment conditions can be set, and cellulose nanofibers having high transparency and low viscosity reduction can be produced.

As the N-oxyl compound used in the present invention, any compound can be used as long as it promotes the target oxidation reaction.
The amount of the N-oxyl compound used is not particularly limited as long as it is a catalyst amount capable of converting the cellulose-based raw material into nanofibers. For example, it is about 0.01 to 10 mmol, preferably 0.01 to 1 mmol, and more preferably about 0.05 to 0.5 mmol with respect to 1 g of cellulosic raw material.

  The method for oxidizing a cellulosic raw material of the present invention is characterized in that it is carried out in water using an oxidizing agent in the presence of an N-oxyl compound and bromide, iodide or a mixture thereof. The oxidized cellulosic raw material thus produced can be efficiently converted into nanofibers. As the bromide or iodide, a compound that can be dissociated and ionized in water, such as an alkali metal bromide or an alkali metal iodide, can be used. The amount of bromide or iodide used can be selected as long as the oxidation reaction can be promoted. For example, it is about 0.1 to 100 mmol, preferably 0.1 to 10 mmol, and more preferably about 0.5 to 5 mmol with respect to 1 g of cellulosic raw material.

  The oxidizing agent may be any oxidizing agent that can promote the target oxidation reaction, such as halogen, hypohalous acid, halous acid, perhalogen acid or salts thereof, halogen oxide, and peroxide. Agents can also be used. From the viewpoint of the production cost of nanofibers, sodium hypochlorite, which is the most widely used oxidant in industrial processes and is low in environmental load, is suitable. The amount of the oxidizing agent used can be selected within a range that can promote the oxidation reaction. For example, it is 0.5 to 500 mmol, preferably 0.5 to 50 mmol, and more preferably about 2.5 to 25 mmol with respect to 1 g of bleached wood pulp.

  The cellulose-based raw material used in the present invention is not particularly limited, and it is purified by chemical treatment such as kraft or sulfite pulp derived from various woods, powdered cellulose obtained by pulverizing them with a high-pressure homogenizer, a mill or the like, or acid hydrolysis. In addition to the use of microcrystalline cellulose powder, plants such as kenaf, hemp, rice, bacus and bamboo may be used.

  The method of the present invention is characterized in that the oxidation reaction can proceed smoothly even under mild conditions. Therefore, even if the reaction temperature is about 15 to 30 ° C., the cellulosic material can be oxidized efficiently. In addition, a carboxyl group produces | generates in a cellulose with progress of reaction, and the pH fall of a reaction liquid is recognized. Therefore, in order to advance the oxidation reaction efficiently, it is desirable to maintain the pH of the reaction solution at about 9 to 12, preferably about 10 to 11.

From the oxidized cellulose obtained in the present invention, cellulose nanofibers can be obtained by fibrillation treatment by a simple method. For example, cellulose nano-materials that have been subjected to oxidation treatment are thoroughly washed with water, and cellulose nano-particles can be treated by using known mixing / stirring / emulsifying / dispersing devices such as a high-speed shear mixer and high-pressure homogenizer alone or in combination of two or more as required. Can be fiberized. Examples of the apparatus include a high-speed rotation type, a colloid mill type, a high-pressure type, a roll mill type, and an ultrasonic type. If the shear rate is 1000 sec ⁻¹ or more, uniform and transparent cellulose nanofibers having no aggregate structure can be obtained.

The cellulose nanofiber produced by the present invention is a cellulose single microfibril having a width of 2 to 5 nm and a length of about 1 to 5 μm. Since this cellulose nanofiber is excellent in barrier property, transparency, and heat resistance, it can be used for various applications such as packaging materials.
[Action]
The reason why the acid-treated bleached pulp of the present invention is excellent in nanofiber formation is presumed as follows. In the pulp, there are alkaline earth metal ions typified by calcium and magnesium ions and transition metal ions typified by iron and copper ions as polyvalent metal ions. Usually, these cationic metal ions are likely to form a complex with a polar group such as a carboxyl group or a carbonyl group. Therefore, it is coordinated with polar groups present in cellulose and hemicellulose, which are constituents of pulp, to form a crosslinked structure between the polysaccharides via ionic bonds. Thus, in the chemical pulp, cross-linked structures formed by complex formation of polyvalent metal ions are scattered in the pulp, and in this microscopic region, the formation of nanofibers is difficult to proceed. In addition, there are hydrated and dissolved polyvalent metal ion species that do not participate in complex formation in the pulp, and these ion species are immediately bonded to the carboxyl groups formed on the surface of the cellulose microfibrils by the oxidation process, and are nano-sized. Causes fiber aggregation. From this point of view, if the polyvalent metal ion species present in the pulp is previously removed by acid treatment, the TEMPO oxidized pulp can be easily defibrated and has no aggregated structure, that is, excellent in dispersibility and transparency. A cellulose nanofiber dispersion is obtained.

