JP2010215872A - Cellulose porous body and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a cellulose porous body which has porosity and open-cell properties by suitably entangling fine cellulose fibers without aggregation and is suitable for an adsorbent, a thermal insulator, a sound-absorbing material, and the like because it has a high specific surface area and is excellent in flexibility, air permeability, dimensional stability, and handleability. <P>SOLUTION: The cellulose porous body includes the fine cellulose fibers having a mean fiber diameter of 2-1,000 nm as a main structural component and has an apparent density of 0.005-0.15 g/cm<SP>3</SP>. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a porous cellulose material obtained by freeze-drying an aqueous suspension containing fine cellulose fibers. More specifically, because the porous molded body maintains the voids between the fibers by appropriately entangled with each other without agglomerating the fine cellulose fibers, the specific surface area is high, and the flexibility, breathability, and dimensional stability are high. The present invention relates to a cellulose porous body that is excellent in properties and handleability and is suitable as an adsorbent, a heat insulating material, and a sound absorbing material, and a method for producing the same.

  Cellulose-based materials are the most mass-produced biomass on the earth, and do not depend on petroleum-based raw materials, and since they do not increase carbon dioxide during incineration, they are attracting attention as materials with little environmental impact. Conventionally, cellulosic materials have been used as high-performance materials such as carbon fibers and activated carbon fibers by utilizing the properties of carbon, plastics, filters, packaging materials, and other general-purpose materials and the ability to carbonize by heating cellulose. Yes. In addition, fine cellulose fibers have been developed due to recent technological advances, and the use of cellulosic materials is expanding further.

  The fine cellulose fiber can be obtained by fibrillating the cellulose fiber by beating or homogenizing the cellulose fiber. The fine cellulose fibers obtained in this way are fine, have a high surface area, and have many entanglements between the fibers, so that they are used for resin reinforcing materials, filter aids, food additives, and the like. Usually, the fine cellulose fiber is maintained in a uniform state by being present as a water suspension in water. However, since cellulose has a plurality of hydroxyl groups in its molecular structure and forms hydrogen bonds within and between molecules, there is a problem that fibers tend to aggregate after drying. As a result, the dried product of fine cellulose fibers has a shrinkage and distortion of size, and has a hard texture with low air permeability. In addition, due to the aggregation of the microfibers, the apparent fiber diameter increases, and the actual surface area of the microcellulose fibers decreases. For example, when considering the use of fine cellulose fibers as an activated carbon adsorbent, a high-density material in which fibers are aggregated has a low contact probability with the adsorbed gas, and it is difficult to express the characteristics of high adsorption capacity. . In this case, in order to improve the surface area by forming micropores in the material surface in the manufacturing process, it is necessary to introduce an activation gas into the material. That is, when trying to utilize fine cellulose fibers as an adsorbent, the structure needs to be porous and breathable in a dry state. However, as for the dried product of fine cellulose fibers, there is a problem of agglomeration at the time of drying as described above, so that only those having a low specific surface area and poor air permeability have been developed.

  In patent document 1, after spraying the water suspension containing a cellulose on the cooled metal plate, rapidly freezing, and sublimating, the granular dried material comprised from a micro cellulose fiber is produced. Moreover, the granular carbide is obtained by heating a granular dried material. In this proposal, the obtained dried product of microcellulose fibers is limited to a granular shape, and the handleability is poor. Further, since the density of granular cellulose carbide is high and the air permeability is low, the permeability of the inert gas during carbonization is inferior. As a result, there is a problem that it is difficult to produce uniform carbide.

  Patent Document 2 forms a thin layer sheet by casting a water suspension containing cellulose and then removing the solvent. In this proposal, since the solvent removal is performed under heating, the surface tension of the solvent always acts between the microfibers, and the fibers are attracted to each other. When the solvent removal proceeds in such a state, the distance between the fibers gradually decreases due to the surface tension of the solvent, and the fibers aggregate during drying. The dimension of the thin layer sheet obtained by this method becomes small and the density increases, so that the porous material is not formed.

  In Patent Document 3, a cellulose porous body is prepared by concentrating a fine cellulose fiber water slurry having a concentration of 4% by weight or more and then freeze-drying. In this proposal, since the fiber concentration in the water slurry is high, the fibers are likely to aggregate, and the apparent fiber diameter after the porous body is produced becomes large. Further, since the density of the porous body is large, the pore diameter is reduced, and the inert gas permeability during carbonization is inferior.

