JP2016188343A - Cellulose nanofiber and production method of the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cellulose nanofiber which is not colored in heating, and a production method of the same.SOLUTION: A cellulose nanofiber has a carboxyl group amount of 0.6-2.0 mmol/g, a number average fiber length of 100-3,000 nm, and a number average fiber diameter of 2-1,000 nm, and when a peak value of ultraviolet absorption spectrum at 260 nm of a film with thickness of 50±5 μm formed of the cellulose nanofiber is indicated by A, a peak value of ultraviolet absorption spectrum at 290 nm thereof is indicated by B, and the film thickness is indicated by X μm, A/X is 0.055/μm or less, and B/X is 0.050/μm or less.SELECTED DRAWING: None

Description

  The present invention relates to a cellulose nanofiber and a method for producing the same. Specifically, the present invention relates to cellulose nanofibers characterized in that the amount of aldehyde groups and ketone groups is reduced by performing a reduction treatment on oxidized cellulose, and coloring does not occur during heating, and a method for producing the same.

  When cellulose raw material is treated in the presence of 2,2,6,6-tetramethyl-1-piperidine-N-oxy radical (hereinafter also referred to as TEMPO) and an inexpensive oxidizing agent, sodium hypochlorite, cellulose micro It is known that a carboxyl group can be efficiently introduced into the fibril surface. The oxidized cellulose thus obtained can be prepared into a uniform and transparent cellulose nanofiber aqueous dispersion by applying a slight fibrillation energy. Various studies have been conducted on such a method for producing oxidized cellulose and a method for producing cellulose nanofiber (Patent Document 1).

JP 2008-001728 A

  However, the cellulose nanofiber disclosed in Cited Document 1 has a problem of coloring when heated. When coloring occurs during heating, there is a problem that when the cellulose nanofiber is industrially used, the product is discolored when it is molded by thermal processing.

Then, an object of this invention is to provide the cellulose nanofiber which is not colored at the time of a heating, and its manufacturing method.

The present invention provides the following [1] to [2].
[1] A cellulose nanofiber having a carboxyl group amount of 0.6 mmol / g to 2.0 mmol / g, a number average fiber length of 100 to 3000 nm, and a number average fiber diameter of 2 to 1000 nm. When the peak value of the 260 nm ultraviolet absorption spectrum of the film having a thickness of 50 ± 5 μm is A, the peak value of the ultraviolet absorption spectrum of 290 nm is B, and the film thickness is X μm, A / X is 0.055 / μm. Cellulosic nanofibers, wherein B / X is 0.050 / μm or less.

[2] The method for producing cellulose nanofiber according to [1], comprising the following steps (i) to (iii):

(I) A step of preparing cellulose oxide by oxidizing a cellulose raw material in water using an oxidizing agent in the presence of an N-oxyl compound and a compound selected from the group consisting of bromide, iodide or a mixture thereof. ii) A step of subjecting the oxidized cellulose to a reduction treatment (iii) A step of defibrating and dispersing the oxidized cellulose subjected to the reduction treatment

ADVANTAGE OF THE INVENTION According to this invention, the cellulose nanofiber which is not colored at the time of a heating, and its manufacturing method can be provided.

  The cellulose nanofiber of the present invention has a carboxyl group amount of 0.6 mmol / g to 2.0 mmol / g, a fiber length of 100 to 3000 nm, a fiber diameter of 2 to 1000 nm, and a thickness of 50 produced from the cellulose nanofiber. When the peak value of the ultraviolet absorption spectrum at 260 nm of the ± 5 μm film is A, the peak value of the ultraviolet absorption spectrum at 290 nm is B, and the film thickness is X μm, A / X is 0.055 / μm or less, and B / X is 0.050 / μm or less.

The ultraviolet absorption spectrum can be measured with a spectrophotometer, and the peak value (absorbance) of the ultraviolet absorption spectrum is represented by the following formula.
Absorbance = −log ₁₀ (I / I ₀ ) I; transmitted light intensity, I ₀ ; incident light intensity

In addition, the cellulose nanofiber of the present invention comprises (i) an N-oxyl compound and a cellulose raw material in water using an oxidizing agent in the presence of a compound selected from the group consisting of bromide, iodide or a mixture thereof. It is manufactured by passing through a step of preparing oxidized cellulose by oxidation, (ii) a step of subjecting the oxidized cellulose to a reduction treatment, and (iii) a step of defibrating and dispersing the oxidized cellulose subjected to the reduction treatment. it can.

