A green preparation method and application of wood fiber-based aerogel

Abstract

The present invention discloses a green preparation method and application of wood fiber-based aerogel, which uses nano-scale wood fiber as raw material, adopts cold alkali pretreatment, washes with water and then dries at normal pressure to obtain wood fiber-based aerogel. The volume shrinkage, density, porosity and thermal conductivity of the wood fiber-based aerogel prepared by the present invention are 26.68-52.93%, 0.039-0.071 g / cm 3 , 95.73-97.57% and 44.76-56.33mW / (m·K). Compared with freeze drying and supercritical drying methods for preparing wood fiber-based aerogels, the present invention prepares wood fiber aerogels under normal pressure conditions, without the need for extreme conditions of low temperature vacuum, high temperature and high pressure and special equipment. The preparation method is simple, green and energy-saving. The obtained aerogel has low volume shrinkage, low density, high porosity and low thermal conductivity, showing great application prospects in the field of thermal insulation.

Description

Technical Field

[0001] The invention relates to the technical field of preparation of wood fiber-based aerogel materials, and in particular to a green preparation method and application of wood fiber-based aerogel. Background Art

[0002] Aerogel is a porous material with a three-dimensional network structure. Due to its inherent complex porous structure, it can reduce the heat flow rate and has great application prospects in the field of thermal insulation. At present, polystyrene foam (EPSF) and polyurethane foam (PUF) are important commercial insulation materials and have been widely used in electrical, automotive and building insulation. However, this type of fossil raw material-based foam material is not easy to degrade and will cause pollution to the environment after use. In order to achieve the lowest carbon footprint, aerogels prepared from renewable and degradable wood fibers have become a hot topic of research.

[0003] The preparation of wood fiber-based aerogels is generally divided into three steps: fiber dispersion, gel formation and drying. Among them, drying technology plays a key role in aerogel formation. At present, freeze drying or supercritical drying is mostly used to dry wood fiber-based aerogels. The freeze drying method is to solidify the solvent in the gel at low temperature, and then sublimate the solvent from solid to gas under vacuum conditions. When the solvent is discharged, the pore structure is maintained to form a porous aerogel. The freeze drying method requires special equipment and low temperature and low pressure conditions, which consumes a lot of energy and limits the product size and production efficiency. The supercritical drying method controls the pressure and temperature so that the solvent reaches its own critical point during the drying process and completes the transition from liquid phase to supercritical fluid. The supercritical drying method requires high temperature and high pressure conditions, has high equipment requirements, is dangerous, and is not suitable for large-scale production. Based on this, the atmospheric pressure drying method has attracted widespread attention due to its energy-saving simplicity, low cost and short drying time. However, during the atmospheric pressure drying process, the capillary action caused by the evaporation of the solvent will cause the aerogel pore structure to shrink or even collapse, seriously affecting the structure and performance of the aerogel. At present, researchers mostly use chemical crosslinking or organic solvent replacement methods to reduce the capillary pressure during atmospheric pressure drying. However, these methods have problems such as chemical consumption, cumbersome solvent replacement process, large environmental pollution and excessive shrinkage. Therefore, the field urgently needs a green preparation method for wood fiber-based aerogels that is energy-saving, economical, simple in process and can be mass-produced. Summary of the invention

[0004] In order to overcome the shortcomings of the above-mentioned prior art, the present invention provides a green preparation method and application of wood fiber-based aerogel. The volume shrinkage, density, porosity and thermal conductivity of the prepared wood fiber-based aerogel are 26.68-52.93%, 0.039-0.071 g / cm 3, 95.73-97.57% and 44.76-56.33mW / (m·K). Compared with freeze drying and supercritical drying methods for preparing wood fiber-based aerogels, the present invention prepares wood fiber aerogels under normal pressure conditions, without the need for extreme conditions of low temperature vacuum, high temperature and high pressure and special equipment. The preparation method is simple, green and energy-saving. The obtained aerogel has low volume shrinkage, low density, high porosity and low thermal conductivity, showing great application prospects in the field of thermal insulation.

[0005] To achieve the above object, the present invention is implemented through the following technical solutions:

[0006] The green preparation method of wood fiber-based aerogel of the present invention uses nano-scale wood fiber as raw material, mixes nano-scale wood fiber suspension with sodium hydroxide solution, freezes, washes with water and dries under normal pressure to prepare the aerogel.