  In Patent Document 4, an aqueous slurry of fine cellulose fibers is dried by a papermaking method to produce a cellulose nonwoven fabric. In this proposal as well, as in Patent Document 2, since the dehydration treatment is performed under heating, the fibers aggregate together with drying. As a result, it is difficult to use the obtained cellulose nonwoven fabric as a porous material.

  In patent document 5, after mixing the removable phase which can remove hydrophilic fiber by freeze-drying, and the binding material required in order to maintain a porous structure, a fibrous absorbent material is produced by freeze-drying. Yes. In this proposal, as the hydrophilic fiber constituting the fibrous absorbent material, a fiber having a larger diameter than that of the fine cellulose fiber is used. For this reason, in order to maintain the porous structure of the fibrous absorbent material, it is necessary to cover and reinforce the entanglement points of the fibers with a binding material. As a result, the substantial surface area of the fibrous absorbent material is reduced. Moreover, although the obtained fibrous absorbent material is low density, since the void size is large, the compression resistance and dimensional stability of the fibrous absorbent material are poor.

  Thus, a material having a high specific surface area and excellent flexibility, breathability, and dimensional stability has not been developed in the dried product of fine cellulose fibers.

JP 2003-82535 A Japanese Patent Laid-Open No. 10-248872 JP 11-255806 A WO2006-004012 WO99-061518

  The object of the present invention is to provide a porous cellulose body composed of fine cellulose fibers having a high specific surface area and excellent flexibility, breathability, and dimensional stability in order to solve the above-mentioned problems of the prior art. There is.

The above-mentioned problem is a porous cellulose body composed of fine cellulose fibers having an average diameter of 2 to 1000 nm, and an apparent density is 0.005 to 0.15 g / cm ^3. Can be solved by the body.

  Moreover, it can employ | adopt suitably that the 25% compression set of a cellulose porous body is 0.5 to 20%.

  Furthermore, the problem can be solved by a method for producing a porous cellulose body, which comprises freeze-drying an aqueous slurry of fine cellulose fibers having a concentration of 0.1 to 3.5% by mass.

  According to the present invention, a porous body having a high surface area and excellent flexibility, dimensional stability, and handleability, which has been conventionally obtained by using fine cellulose fibers, can be obtained. In addition, since it is a porous, open-celled material, it has excellent air permeability and can be carbonized and activated by inert gas ventilation under heating, making it suitable as an adsorbent, heat insulating material, and sound absorbing material. Can be used.

  Hereafter, the porous body comprised from the micro cellulose fiber excellent in the high specific surface area and the softness | flexibility, air permeability, and dimensional stability of this invention is demonstrated in detail.

  Cellulose in the present invention originates from plants such as wood, cotton, hemp, flax, ramie, jute, kenaf, animals such as sea squirts, algae such as seaweed, microorganisms such as acetic acid bacteria, etc. It may be. Among them, refined pulp, cotton-derived cotton linter and cotton lint, and bacterial cellulose derived from acetic acid bacteria can be suitably employed because of their high cellulose purity.

  The cellulose in the present invention is preferably a cellulose fiber. Since the specific surface area is increased by taking the fiber structure, the adsorption performance is improved when used as an adsorbent.

  The average fiber diameter of the fine cellulose fibers is preferably 2 to 1000 nm. An average fiber diameter of 2 nm or more is preferable because cellulose fibers can be stably produced by refining cellulose. The average fiber diameter is more preferably 5 nm or more, still more preferably 10 nm or more. On the other hand, an average fiber diameter of 1000 nm or less is preferable because a porous body of fine cellulose fibers having excellent form recoverability and flexibility can be obtained by entanglement of fibers. When the average fiber diameter of the fine cellulose fibers is larger than 1000 nm, the fibers are less entangled, and the sample after freeze-drying is easily loosened, and the handling property may be poor. The average fiber diameter is more preferably 700 nm or less, and further preferably 500 nm or less. Moreover, although the cellulose fiber whose fiber diameter is outside the range of 2-1000 nm may be contained, it is preferable that the fiber diameter of 70% or more with respect to the total number of a cellulose fiber is 2-1000 nm.