The reason why the cellulose nanofiber of the present invention exhibits an excellent effect (low coloring during heating) is presumed as follows.

  Cellulose nanofibers having a carboxyl group amount of 0.6 mmol / g to 2.0 mmol / g are obtained by oxidizing the cellulose raw material using an oxidation catalyst such as an N-oxyl compound (such as TEMPO or a TEMPO derivative) and an oxidizing agent, or ozone. It can be obtained by defibrating oxidized cellulose obtained by oxidation of the used cellulose raw material.

  However, for example, in the production of oxidized cellulose using an N-oxyl compound as an oxidation catalyst, a carboxyl group is selectively generated at the C6-position, while cellulose is produced by N-oxyl compound or sodium hypochlorite as a side reaction. Oxidation of the secondary alcohols at the C2 and C3 positions occurs, and ketone groups are generated at the C2 and C3 positions. Moreover, the oxidation to the carboxyl group at the C6 position by the TEMPO catalyst is a two-step reaction via an aldehyde, and a part of the aldehyde group remains without being oxidized to the carboxyl group. When a ketone group or an aldehyde group is present in this way, a β elimination reaction occurs using the ketone group or aldehyde group as a scaffold when heated. As a result, 2,3-diketone is produced from the ketone group and α, β-unsaturated aldehyde is produced from the aldehyde at the non-reducing end of the newly produced cellulose. Furthermore, a peeling reaction occurs from the reducing end of the newly produced cellulose, and a colored substance having 2,3-diketone accumulates.

  Due to such a mechanism, there has been a problem that conventional cellulose nanofibers having a carboxyl group content of 0.6 mmol / g to 2.0 mmol / g are colored when heated. In contrast, the cellulose nanofiber of the present invention is presumed to exhibit excellent effects by reducing the amount of aldehyde groups and ketone groups by subjecting oxidized cellulose to a reduction treatment.

In the absorbance of the ultraviolet absorption spectrum, 260 nm is a peak peculiar to an aldehyde, and 290 nm is a peak peculiar to a ketone group.


(1) Cellulose raw material In the present invention, the cellulose raw material refers to materials in various forms mainly composed of cellulose, and pulp (bleached or unbleached wood pulp, bleached or unbleached non-wood pulp, refined linter, jute, Manila hemp , Pulp derived from herbs such as kenaf), natural cellulose such as cellulose produced by microorganisms such as acetic acid bacteria, regenerated cellulose spun after dissolving cellulose in some solvent such as copper ammonia solution, morpholine derivative, and the above Examples thereof include fine cellulose obtained by depolymerizing cellulose by subjecting the cellulose raw material to mechanical treatment such as hydrolysis, alkaline hydrolysis, enzymatic decomposition, explosion treatment, and vibration ball mill.

(2) Oxidation treatment Carboxyl is obtained by oxidizing the above cellulose raw material in water using an oxidizing agent in the presence of an N-oxyl compound and a compound selected from the group consisting of bromide, iodide or a mixture thereof. Oxidized cellulose having a group introduced into cellulose can be obtained.

  An N-oxyl compound is a compound capable of generating a nitroxy radical. 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 catalytic amount capable of forming cellulose into nanofibers. For example, 0.01 to 10 mmol is preferable, 0.01 to 1 mmol is more preferable, and 0.05 to 0.5 mmol is more preferable with respect to 1 g of the absolutely dry cellulose raw material.

  Bromide is a compound containing bromine, examples of which include alkali metal bromides that can dissociate and ionize in water. Further, an iodide is a compound containing iodine, and examples thereof include alkali metal iodide. The amount of bromide or iodide used can be selected as long as the oxidation reaction can be promoted. The total amount of bromide and iodide is, for example, preferably from 0.1 to 100 mmol, more preferably from 0.1 to 10 mmol, and even more preferably from 0.5 to 5 mmol, with respect to 1 g of an absolutely dry cellulose raw material.

