JP7670538B2 - Cellulose dissolving solvent, cellulose dissolving method, and method for producing cellulose molded body - Google Patents

Description

The present invention relates to a solvent used to dissolve cellulose, and a method for producing a molded body using the solvent.

The applicant has filed a patent application for a mixture of tetraalkylammonium acetate (TAAA) and an aprotic polar solvent as a solvent for dissolving cellulose (Patent Document 1).
However, the solvent of Patent Document 1 has a large effect of water on its cellulose solubility. When about 5 wt% of water is contained, the solubility of cellulose is significantly reduced. However, since the solvent is highly hygroscopic, waterproofing management is strict in the series of processes from the solvent to the cellulose solution. Furthermore, water is often used as a coagulating liquid when spinning a cellulose solution or forming a film. After the coagulating liquid is used, it is common to distill off the water and recover and reuse only the cellulose solvent. Both TAAA and aprotic polar solvents have strong interactions with water molecules, so when the moisture content falls below a certain value, it is not easy to remove the moisture. Therefore, long-term distillation is required while using vacuum and heating in combination. As a result, TAAA and aprotic polar solvents decompose, or a large amount of energy is consumed by distillation. The solubility of cellulose decreases when TAAA and aprotic polar solvents decompose. As a result, the number of times the cellulose solvent is recycled decreases.

JP 2012-211302 A

The present invention provides a solvent capable of uniformly dissolving cellulose in a short time at around room temperature without requiring special pretreatment or moisture management, a method for dissolving cellulose using the solvent, and a method for producing a cellulose molded body.

式中、Ｒ１、Ｒ２、Ｒ３およびＲ４はそれぞれ独立して、炭素数１～４のアルキル基を表す。
〔２〕 水酸化テトラアルキルアンモニウムが、水酸化テトラエチルアンモニウム、水酸化テトラプロピルアンモニウムおよび水酸化テトラブチルアンモニウムの少なくとも一つを含むことを特徴とする前記〔１〕の溶媒。
〔３〕 セルロースの溶解方法であって、セルロースと前記〔１〕または〔２〕に記載の溶媒を接触させてセルロースを溶解するセルロース溶解方法。
〔４〕 前記〔３〕に記載のセルロースの溶解方法で得られたセルロース溶解溶液を用いてセルロース成形体を製造するセルロース成形体製造方法。 As a result of extensive investigations aimed at achieving the above object, the present inventors have completed the invention described below.
[1] A solvent used for dissolving cellulose,
The solvent comprises a tetraalkylammonium hydroxide represented by the following formula, water, and dimethyl sulfoxide:
The concentration of each of the solvents in the solvent is within the ranges of 2.0 to 25 wt % for tetraalkylammonium hydroxide, 1.0 to 35 wt % for water, and 40 to 97 wt % for dimethylsulfoxide.

In the formula, R1, R2, R3 and R4 each independently represent an alkyl group having 1 to 4 carbon atoms.
[2] The solvent according to [1] above, wherein the tetraalkylammonium hydroxide comprises at least one of tetraethylammonium hydroxide, tetrapropylammonium hydroxide, and tetrabutylammonium hydroxide.
[3] A method for dissolving cellulose, comprising contacting cellulose with the solvent according to [1] or [2] above to dissolve the cellulose.
[4] A method for producing a cellulose molded body, comprising the steps of: producing a cellulose molded body by using a cellulose dissolving solution obtained by the method for dissolving cellulose according to [3] above.

The solvent of the present invention is capable of dissolving cellulose uniformly in a short time without special pretreatment, regardless of the crystalline form of the cellulose.

Compared to the solvent of Patent Document 1, the solubility of the solvent of the present invention does not decrease even when it contains 30% water, so harsh distillation and concentration conditions are not required when handling the dissolution process or recycling the solidification liquid, and decomposition of solvent components such as DMSO is avoided. In addition, high energy is not used in the process of recycling the solidification liquid.

In addition, the solution in which cellulose is dissolved using the solvent of the present invention has fluidity even at room temperature and has excellent moldability.

Photographs of the cellulose solutions prepared in Examples 1, 2, 3, 6 and 7 (the stirring bar of the magnetic stirrer used for stirring can be seen in some photographs. The same is true for Figures 2 and 4.) Photographs of the cellulose spinning process and the resulting cellulose fibers obtained in Example 1 UV spectra of the cellulose solutions prepared in Examples 2-4 and 7-9 Photographs of the cellulose solutions prepared in Examples 4, 8 and 9

The tetraalkylammonium hydroxide, which is a component of the cellulose dissolving solvent of the present invention, is not limited in any way so long as it is a tetraalkylammonium hydroxide of the chemical formula shown in the above [1], and may be used alone or in combination of two or more kinds.
Specific examples of the compounds include the following compounds.
Examples of the hydroxide include tetraethylammonium hydroxide, tetrapropylammonium hydroxide, tetrabutylammonium hydroxide, methyltripropylammonium hydroxide, methyltributylammonium hydroxide, ethyltripropylammonium hydroxide, ethyltributylammonium hydroxide, propyltributylammonium hydroxide, dimethyldipropylammonium hydroxide, dimethyldibutylammonium hydroxide, diethyldipropylammonium hydroxide, diethyldibutylammonium hydroxide, and the like. More preferred are tetraethylammonium hydroxide (TEAH), tetrapropylammonium hydroxide (TPAH), and tetrabutylammonium hydroxide (TBAH). Particularly preferred are tetraethylammonium hydroxide and tetrapropylammonium hydroxide.