[0007] The water washing is to thaw the frozen wood fiber suspension and sodium hydroxide solution mixture at room temperature and then wash with distilled water until it becomes neutral.

[0008] The freezing conditions are a temperature of -5 to -40°C and a freezing time of 5 to 12 hours.

[0009] The drying conditions are a temperature of 25 to 120° C. and a drying time of 6 to 24 hours.

[0010] The fiber diameter of the nano-scale wood fiber is 50-150 nm.

[0011] The concentration of the nano-scale wood fiber suspension is 2-5 wt %.

[0012] The added amount of the sodium hydroxide is 12.5-75.0 wt% of the absolute dry weight of the wood fiber.

[0013] The wood fiber source is at least one of eucalyptus, poplar, bamboo, bagasse, corn stalks, rice straw and sisal.

[0014] The green preparation method of the wood fiber-based aerogel is used to prepare the wood fiber-based aerogel for use in thermal insulation.

[0015] Compared with the prior art, the present invention has the following beneficial effects:

[0016] (1) The present invention uses wood fiber as raw material, which is widely available, renewable and biodegradable.

[0017] (2) The preparation method of the present invention is simple, uses distilled water as a solvent for replacement and is dried under normal pressure, is green and environmentally friendly, has low production cost, and can be mass-produced.

[0018] (3) The wood fiber-based aerogel prepared by the present invention has low volume shrinkage (26.68-52.93%), low density (0.039-0.071 g / cm 3 ), high porosity (95.73-97.57%) and low thermal conductivity (44.76-56.33 mW / (m·K)), showing great application prospects in the field of thermal insulation. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a TEM image of the wood fiber prepared in Example 1 provided by the present invention.

[0020] Figure 2 This is a physical picture of the sample of Example 1 provided by the present invention.

[0021] Figure 3 This is a SEM image of the sample of Example 1 provided by the present invention. DETAILED DESCRIPTION

[0022] The present invention is further described below in conjunction with the examples, but it should not be construed as limiting the present invention. Without departing from the spirit and substance of the present invention, simple modifications or replacements made to the methods, steps or conditions of the present invention are within the scope of the present invention. Unless otherwise specified, the technical means used in the examples are conventional means well known to those skilled in the art.

[0023] Unless otherwise specified, the reagents, methods and equipment used in the present invention are conventional reagents, methods and equipment in the art. Unless otherwise specified, the reagents and materials used in the following examples are commercially available.

[0024] Example 1

[0025] A method for preparing wood fiber-based aerogel, the specific steps are as follows:

[0026] S1. Prepare a eucalyptus fiber suspension with a mass fraction of 2wt% and a fiber diameter of 50 to 150nm;

[0027] S2. Add sodium hydroxide to the wood fiber suspension obtained in step S1 in an amount of 12.5 wt% of the wood fiber absolute dry weight, and stir at room temperature for 30 min;

[0028] S3. The mixed solution obtained in step S2 was frozen in a refrigerator at -5°C for 12h;

[0029] S4. Thaw the sample frozen in step S3 at room temperature and wash it with distilled water until it becomes neutral;

[0030] S5. Dry the sample washed with water in step S4 at 25° C. (room temperature) for 24 h to obtain the wood fiber-based aerogel.

[0031] The cellulose and hemicellulose contents of the wood fiber-based aerogel prepared in this example were measured using the T203 OS-74 TAPPI standard, and the lignin content was measured using the T222 OS-83 TAPPI standard. The cellulose, hemicellulose and lignin contents were 70.85, 9.05 and 11.33%, respectively.

[0032] The crystallinity of the wood fiber-based aerogel prepared in this example was measured by X-ray diffraction (XRD), and the crystallinity was 70.72%.

[0033] The volume shrinkage of the wood fiber-based aerogel prepared in this example was measured at room temperature and pressure, and the volume shrinkage was 26.68%.

[0034] The density of the wood fiber-based aerogel prepared in this example was measured by weighing method at room temperature and pressure. The density at room temperature and pressure was 0.039 g / cm 3 .

[0035] The porosity of the wood fiber-based aerogel prepared in this example was measured by mass density method at room temperature and pressure. The porosity was 97.57% at room temperature and pressure.