  As the oxidizing agent, known ones can be used, and for example, halogen, hypohalous acid, halous acid, perhalogen acid or salts thereof, halogen oxide, peroxide and the like can be used. Among these, from the viewpoint of cost, sodium hypochlorite, which is the most widely used in industrial processes and has a low environmental load, is particularly preferable. The appropriate amount of the oxidizing agent used is, for example, preferably 0.5 to 500 mmol, more preferably 0.5 to 50 mmol, still more preferably 1 to 25 mmol, and most preferably 3 to 10 mmol with respect to 1 g of the absolutely dry cellulose raw material. preferable.

  The cellulose oxidation process allows the reaction to proceed efficiently even under relatively mild conditions. Therefore, the reaction temperature may be a room temperature of about 15 to 30 ° C. As the reaction proceeds, a carboxyl group is generated in the cellulose constituting the pulp, so that the pH of the reaction solution is reduced. In order to advance the oxidation reaction efficiently, it is preferable to add an alkaline solution such as an aqueous sodium hydroxide solution to maintain the pH of the reaction solution at about 9 to 12, preferably about 10 to 11. The reaction medium is preferably water because it is easy to handle and hardly causes side reactions.

  The reaction time in the oxidation reaction can be appropriately set according to the progress of oxidation, and is usually 0.5 to 6 hours, preferably 2 to 6 hours, more preferably about 4 to 6 hours. However, in the present invention, since the oxidation time can be reduced as described above, the reaction time is preferably 30 minutes to 120 minutes, more preferably 30 to 100 minutes, and even more preferably 30 to 70 minutes.

  The oxidation reaction may be performed in two stages. For example, oxidized cellulose obtained by filtration after the completion of the first stage reaction is oxidized again under the same or different reaction conditions, so that the cellulose is not subject to reaction inhibition by the salt produced as a by-product in the first stage reaction. A carboxyl group can be efficiently introduced into the raw material, and oxidation of the cellulose raw material can be promoted.

The conditions are preferably set so that the amount of carboxyl groups of oxidized cellulose is 0.2 mmol / g or more with respect to the absolute dry mass of cellulose. In this case, the carboxyl group amount is more preferably 0.6 mmol / g to 2.0 mmol / g, and still more preferably 1.0 mmol / g to 1.8 mmol / g. The amount of the carboxyl group can be adjusted by adjusting the oxidation reaction time, adjusting the oxidation reaction temperature, adjusting the pH during the oxidation reaction, adjusting the addition amount of the N-oxyl compound, bromide, iodide, or oxidizing agent.

The amount of carboxyl groups in the oxidized cellulose can be measured by the following procedure:
Prepare 60 ml of 0.5% by mass slurry of oxidized cellulose, add 0.1 M hydrochloric acid aqueous solution to pH 2.5, then add 0.05 N aqueous sodium hydroxide solution dropwise until the pH reaches 11. Is calculated from the amount of sodium hydroxide (a) consumed in the neutralization step of the weak acid with a gradual change in electrical conductivity, using the following formula:
Carboxyl group amount [mmol / g oxidized cellulose] = a [ml] × 0.05 / oxidized cellulose mass [g].

(3) Reduction treatment As the reducing agent used in the reduction treatment of the present invention, any reducing agent can be used as long as it can reduce partially generated aldehyde groups and ketone groups of oxidized cellulose to alcohol. Thiourea, hydrosulfite, sodium hydrogen sulfite, sodium borohydride, sodium cyanoborohydride, lithium borohydride and the like. As a usage-amount of a reducing agent, it is 0.1-150 mass parts with respect to 100 mass parts of oxidized cellulose raw materials, Preferably it is 0.5-100 mass parts, More preferably, it is about 1-50 mass parts. The reduction treatment temperature is preferably about 10 to 90 ° C., more preferably 20 to 70 ° C., from the viewpoint of the efficiency of reduction treatment and suppression of fiber deterioration. The pH during the reduction treatment may be adjusted as appropriate depending on the reducing agent used, but is usually pH 2-12, preferably pH 3-10. The reaction time in the reduction reaction can be appropriately set according to the progress of the reduction, and is not particularly limited, but is usually 0.5 to 6 hours, preferably 1 to 5 hours, and more preferably 1 to 4 hours.