  The average fiber length of the fine cellulose fibers is preferably 0.1 to 1 mm. When the average fiber length is 0.1 mm or more, it is preferable that when formed into a porous material, voids are generated between the fibers due to entanglement between the fibers, and the porosity and open cell property of the porous material are maintained. In addition, although the cellulose fiber whose fiber length is less than 0.1 mm may be contained, it is preferable that the fiber length of 70% or more with respect to the total number of a cellulose fiber is 0.1 mm-1 mm. As an upper limit, it is preferable that it is 1 mm or less from the point which can produce the uniform water slurry of a micro cellulose fiber. More preferably, it is 0.7 mm or less.

The apparent density of the porous cellulose body of the present invention is 0.005 to 0.15 g / cm ³ . More preferably 0.01~0.13g / cm ^3, most preferably from 0.02~0.11g / cm ^3. When the apparent density of the cellulose porous body is higher than 0.15 g / cm ³ , the porosity of the porous body after lyophilization is small and the flexibility may be poor. Moreover, in the activation process at the time of adsorbent preparation, the activation gas may not easily penetrate uniformly into the porous body. On the other hand, when the apparent density of the porous cellulose body is lower than 0.005 g / cm ³ , the porosity of the porous body increases, and the mechanical properties and dimensional stability of the porous body may be poor.

  The 25% compression set of the porous cellulose body of the present invention is preferably 0.5 to 20%. More preferably, it is 2.0-15%, Most preferably, it is 3.0-10%. If the 25% compression set of the cellulose porous body is higher than 20%, the porous body is difficult to recover from the compressed state to the shape before compression, and thus the porosity and dimensional stability may be poor.

  Next, the manufacturing method of the cellulose porous body of this invention is demonstrated.

  The fine fibrous cellulose having an average fiber diameter of 2 to 1000 nm according to the present invention is a chemical method by acid hydrolysis of cellulose using an acid such as sulfuric acid or hydrochloric acid according to a known method, or a high pressure homogenizer, an ultrahigh pressure homogenizer, Although it is obtained by a physical method of beating cellulose with a refiner, grinder, stone mill or the like to defibrate or refine the cellulose, it is not limited thereto. It is also possible to use commercially available cellulose fibers that have been subjected to treatment by chemical methods or physical methods.

  The cellulosic porous material of the present invention is a porous material having porosity and open-celling properties by, for example, preparing a water slurry by dispersing fine cellulose fibers in water, and lyophilizing the obtained water slurry to remove the solvent. It can be obtained as a mass.

  Specifically, a water slurry of fine cellulose fibers is cast into a mold having an appropriate size and shape, and freezing is performed in this state. The size and shape of the resulting cellulose porous body can be controlled by the shape of the mold at this time. As a freeze-drying condition, the freezing temperature of the water slurry of fine cellulose fibers is preferably −196 to 0 ° C., and a refrigerant such as methanol containing liquid nitrogen or dry ice is appropriately used according to the freezing temperature of the water slurry. Can be used. When the freezing temperature of the water slurry of fine cellulose fibers is higher than 0 ° C., the water slurry melts during lyophilization and a porous material cannot be obtained. If the freezing temperature of the water slurry is lower than -196 ° C, the freeze-drying process takes time, and the production efficiency of the porous body may be lowered. The freezing temperature of the water slurry of fine cellulose fibers is more preferably −180 to −10 ° C., and most preferably −160 to −20 ° C. The pressure during freeze-drying is preferably 0.3 Torr or less.

  It is preferable that the water slurry density | concentration of a micro cellulose fiber is 0.1-3.5 mass%. More preferably, it is 0.3-3.0 mass%, Most preferably, it is 0.5-2.5 mass%. By setting it as the said range, it can be in the range of the apparent density prescribed | regulated by this invention. When the concentration of the fine cellulose fiber water slurry is higher than 3.5% by mass, the fine cellulose fibers are not uniformly dispersed in the water slurry, and aggregation of the fibers may be observed. As a result, the apparent density increases, the porosity of the porous body after lyophilization is small, and the flexibility may be poor. Further, when the concentration of the water slurry is lower than 0.5% by mass, not only the apparent density is decreased, but also the porosity of the porous body is increased, and the mechanical properties and dimensional stability of the porous body may be poor. is there.