[0036] The pore size of the wood fiber-based aerogel prepared in this example was measured by a fully automatic mercury intrusion instrument, and the pore size was 108.25 μm.

[0037] The thermal conductivity of the wood fiber-based aerogel prepared in this example was measured by a thermal conductivity meter, and the thermal conductivity was 56.33 mW / (m·K).

[0038] Example 2

[0039] A method for preparing wood fiber-based aerogel, the specific steps are as follows:

[0040] S1. Prepare a bagasse fiber suspension having a mass fraction of 3 wt% and a fiber diameter of 50 to 150 nm;

[0041] S2. Add sodium hydroxide to the wood fiber suspension obtained in step S1 in an amount of 25 wt% of the wood fiber absolute dry weight, and stir at room temperature for 45 min;

[0042] S3. The mixed solution obtained in step S2 was frozen in a refrigerator at -20°C for 10h;

[0043] S4. Thaw the sample frozen in step S3 at room temperature and wash it with distilled water until it becomes neutral;

[0044] S5. Dry the sample washed with water in step S4 at 60° C. for 18 h to obtain the wood fiber-based aerogel.

[0045] The cellulose and hemicellulose contents of the wood fiber-based aerogel prepared in this example were measured using the T203 OS-74 TAPPI standard, and the lignin content was measured using the T222 OS-83 TAPPI standard. The cellulose, hemicellulose and lignin contents were 75.35, 5.36 and 11.04%, respectively.

[0046] The crystallinity of the wood fiber-based aerogel prepared in this example was measured by X-ray diffraction (XRD), and the crystallinity was 66.51%.

[0047] The volume shrinkage of the wood fiber-based aerogel prepared in this example was measured at room temperature and pressure, and the volume shrinkage was 31.04%.

[0048] The density of the wood fiber-based aerogel prepared in this example was measured by weighing method at room temperature and pressure. The density at room temperature and pressure was 0.041 g / cm 3 .

[0049] The porosity of the wood fiber-based aerogel prepared in this example was measured by mass density method at room temperature and pressure. The porosity was 97.28% at room temperature and pressure.

[0050] The pore size of the wood fiber-based aerogel prepared in this example was measured by a fully automatic mercury intrusion porosimeter, and the pore size was 92.52 μm.

[0051] The thermal conductivity of the wood fiber-based aerogel prepared in this example was measured by a thermal conductivity meter, and the thermal conductivity was 53.12 mW / (m·K).

[0052] Example 3

[0053] A method for preparing wood fiber-based aerogel, the specific steps are as follows:

[0054] S1. Prepare a corn straw fiber suspension with a mass fraction of 4wt% and a fiber diameter of 50 to 150nm;

[0055] S2. Add sodium hydroxide to the wood fiber suspension obtained in step S1 in an amount of 50 wt% of the wood fiber absolute dry weight, and stir at room temperature for 45 min;

[0056] S3. The mixed solution obtained in step S2 was frozen in a refrigerator at -30°C for 8h;

[0057] S4. Thaw the sample frozen in step S3 at room temperature and wash it with distilled water until it becomes neutral;

[0058] S5. Dry the sample washed with water in step S4 at 80° C. for 12 h to obtain the wood fiber-based aerogel.

[0059] The cellulose and hemicellulose contents of the wood fiber-based aerogel prepared in this example were measured using the T203 OS-74 TAPPI standard, and the lignin content was measured using the T222 OS-83 TAPPI standard. The cellulose, hemicellulose and lignin contents were 76.87, 3.11 and 10.75%, respectively.

[0060] The crystallinity of the wood fiber-based aerogel prepared in this example was measured by X-ray diffraction (XRD), and the crystallinity was 49.75%.

[0061] The volume shrinkage of the wood fiber-based aerogel prepared in this example was measured at room temperature and pressure, and the volume shrinkage was 42.72%.

[0062] The density of the wood fiber-based aerogel prepared in this example was measured by weighing method at room temperature and pressure. The density at room temperature and pressure was 0.056 g / cm 3 .

[0063] The porosity of the wood fiber-based aerogel prepared in this example was measured by mass density method at room temperature and pressure. The porosity was 96.65% at room temperature and pressure.

[0064] The pore size of the wood fiber-based aerogel prepared in this example was measured by a fully automatic mercury intrusion porosimeter, and the pore size was 89.51 μm.