(4) Defibration The oxidized pulp subjected to the reduction treatment is defibrated and converted into cellulose nanofibers. Defibration can be performed, for example, by mixing or stirring, emulsifying / dispersing devices such as a high-speed shear mixer or a high-pressure homogenizer, singly or in combination of two or more. At this time, the size of the oxidized pulp (fiber length and fiber diameter) is reduced at the same time as the fibers are loosened to form a single microfibril. In particular, when an ultrahigh pressure homogenizer that enables a pressure of 100 MPa or more, preferably 120 MPa or more, more preferably 140 MPa or more is used, the shortening and dispersion of cellulose nanofibers proceed efficiently, and when an aqueous dispersion is obtained. This is preferable because cellulose nanofibers having a low viscosity can be produced efficiently.

The fiber length of the cellulose nanofiber of the present invention is preferably 100 to 3000 nm, more preferably 150 to 1500 nm, still more preferably 200 nm to 1000 nm. The fiber diameter is preferably 2 to 1000 nm, more preferably 2.5 to 100 nm, and still more preferably 3 to 10 nm.

The cellulose nanofiber of the present invention can be used as a dispersion. Moreover, the cellulose nanofiber of this invention can be used by making it contain in a sheet | seat or a molding.

(5) Film In the present invention, a film made of cellulose nanofibers can be obtained by drying or solidifying without applying a force such as stretching.

EXAMPLES Next, although this invention is demonstrated in detail based on an Example, this invention is not limited to a following example.

[Example 1]
(Oxidation of pulp)
Bleached unbeaten pulp (Nippon Paper Co., Ltd.) 5 g (absolutely dried) derived from coniferous trees, TEMPO (Tokyo Kasei Co., Ltd.) 78 mg (0.5 mmol) and sodium bromide (Wako Pure Chemical Industries, Ltd.) 756 mg (7.35 mmol) ) Was dissolved in 500 ml of the dissolved aqueous solution and stirred until the pulp was uniformly dispersed. To this, 2.3 mmol of sodium hypochlorite (manufactured by Wako Pure Chemical Industries, Ltd.) was added in the form of an aqueous solution, and then a liquid feed pump was added so that sodium hypochlorite was added at a rate of 0.23 mmol / min per gram of pulp. Was added gradually to oxidize the pulp. The addition was continued until the total amount of sodium hypochlorite added was 22.5 mmol. During the reaction, the pH in the system was lowered, but a 3N sodium hydroxide aqueous solution was successively added to adjust the pH to 10. From the start of the addition of the aqueous sodium hydroxide solution (that is, from the time when the oxidation reaction is started and a decrease in pH is observed) until the addition is completed (that is, the oxidation reaction is completed and no decrease in pH is observed) The reaction time was taken as the reaction time. The liquid after the reaction was filtered through a glass filter, and sufficiently oxidized to obtain an oxidized pulp.

(Measurement of carboxyl group content of oxidized pulp)
The carboxyl group content of oxidized pulp was measured by the following method:
Prepare 60 ml of 0.5% by weight slurry of oxidized pulp, add 0.1 M hydrochloric acid aqueous solution to pH 2.5, then add 0.05 N aqueous sodium hydroxide solution dropwise until the pH is 11 Was calculated from the amount of sodium hydroxide (a) consumed in the neutralization step of the weak acid with a gradual change in electrical conductivity using the following formula:
Amount of carboxyl group [mmol / g oxidized pulp] = a [ml] × 0.05 / oxidized pulp mass [g].

As a result of this measurement, the carboxyl group content of the obtained oxidized pulp was 1.60 mmol / g.

(Reduction of oxidized pulp)
2 g of sodium hydrogen sulfite (manufactured by Wako Pure Chemical Industries, Ltd.) was added to 500 mL of oxidized pulp slurry having a concentration of 1% (w / v), and reduction treatment was performed for 3 hours while maintaining the temperature at 40 ° C. The pulp slurry after the reduction treatment was filtered with a glass filter and sufficiently washed with water to obtain oxidized pulp subjected to the reduction treatment.

(Oxidized pulp defibration and dispersion preparation)
When 500 mL of 1% (w / v) oxidized pulp slurry subjected to the reduction treatment was treated five times with an ultra-high pressure homogenizer (20 ° C., 140 MPa), a transparent gel-like cellulose nanofiber dispersion was obtained. It was.