Next, based on an Example, this invention is demonstrated still in detail.
[Example 1]
10 g of bleached unbeaten sulfite pulp (Nippon Paper Chemical Co., Ltd.) derived from conifers (absolutely dry) was adjusted with dilute sulfuric acid and water to a pH of 3 and a pulp concentration of 10%. After acid treatment in an autoclave at 80 ° C. for 2 hours, it was washed with water and dehydrated. 5 g of acid-treated pulp (absolutely dried) was added to 500 ml of an aqueous solution in which 94 mg (0.5 nmol) of TEMPO (Sigma Aldrich) and 755 mg (5 mmol) of sodium bromide had been dissolved, and stirred until the pulp was uniformly dispersed. After adding 18 ml of sodium hypochlorite aqueous solution (effective chlorine 5%) to the reaction system, the pH was adjusted to 10.3 with 0.5N hydrochloric acid aqueous solution, and the oxidation reaction was started. During the reaction, the pH in the system was lowered, but a 0.5N aqueous sodium hydroxide solution was successively added to adjust the pH to 10. After reacting for 2 hours, it was filtered through a glass filter and sufficiently washed with water to obtain an oxidized pulp. When a 0.3% (w / v) slurry of the oxidized pulp was stirred at 12,000 rpm for 15 minutes, a transparent gel dispersion was obtained. The B-type viscosity (60 rpm, 20 ° C.) of the 0.25% (w / v) cellulose nanofiber dispersion was 830 mPa · s.
[Example 2]
Acid treatment was performed in the same manner as in Example 1 except that the pH was 5 and the treatment time was 3 hours, and TEMPO oxidation was performed. When the obtained oxidized pulp was stirred at 12,000 rpm for 15 minutes, a transparent nanofiber dispersion was obtained. The B-type viscosity (60 rpm, 20 ° C.) of the 0.25% (w / v) cellulose nanofiber dispersion was 850 mPa · s.
[Example 3]
The acid treatment was performed in the same manner as in Example 1 except that the pH was 2, the treatment temperature was 30 ° C., and the treatment time was 1 hour, and TEMPO oxidation was performed. When the obtained oxidized pulp was stirred at 12,000 rpm for 15 minutes, a transparent nanofiber dispersion was obtained. The B-type viscosity (60 rpm, 20 ° C.) of the 0.25% (w / v) cellulose nanofiber dispersion was 815 mPa · s.
[Comparative Example 1]
The acid treatment was performed in the same manner as in Example 1 except that the treatment temperature was 130 ° C., and TEMPO oxidation was performed. When the resulting oxidized pulp was stirred at 12,000 rpm for 15 minutes, the nanofiber dispersion was opaque. Further, the B-type viscosity (60 rpm, 20 ° C.) of the aqueous 0.25% (w / v) cellulose nanofiber fiber was 335 mPa · s.
[Comparative Example 2]
The acid treatment was performed in the same manner as in Example 1 except that the treatment temperature was 25 ° C., and TEMPO oxidation was performed. When the resulting oxidized pulp was stirred at 12,000 rpm for 15 minutes, the nanofiber dispersion was opaque. Further, the B-type viscosity (60 rpm, 20 ° C.) of the aqueous 0.25% (w / v) cellulose nanofiber fiber was 866 mPa · s.
[Comparative Example 3]
The acid treatment was performed in the same manner as in Example 1 except that the pH was 1.5, and TEMPO oxidation was performed. When the resulting oxidized pulp was stirred at 12,000 rpm for 15 minutes, the nanofiber dispersion was opaque. Further, the B-type viscosity (60 rpm, 20 ° C.) of the 0.25% (w / v) cellulose nanofiber solution was 350 mPa · s.
[Comparative Example 4]
Except that the pH was 7, acid treatment was performed in the same manner as in Example 1 to perform TEMPO oxidation. When the resulting oxidized pulp was stirred at 12,000 rpm for 15 minutes, the nanofiber dispersion was opaque. Further, the B-type viscosity (60 rpm, 20 ° C.) of the 0.25% (w / v) cellulose nanofiber solution was 870 mPa · s.

2 is a photograph of cellulose nanofiber dispersion liquids of Example 1 and Comparative Examples 3 and 4. FIG.

Claims (2)

An aqueous slurry containing a cellulosic raw material washed after acid treatment under the conditions of pH 2-6, temperature 30 ° C. or higher and 120 ° C. or lower, an oxidant in the presence of an N-oxyl compound and bromide, iodide or a mixture thereof. A method for producing cellulose nanofibers comprising: adding cellulose, preparing oxidized cellulose by treating the cellulose-based raw material, and defibrating the oxidized cellulose into nanofibers. The method for producing cellulose nanofibers according to claim 1, wherein the cellulosic material is bleached kraft pulp or bleached sulfite pulp.

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