  In the present invention, the compression set can be within a preferable range by forming a water slurry of fine cellulose fibers having a concentration in the above specified range into a porous body by freeze drying. By using the fine cellulose fibers as described above, the fine cellulose fibers can be uniformly dispersed in water while being appropriately entangled, and local variations in the water slurry concentration can be suppressed. Moreover, since the water removal part becomes a space | gap at the time of the water slurry of a micro cellulose fiber, the apparent density of a dried material can be controlled by making water slurry density | concentration into the range of this invention. Drying of the water slurry of fine cellulose fibers can employ freeze-drying to suppress the surface tension to the fine cellulose fibers during drying, and to obtain a porous body having porosity and open cell properties. . Such a cellulose porous body of the present invention has flexibility and elastic recovery, and can be within the compression set range defined in the present invention.

  Regarding the cellulose porous body of the present invention, a slurry of fine cellulose fibers was cast into a fiber structure, for example, a warp knitted fabric, a weft knitted fabric, a woven fabric, a non-woven fabric, a braid or the like alone or a mixture of two or more. Thereafter, the porous body can be reinforced by freeze-drying.

  In the present invention, as described above, the water slurry containing a fine cellulose fiber such as a fine cellulose fiber at a relatively low concentration suppresses the aggregation of the cellulose fiber and has a relatively dense and relatively uniform dispersion state. Since it is formed, a porous body in which fine voids are formed while fine cellulose fibers are entangled can be obtained by freeze-drying this. Therefore, the cellulose porous body of the present invention has a high surface area and is excellent in flexibility, air permeability, and dimensional stability. In addition, since it has characteristics such as porosity and open cell property, it can be uniformly carbonized and activated, and can be suitably used as an adsorbent or a heat insulating material.

  Hereinafter, the present invention will be described in more detail with reference to examples. Each characteristic value in the examples is obtained by the following method.

A. Appearance of Porous Body The appearance of the porous body was evaluated as to whether it could be formed into one porous body shape by drying the water slurry of fine cellulose fibers or whether the shape could be maintained by the tentacles.
`` If it can be made into a porous body shape and can be held in shape by a tentacle '' ○, `` If it can be made into a porous body shape but cannot be held in shape by a tentacle '' or “Case where porous body shape cannot be produced” was evaluated as x, and “Case where porous body shape could be produced and shape could be maintained by tentacles” was evaluated as acceptable.

B. Volume change rate The volume change rate was measured by casting 100 cm ^{3 of} an aqueous slurry of fine cellulose fibers on a 5 cm × 5 cm × 5 cm height mold of the bottom surface, freeze-drying, and then drying the sample. The volume was measured, and the volume change rate was calculated from the following formula.
Volume change rate (%) = (V0−V1) / V0 × 100
V0 = volume of water slurry before drying V1 = volume of sample after drying.

C. Average fiber diameter Using a scanning electron microscope (SEM), take a photo of the cross section of the microfibrous cellulose porous material (magnification: 5000 times), and randomly select 20 microfibers from the photo taken. The fiber diameter was measured. Next, an average value of 20 diameters was calculated and used as an average fiber diameter (nm).
SEM device: Hitachi S-4000 type

D. Apparent density The apparent density of the microfibrous cellulose porous material was calculated based on JIS K7222: 2005 (foamed plastic and rubber-how to determine the apparent density).

E. The flexible cellulose porous body was sensory-evaluated for the softness felt by ten subjects with tentacles. According to sensory evaluation, “very excellent flexibility” is ◎, “excellent flexibility” is ○, “slightly flexible” is △, and “not flexible” is ×. , "Good or better" ○ or better.

F. 25% compression set The 25% compression set compressed the porous material 25% and held it in the compressed state for 1 hour before releasing the compression. In this method, the thickness of the porous material before compression and the thickness of the porous material at the time of compression release were measured, and the compression set was calculated from the following formula.
Compression set (%) = (t0−t1) / t0 × 100
t0 = thickness of porous material before compression (mm)
t1 = Thickness (mm) of the porous material at the time of compression release

G. Breathability Breathability of the porous cellulose body was calculated based on JIS K6400-7: 2004 (soft foam material—how to obtain physical properties—Part 7: Breathability).

Example 1
As a cellulose fiber, 400 parts by mass of water is added to 100 parts by mass of “Cerish KY-100G” (water dispersion having a cellulose concentration of 10% by mass) manufactured by Daicel Chemical Industries, which is a fine cellulose fiber obtained by high-pressure homogenizer treatment of wood pulp. And stirred to prepare a water slurry of 2% by mass of microfibrous cellulose. The water slurry was cast on a mold having an inner size of 5 cm × 5 cm × height 5 cm on the bottom surface, frozen at −150 ° C., and freeze-dried under a condition of 0.1 Torr.