[0065] The thermal conductivity of the wood fiber-based aerogel prepared in this example was measured by a thermal conductivity meter, and the thermal conductivity was 47.43 mW / (m·K).

[0066] Example 4

[0067] A method for preparing wood fiber-based aerogel, the specific steps are as follows:

[0068] S1. Prepare a sisal fiber suspension having a mass fraction of 5 wt% and a fiber diameter of 50 to 150 nm;

[0069] S2. Add sodium hydroxide to the wood fiber suspension obtained in step S1 in an amount of 75 wt% of the wood fiber absolute dry weight, and stir at room temperature for 60 min;

[0070] S3. The mixed solution obtained in step S2 was frozen in a refrigerator at -40°C for 5h;

[0071] S4. Thaw the sample frozen in step S3 at room temperature and wash it with distilled water until it becomes neutral;

[0072] S5. Dry the sample washed with water in step S4 at 120° C. for 6 h to obtain the wood fiber-based aerogel.

[0073] The cellulose and hemicellulose contents of the wood fiber-based aerogel prepared in this example were measured using the T203 OS-74 TAPPI standard, and the lignin content was measured using the T222 OS-83 TAPPI standard. The cellulose, hemicellulose and lignin contents were 78.54, 2.86 and 10.13%, respectively.

[0074] The crystallinity of the wood fiber-based aerogel prepared in this example was measured by X-ray diffraction (XRD), and the crystallinity was 46.15%.

[0075] The volume shrinkage of the wood fiber-based aerogel prepared in this example was measured at room temperature and pressure, and the volume shrinkage at room temperature and pressure was 52.93%.

[0076] The density of the wood fiber-based aerogel prepared in this example was measured by weighing method at room temperature and pressure. The density at room temperature and pressure was 0.071 g / cm 3 .

[0077] The porosity of the wood fiber-based aerogel prepared in this example was measured by mass density method at room temperature and pressure. The porosity was 95.73% at room temperature and pressure.

[0078] The pore size of the wood fiber-based aerogel prepared in this example was measured by a fully automatic mercury intrusion instrument, and the pore size was 58.84 μm.

[0079] The thermal conductivity of the wood fiber-based aerogel prepared in this example was measured by a thermal conductivity meter, and the thermal conductivity was 44.76 mW / (m·K).

[0080] Comparative Example 1

[0081] Different from Example 1, step S2 is not performed in this comparative example, and the conditions of other steps are the same as those in Example 1.

[0082] The cellulose and hemicellulose contents of the wood fiber-based aerogel prepared in this example were measured using the T203 OS-74 TAPPI standard, and the lignin content was measured using the T222 OS-83 TAPPI standard. The cellulose, hemicellulose and lignin contents were 70.60, 13.15 and 13.77%, respectively.

[0083] The crystallinity of the wood fiber-based aerogel prepared in this example was measured by X-ray diffraction (XRD), and the crystallinity was 73.50%.

[0084] The volume shrinkage of the wood fiber-based aerogel prepared in this comparative example was measured at room temperature and pressure, and the volume shrinkage at room temperature and pressure was 90.23%.

[0085] The density of the wood fiber-based aerogel prepared in this comparative example was measured by weighing method at room temperature and pressure. The density at room temperature and pressure was 0.0917 g / cm 3 .

[0086] The porosity of the wood fiber-based aerogel prepared in this comparative example was measured by mass density method at room temperature and pressure, and the porosity was 30.26% at room temperature and pressure.

[0087] The pore size of the wood fiber-based aerogel prepared in this comparative example was measured by a fully automatic mercury intrusion instrument, and no obvious void structure was found.

[0088] The thermal conductivity of the wood fiber-based aerogel prepared in this comparative example was measured by a thermal conductivity meter, and the thermal conductivity was 80.56 mW / (m·K).

[0089] The following table is a comparison table of cellulose content, hemicellulose content, lignin content, crystallinity, volume shrinkage, density, porosity, pore size and thermal conductivity of the wood fiber-based aerogels prepared in Examples 1 to 4 and Comparative Example 1.