(Measurement of viscosity of cellulose nanofiber)
The concentration (% (w / v)) of the cellulose nanofiber aqueous dispersion with a B-type viscosity (60 rpm, 20 ° C.) measured using a TV-10 viscometer (manufactured by Toki Sangyo Co., Ltd.) of 1000 mP · a. Used as an index of viscosity. When the value becomes 3% (w / v) or more under these conditions, it can be said that “the B-type viscosity (at 60 rpm, 20 ° C.) at a concentration of 3% (w / v) is 1000 mPa · s or less”. .

(Measurement of average fiber length of cellulose nanofiber)
The fiber length was measured from the atomic force microscope image (3000 nm × 3000 nm) of the cellulose nanofiber fixed on the mica section, and the number average fiber length was calculated. The fiber length measurement was performed using image analysis software WinROOF (manufactured by Mitani Corporation) in the range of 100 nm to 2000 nm in length.

(Measurement of average fiber diameter of cellulose nanofiber)
A cellulose nanofiber aqueous dispersion diluted so that the concentration of the cellulose nanofibers was 0.001% by mass was prepared. This diluted dispersion is thinly spread on a sample table manufactured by Mica, heated and dried at 50 ° C. to prepare a sample for observation, and the cross-sectional height of the shape image observed with an atomic force microscope (AFM) is measured. The average fiber diameter was calculated.

(Film formation of cellulose nanofiber)
After weighing 15 ml of the above-mentioned cellulose nanofiber having a concentration of 1% and pouring it into a petri dish having a diameter of 5 cm, it was allowed to stand at a temperature of 40 ° C. for 2 days in a blow dryer. Thereafter, a film of cellulose nanofibers having a thickness of 50 μm was obtained.

(Evaluation of coloring after heating of cellulose nanofiber)
The filmed product obtained above was heat-treated at a temperature of 120 ° C. for 15 minutes in a blow dryer. The film-formed product after the heat treatment was placed on a white sheet, and the color was visually confirmed.

(Confirmation of aldehyde group and ketone group)
The film of cellulose nanofibers obtained by film formation was subjected to heat treatment at a temperature of 150 ° C. for 30 minutes in a blow dryer. The absorbance of the ultraviolet absorption spectrum (in the range of 190 nm to 400 nm) was measured using a UV-visible near-infrared spectrophotometer V-670 (manufactured by JASCO Corporation) for the film-formed cellulose nanofiber after the heat treatment. . A peak at 290 nm peculiar to the ketone group and a peak value at a wavelength of 260 nm peculiar to the aldehyde group were measured.

[Example 2]
A cellulose nanofiber dispersion was obtained in the same manner as in Example 1 except that 0.25 g of sodium borohydride was added as a reducing agent and a reduction reaction was performed at 25 ° C. and pH 10 for 1 hour.

[Comparative Example 1]
A cellulose nanofiber dispersion was obtained in the same manner as in Example 1 except that the oxidation pulp reduction reaction was not performed.

The cellulose nanofibers of Examples 1 and 2 are not significantly different in physical properties in terms of average fiber length, average fiber diameter, and viscosity as compared with the cellulose nanofibers of Comparative Example 1, but are clearly different in coloring after heating. There is. It can be said that the fact that coloring after heating does not occur is advantageous in that it is not affected by discoloration due to heat treatment in industrial use of cellulose nanofibers.

Claims (2)

A cellulose nanofiber having a carboxyl group amount of 0.6 mmol / g to 2.0 mmol / g, a number average fiber length of 100 to 3000 nm, and a number average fiber diameter of 2 to 1000 nm, the thickness created from the cellulose nanofiber When the peak value of the ultraviolet absorption spectrum at 260 nm of a 50 ± 5 μm film is A, the peak value of the ultraviolet absorption spectrum at 290 nm is B, and the film thickness is X μm, A / X is 0.055 / μm or less, and Cellulose nanofiber, wherein B / X is 0.050 / μm or less.
The manufacturing method of the cellulose nanofiber of Claim 1 including the process of following (i)-(iii).
(I) A step of preparing cellulose oxide by oxidizing a cellulose raw material in water using an oxidizing agent in the presence of an N-oxyl compound and a compound selected from the group consisting of bromide, iodide or a mixture thereof. ii) A step of subjecting the oxidized cellulose to a reduction treatment (iii) A step of defibrating and dispersing the oxidized cellulose subjected to the reduction treatment