  Table 1 shows the characteristic results of the obtained samples. The volume change after lyophilization and the aqueous slurry of microfibrous cellulose was small, and a porous body could be produced by lyophilization. The obtained porous body was a rectangular parallelepiped shape well reflecting the shape of the mold. The average fiber diameter of the fine cellulose fibers constituting the obtained porous body was 55 nm, and there was little aggregation between the fibers. Further, the apparent density of the obtained porous body was 0.075 g / mL, and the flexibility, 25% compression set, and air permeability were extremely excellent.

Examples 2-6
The water slurry concentration of the fine cellulose fibers was 0.2% by mass in Example 2, 0.4% by mass in Example 3, 1.0% by mass in Example 4, and 2.7% by mass in Example 5. In Example 6, the water slurry of fine cellulose fibers was freeze-dried in the same manner as in Example 1 except that 3.3% by mass was used.

  Table 1 shows the characteristic results of the obtained samples. Each sample was able to produce a porous body by freeze-drying and had a rectangular parallelepiped shape that well reflected the shape of the mold. Moreover, it had the characteristic outstanding in all the porous bodies.

Example 7
About the water slurry of the 2 mass% micro cellulose fiber, the freeze slurry of the water slurry of the micro cellulose fiber was performed like Example 1 except having made the conditions at the time of freezing into -30 degreeC.
As shown in Table 1, even when the freezing temperature was changed to −30 ° C., a porous body could be produced by lyophilization, which was a rectangular parallelepiped shape well reflecting the shape of the mold. Further, the obtained porous body had extremely excellent characteristics.

Comparative Examples 1 and 2
The water slurry concentration of the microcellulose fibers was freeze-dried in the same manner as in Example 1 except that the concentration of the water slurry of the microcellulose fibers was 0.05% by mass in Comparative Example 1 and 10% by mass in Comparative Example 2.

  Table 2 shows the characteristic results of the obtained samples. All of the obtained samples had a porous body shape. However, the porous body obtained in Comparative Example 1 had a low apparent density, low physical properties of the porous body, and poor 25% compression set. On the other hand, since the porous body obtained in Comparative Example 2 had a high density, there were many entanglements between fibers, and aggregation of fibers was also observed. Therefore, the average fiber diameter of the fine cellulose fibers in the porous body was slightly large as 137 nm. The physical properties of the porous body were poor in flexibility, and the apparent density of the porous body was high, so that the air permeability was poor.

Comparative Example 3
Lyophilization was performed under the same conditions as in Example 1 except that dissolving pulp (manufactured by Nippon Paper Chemicals Co., Ltd.) was used as the cellulose fiber.

  Table 2 shows the characteristic results of the obtained samples. The average fiber diameter of the cellulose fibers was as large as 33700 nm, and the entanglement between the fibers was small. The sample after freeze-drying was cotton-like and flexible, but it had a shape that was broken apart by the tentacles and did not become one porous body.

Comparative Example 4
The water slurry of fine cellulose fibers was vacuum-dried at 20 ° C. without freezing the water slurry of 2% by mass of fine fibrous cellulose.

Table 2 shows the characteristic results of the obtained samples. Since the water slurry was vacuum dried without freezing, the volume change rate of the sample after drying was extremely large, and the sheet shape was not porous. The fine cellulose fibers constituting the sheet sample had many aggregations, and the average fiber diameter was as large as 1260 nm. Further, the obtained sheet sample had a hard texture and poor flexibility, and the air permeability was as low as 0.3 cc / cm ² · sec.

  Since the cellulose porous body of the present invention is composed of fine cellulose fibers, it has a high surface area and is excellent in flexibility, breathability, dimensional stability, and handleability. Moreover, since it is a porous and open-cell material, uniform carbonization and activation are possible, and it can be suitably used as an adsorbent, a heat insulating material, and a sound absorbing material.

Claims (3)

A porous porous body comprising fine cellulose fibers having an average fiber diameter of 2 to 1000 nm, and an apparent density of 0.005 to 0.15 g / cm ³ .   The porous cellulose body according to claim 1, wherein 25% compression set is 0.5 to 20%.   A method for producing a porous cellulose material according to claim 1 or 2, wherein a water slurry containing fine cellulose fibers having an average fiber diameter of 2 to 1000 nm in a concentration of 0.1 to 3.5% by mass is freeze-dried. A method for producing a porous cellulose material.