[0090]

[0091] After mechanical grinding, wood fiber pulp exposes more hydroxyl groups, which is conducive to the formation of hydrogen bonds between wood fiber molecules. Under low-temperature sodium hydroxide treatment, hemicellulose and lignin in wood fiber are partially removed, and the amorphous region and partial crystalline region of cellulose are destroyed, exposing more hydroxyl groups. In addition, cellulose fibers swell, increasing fiber entanglement and aggregation, thereby promoting hydrogen bond formation and gelation. During the drying process, since lignin in wood fiber has adhesion, it can reduce the capillary force during the drying process, so that the aerogel maintains a good structure during normal pressure drying.

[0092] As the amount of NaOH added increases, the removal of lignin and hemicellulose from wood fibers increases, and the crystallinity decreases, because the crystalline structure of cellulose partially transforms to type II. At higher NaOH concentrations, fiber entanglement accelerates, accompanied by a sharp contraction and agglomeration of adjacent fibers, so the aerogel structure becomes denser, increasing its volume shrinkage and density, and reducing its porosity and pore size.

[0093] Generally speaking, the total heat transfer (λt) of aerogel includes three parts: solid phase heat transfer (λs), gas phase heat transfer (λg) and pore radiation heat transfer (λr). Heat transfer through λs and λr mainly depends on the density of aerogel. Solid phase heat transfer (λs) increases with increasing density, and pore radiation heat transfer (λr) decreases with increasing density. Gas phase heat transfer (λg) is independent of density and can be reduced by reducing the pore size. With the increase of NaOH addition, the density of aerogel increases, the pore size decreases, and the complexity of aerogel network structure increases. Its solid phase heat transfer (λs) increases, and pore radiation heat transfer (λr) and gas phase heat transfer (λg) decrease, so the thermal conductivity of aerogel decreases.

[0094] Technical principle of the present invention: The present invention uses wood fiber (eucalyptus, poplar, bamboo, bagasse, corn stalks, straw, sisal) as raw materials, and obtains nano-scale wood fiber after mechanical grinding. The fiber exposes more hydroxyl groups and is small in size, which is conducive to the formation of gelation. Under low-temperature sodium hydroxide treatment, hemicellulose and lignin in the wood fiber are partially removed, so that more cellulose fibers are exposed. Here, the NaOH solution fully contacts the cellulose fiber and destroys the amorphous region and partial crystalline region of cellulose, exposing more hydroxyl groups, which is conducive to the formation of intermolecular hydrogen bonds. In addition, under cold alkali treatment, the cellulose fiber swells, and the cellulose molecules are closer to each other, which increases the entanglement and aggregation of the fibers, thereby promoting the formation of hydrogen bonds and gelation. At the same time, because the lignin in the wood fiber has adhesion, it hinders the formation of hydrogen bonds between cellulose and hemicellulose on the one hand, so that the pores of the fiber network structure increase; on the other hand, lignin is hydrophobic and has low surface tension. According to the Young-Laplace equation, the two effects of lignin reduce the capillary force during the drying process, which is beneficial to reduce the volume shrinkage of the aerogel, thereby maintaining its good structure during the normal pressure drying process.

Claims

1. A green preparation method for wood fiber-based aerogel, It is characterized in that The nano-scale wood fiber suspension is mixed with a sodium hydroxide solution, frozen, washed with water and dried under normal pressure; The freezing conditions are a temperature of -5 to -40°C and a freezing time of 5 to 12 hours; The amount of sodium hydroxide added is 12.5 to 75 wt% of the absolute dry weight of the wood fiber; The water washing is to thaw the frozen wood fiber suspension and sodium hydroxide solution mixture at room temperature and then wash with distilled water until it becomes neutral.

2. A green preparation method of wood fiber-based aerogel according to claim 1, It is characterized in that The drying conditions are a temperature of 25 to 120° C. and a drying time of 6 to 24 hours.

3. A green preparation method of wood fiber-based aerogel according to claim 1, It is characterized in that The fiber diameter of the nano-scale wood fiber is 50-150 nm.

4. A green preparation method of wood fiber-based aerogel according to claim 1, It is characterized in that The concentration of the nano-scale wood fiber suspension is 2-5 wt %.

5. A green preparation method of wood fiber-based aerogel according to claim 1, It is characterized in that The wood fiber source is at least one of eucalyptus, poplar, bamboo, bagasse, corn stalks, rice straw and sisal.

6. Application of the wood fiber-based aerogel prepared by the green preparation method of any one of claims 1 to 5 in thermal insulation.

Images