Directional heat conduction plant fiber composite material as well as preparation method and application thereof

Abstract

The invention provides a directional heat-conducting plant fiber composite material and a preparation method and application thereof.The preparation method comprises the steps that plant fibers are directionally arranged to have the consistent orientation, then a two-dimensional material with the good heat-conducting property and an adhesive are mixed to prepare heat-conducting fluid, the plant fibers are soaked in the fluid, the surfaces of the plant fibers are evenly coated with the fluid, and the heat-conducting fluid is formed; and placing into an air-dry state, and putting into a mold for hot pressing. According to the invention, a simple and convenient processing method is adopted, the surface of the plant fiber is coated with a material with high heat-conducting property, a green and sustainable biomass material is used as a substrate, and the heat-conducting property of the heat-conducting material and the directional heat-conducting property of the plant fiber can be effectively exerted, so that the heat conductivity is greatly improved, and meanwhile, industrial production is facilitated.

Description

Technical Field

[0001] The present invention belongs to the technical field of thermal conductive materials, and particularly relates to an oriented thermal conductive plant fiber composite material, a preparation method thereof, and an application thereof. Background Art

[0002] With the continuous improvement of the demand for efficient thermal management in fields such as electronic devices, automobiles, buildings, aerospace, etc., the research on thermal conductive composite materials has received increasing attention. Traditional polymer-based thermal conductive materials have certain cost and environmental problems. At the same time, as non-biomass materials, their thermal conductivities are not high. After using a large amount of thermal conductive filler materials, the improvement of their thermal conductivity is not significant. Biomass thermal conductive composite materials, as a new type of material, have the advantages of low cost, environmental protection, and renewable, and have become a strong candidate to replace traditional non-biomass thermal conductive materials.

[0003] Wood materials, as a renewable resource, have the characteristics of natural biodegradability, good mechanical properties, and low density. However, the thermal conductivity of wood itself is poor. In order to improve the thermal conductivity of wood materials, wood high-thermal-conductivity composite materials have emerged. This material is made by compounding plant fibers with materials having high thermal conductivity using an adhesive to achieve modification and performance improvement. At present, there are still some common problems in the existing thermal management materials in terms of improving thermal conductivity, reducing costs, and enhancing durability. Therefore, developing a low-cost, green and sustainable composite material based on plant fibers with a thermal conduction track and a thermal conduction path and having a metal-level high-oriented thermal conductivity and being suitable for industrialization is an important and extremely challenging task. Summary of the Invention

[0004] The present invention provides an oriented thermal conductive plant fiber composite material made of plant fibers and a preparation method thereof, aiming to solve the problem of poor thermal conductivity of traditional polymer-based thermal conductive materials. This composite material is made by compounding plant fibers with two-dimensional materials having good thermal conductivity using an adhesive. Under the thermal conduction track and thermal conduction path provided by the oriented fibers, the thermal conductivity of wood materials is significantly improved, while maintaining the excellent mechanical properties of the fibers, and having the advantages of environmental protection and sustainability, and is suitable for fields such as electronic heat dissipation and thermal management.

[0005] One object of the present invention is to provide an oriented thermal conductive plant fiber composite material, including plant fibers arranged in parallel and two-dimensional thermal conductive materials coated on the surfaces of the plant fibers.

[0006] In the above-mentioned oriented thermal conductive plant fiber composite material:

[0007] The plant fiber described above is selected from at least one of bamboo fiber (for example, bamboo fiber obtained by delignification treatment), hemp fiber (abaca, flax, hemp, jute, ramie, sisal, etc.), coconut shell fiber, and cotton fiber; the plant fiber can use existing fiber products or can be prepared by the preparation methods of the prior art. For example, bamboo fiber can be prepared by methods such as formic acid + hydrogen peroxide, acetic acid + hydrogen peroxide, sodium hydroxide, sodium sulfide + sodium chloride, etc. to remove lignin;

[0008] The length of the plant fiber is greater than 8 cm, preferably 8 - 15 cm;

[0009] The plant fiber described above is pre-treated with acid or alkali; after the surface of the plant fiber is treated, its surface roughness increases, increasing the adhesion degree of its surface to the heat-conducting filler;

[0010] The adhesive is selected from at least one of phenolic resin, PAE adhesive, urea-formaldehyde resin, and melamine;

[0011] The two-dimensional heat-conducting material is selected from at least one of graphite flakes, boron nitride, and Mxene. Preferably, the graphite flakes include graphite flakes with different particle sizes; more preferably, the graphite flakes include graphite flake A with a particle size range of 180 - 220 μm, graphite flake B with a particle size range of 20 - 30 μm, and graphite flake C with a particle size of 5 - 10 μm. Preferably, the mass ratio of graphite flakes A, B, and C is (1 - 6):(1 - 3):1, preferably (2 - 5):(1 - 2):1.