The tetraalkylammonium hydroxide (TAAH) used in the present invention to dissolve cellulose is water-soluble and is readily available as a commercially available chemical in aqueous solution.
Since the cellulose dissolving solvent of the present invention contains tetraalkylammonium hydroxide (TAAH), water and dimethylsulfoxide (DMSO), TAAH can be mixed in the form of an aqueous solution with DMSO to form a dissolving solvent.

The cellulose dissolving solvent of the present invention is composed of TAAH, water and DMSO, and the composition ratio is adjusted to be within the concentration range of 2.0 to 25 wt% TAAH, 1.0 to 35 wt% water, and 40 to 97 wt% DMSO, more preferably, 2.0 to 22 wt% TAAH, 3.0 to 30 wt% water, and 50 to 95 wt% DMSO.
A TAAH concentration lower than 2 wt % is undesirable because the solubility and dissolution rate are low, while a TAAH concentration higher than 25 wt % is undesirable because the solubility of cellulose is reduced and there is a risk of cellulose decomposition during dissolution or storage of the resulting cellulose solution.

If the water concentration is lower than 1.0%, it is not preferable because the solubility of cellulose decreases and the TAAH becomes unstable and may decompose. On the other hand, if the water concentration is higher than 35%, it is not preferable because the solubility of cellulose decreases.

The solvent of the present invention may contain other organic solvents in addition to tetraalkylammonium hydroxide, water, and dimethyl sulfoxide. Examples include alcohol, N,N-dimethylacetamide, N,N-dimethylformamide, N-methylpyrrolidone, and pyridine. Addition of these solvents can improve the dissolution of cellulose and adjust the viscosity and reactivity of the cellulose solution.

There are no particular limitations on the method for preparing the cellulose dissolving solvent of the present invention. For example, a cellulose solvent can be obtained by preparing a commercially available TAAH aqueous solution to the desired concentration, then adding DMSO to it and stirring it. If the TAAH concentration of the commercially available TAAH aqueous solution is lower than the desired concentration when the water concentration is in the optimal range, it is preferable to distill it before use and concentrate it to the desired moisture content before use. If the concentration is higher than the desired concentration, water is added to dilute it before use. There are no particular limitations on the temperature when mixing, and room temperature is sufficient.

There are no particular limitations on the method of dissolving cellulose in the cellulose dissolving solvent of the present invention. For example, a cellulose solution is prepared by adding cellulose to a predetermined amount of the cellulose dissolving solvent of the present invention and stirring until a transparent solution is obtained. Stirring may be performed with a commonly used mechanical stirrer. For a beaker scale, stirring with a magnetic stirrer is sufficient. The temperature during dissolution may be 10 to 80°C, and dissolution may be performed at room temperature without temperature adjustment. Temperatures lower than 10°C are not preferred because the solubility or dissolution rate of cellulose is low. Temperatures higher than 80°C are not preferred because there is a risk of decomposition or decomposition of cellulose, TPAH, or TEAH. More preferably, the temperature is 15 to 70°C, and most preferably, the temperature is 20 to 60°C.

There are no particular limitations on the amount of cellulose dissolved. It may be adjusted as appropriate depending on the degree of polymerization (molecular weight) of the cellulose and the intended use. For example, it is 1 to 20 wt %. If the amount dissolved is too low, productivity will be low. On the other hand, if the amount dissolved is too high, the cellulose may not be completely dissolved, the uniformity of the solution may decrease, and the fluidity of the resulting cellulose solution may be lost, which is not preferable.

There are no particular limitations on the cellulose that can be dissolved in the cellulose solvent of the present invention. Examples of cellulose that can be dissolved include wood pulp, cotton, cotton linter pulp, cellulose powder, bacterial cellulose, sea squirt cellulose, and bagasse cellulose, regardless of the crystal structure, degree of crystallization, and degree of polymerization of the cellulose.

The cellulose solution obtained by dissolving cellulose in the cellulose dissolving solvent of the present invention is degassed and used as a molding solution. There is no particular limitation on the molding method, but a wet molding method can generally be applied. For example, the cellulose solution is sucked into a syringe, a nozzle is attached to the syringe pump or a microfeeder, and the cellulose solution is discharged from the nozzle into a water bath at room temperature while moving the syringe, and the fiber is washed and then wound up by a winder to prepare cellulose fiber.
When forming a cellulose film, for example, a cellulose solution is cast onto a glass substrate, and the liquid film is washed while being coagulated by immersion in a coagulating liquid such as water or alcohol, and then dried to prepare a film.

The present invention will be further explained using examples. Note that the present invention is not limited to these examples.

(Materials and solvents used)
Cellulose pulp: NBKP pulp (Canfor) and cotton linter pulp were used, both of which were obtained from Marubeni Corporation.
Absorbent cotton: purchased from Nacalai Tesque.
40% tetrapropylammonium hydroxide aqueous solution: manufactured by Tokyo Chemical Industry Co., Ltd.
55% aqueous solution of tetrapropylammonium hydroxide: Obtained by concentrating a 40% product of Tokyo Chemical Industry Co., Ltd.
35% tetraethylammonium hydroxide aqueous solution: manufactured by Tokyo Chemical Industry Co., Ltd.
56% tetraethylammonium hydroxide aqueous solution: Obtained by concentrating a 35% Tokyo Chemical Industry Co., Ltd. product.
Other reagents were purchased from Nacalai Tesque.

(Measurement of visible light transmittance)
The cellulose solution was sandwiched between two glass slides and measured using a UV-visible spectrophotometer UV-visible transmission spectrum chart to evaluate the visible light transmittance at a wavelength of 450 nm. The device used for the measurement was Shimadzu UV-36003000, and the measurement wavelength range was 400 nm to 800 nm. The thickness of the liquid film of the cellulose solution was 10 μm.

(Viscosity measurement of cellulose solution)
The viscosity of the cellulose solution was measured using a vibration viscometer (VISOCOMATE MODEL VM-10A series) manufactured by Sekonic Corporation, and the measurement temperature was room temperature set at 23°C.

[Example 1]
0.25 g of 40% tetrapropylammonium hydroxide (TPAH) aqueous solution and 4.75 g of dimethyl sulfoxide were added to a 10 ml vial, and 0.1 g of linter pulp was added and dissolved while stirring with a magnetic stirrer at room temperature. The appearance of the solution was observed at regular intervals, and the time when the solution became uniform and transparent was taken as the time when the cellulose was completely dissolved, and the dissolution time was measured. The measured dissolution time and the measurement results of the color and transmittance of the obtained cellulose solution are shown in Table 1. A photo of the appearance of the cellulose solution is shown in Figure 1.

The resulting cellulose solution was drawn into a syringe and discharged into a distilled water bath at room temperature using a nozzle with a pore diameter of 0.5 mm, while stretching the fibrous transparent gel. The fibrous transparent gel was then washed with distilled water to remove the tetrapropylammonium hydroxide and dimethyl sulfoxide, and air-dried at room temperature to obtain cellulose fibers. A photograph of the resulting cellulose fibers is shown in Figure 2.

[Examples 2 to 12]
The weights of the components shown in Table 1 were weighed, and the cellulose was dissolved and evaluated in the same manner as in Example 1. The results are shown in Table 1 and Figures 1, 3 and 4. All of the obtained cellulose solutions were transparent.

[Comparative Examples 1 to 5]
The weight of each component shown in Table 1 was weighed, and the cellulose was dissolved and evaluated in the same manner as in Example 1. The results are shown in Table 1. No transparent cellulose solution was obtained in any case.

The dissolution conditions and evaluation results for Examples 1 to 12 and Comparative Examples 1 to 5 are summarized in Table 1, and the solvent composition ratios are shown in Table 2.

As shown in Table 1, in Examples 1 to 12, the cellulose was not pretreated and was able to be completely dissolved uniformly and in a short time at room temperature. On the other hand, in Comparative Examples 1 to 5, the cellulose could not be dissolved. The viscosities of the cellulose solutions prepared in Examples 5, 8, 9, and 12 were high and exceeded the measurement range of the vibration viscometer (1000 mPa.s or less), so they could not be measured.

As described above, the cellulose dissolving solvent of the present invention can dissolve cellulose uniformly in a short time, regardless of the degree of polymerization and crystalline form of cellulose, and does not require the conventional strict waterproofing and water content control processes and operations. Furthermore, the cellulose-containing solution dissolved in the solvent of the present invention has excellent fluidity and moldability, and can be widely applied to the technical field of manufacturing cellulose molded bodies. In addition, the cellulose-containing solution dissolved in the solvent of the present invention can also be applied to the synthesis of acylated cellulose derivatives such as cellulose acetate because of its basicity.

Claims (4)

A solvent used to dissolve cellulose,
The solvent consists of a tetraalkylammonium hydroxide represented by the following formula, water, and dimethyl sulfoxide,
The concentration of each of the solvents in the solvent is within the ranges of 2.0 to 25 wt % for tetraalkylammonium hydroxide, 1.0 to 35 wt % for water, and 40 to 97 wt % for dimethylsulfoxide.

In the formula, R1, R2, R3 and R4 each independently represent an alkyl group having 1 to 4 carbon atoms. The solvent of claim 1, characterized in that the tetraalkylammonium hydroxide includes at least one of tetraethylammonium hydroxide, tetrapropylammonium hydroxide, and tetrabutylammonium hydroxide. A method for dissolving cellulose, comprising contacting cellulose with the solvent according to claim 1 or 2 to dissolve the cellulose. A method for producing a cellulose molded body, comprising the steps of: producing a cellulose molded body using a cellulose dissolving solution obtained by the cellulose dissolving method according to claim 3;