[0012] In the oriented heat-conducting plant fiber composite material, the mass percentage content of the two-dimensional heat-conducting material is 10 - 70 wt%, preferably 40 - 60 wt%; the mass percentage content of the adhesive is 5 - 20 wt%, preferably 5 - 15 wt%; the mass percentage content of the plant fiber is 10 - 70 wt%, preferably 25 - 55 wt%.

[0013] The second object of the present invention is to provide a preparation method of the above-mentioned oriented heat-conducting plant fiber composite material, including the step of coating the two-dimensional heat-conducting material on the surface of parallel-arranged plant fibers. Preferably, the plant fiber is pre-treated with acid or alkali.

[0014] The preparation method of the above-mentioned oriented heat-conducting plant fiber composite material specifically includes the following steps:

[0015] Step 1: Orient the plant fiber after being treated with acid or alkali to obtain parallel-arranged plant fibers;

[0016] Step 2: Add the adhesive and two-dimensional thermal conductive material into a solvent to obtain a thermal conductive fluid. Immerse the plant fibers arranged in parallel in Step 1 into the thermal conductive fluid for soaking, and then take them out and place them until they are air-dried.

[0017] Step 3: Hot-press the air-dried plant fibers in Step 2 to obtain the oriented thermal conductive plant fiber composite material.

[0018] In the above preparation method of the oriented thermal conductive plant fiber composite material:

[0019] The adhesive is selected from at least one of phenolic resin, PAE adhesive, urea-formaldehyde resin, and melamine;

[0020] The solvent is selected from at least one of ethanol, methanol, acetone, and deionized water.

[0021] In the above preparation method of the oriented thermal conductive plant fiber composite material, in Step 1:

[0022] The acid used for the acid treatment is selected from at least one of formic acid, acetic acid, hydrochloric acid, sulfuric acid, and oxalic acid. The concentration of the acid solution used for the acid treatment is not particularly limited and can be adjusted according to actual needs;

[0023] The base used for the base treatment is selected from at least one of sodium hydroxide, potassium hydroxide, and ammonia water. The concentration of the base solution used for the base treatment is not particularly limited and can be adjusted according to actual needs.

[0024] In the above preparation method of the oriented thermal conductive plant fiber composite material, in Step 2:

[0025] The mass ratio of the adhesive to the two-dimensional thermal conductive material is 1:(1 - 7), preferably 1:(4 - 6);

[0026] The dosage of the solvent in Step 2 is not particularly limited as long as it can fully disperse the two-dimensional thermal conductive material. For example, the mass ratio of the two-dimensional thermal conductive material to the solvent is 1:(1 - 10), preferably 1:(1 - 3);

[0027] The soaking time is 10 - 60 min, preferably 30 - 60 min; there is no special requirement for the soaking temperature, and it can be completed at room temperature.

[0028] In the above preparation method of the oriented thermal conductive plant fiber composite material, the hot-pressing operation in Step 3 can be completed on common hot-pressing equipment. The hot-pressing conditions are: temperature 120 - 150 °C, pressure 5 - 10 MPa, and time 10 - 30 min.

[0029] A third object of the present invention is to provide the above-mentioned directionally thermally conductive plant fiber composite material for use in heat dissipation materials or packaging materials. For example, it can be used in the electronics and semiconductor industries, electric vehicles (EVs) and battery systems, aerospace fields, automotive industries, wearable devices, high-power lasers and optical systems, thermoelectric energy and thermal management, medicine and bioengineering, LED lighting and displays, architecture and household appliances. Specifically, it can be applied to mobile phone cases, electrical appliance housings, electric vehicle battery pack housings, etc.

[0030] The present invention utilizes the structural characteristics of plant fibers and significantly improves the thermal conductivity of the composite material by compounding with two-dimensional thermally conductive materials. The thermal conductivity of plant fibers themselves is poor, but they have the characteristic of directional arrangement, and their heat conduction paths can be enhanced through reasonable arrangement methods and optimization processes. When plant fibers are directionally arranged, the heat conduction in the fiber growth direction is more efficient. Two-dimensional thermally conductive materials, such as graphite sheets, boron nitride, MXene, etc., have extremely high thermal conductivities. After combining these thermally conductive materials with plant fibers, a continuous thermal conduction network can be formed between the fibers, greatly improving the thermal conductivity of the composite material. The role of the adhesive in the composite material is to firmly bond the plant fibers and the two-dimensional thermally conductive materials to ensure the structural stability and durability of the material. Different types of adhesives (such as phenolic resin, urea-formaldehyde resin, PAE (polyamide-epoxy) adhesive, etc.) can provide different degrees of bonding strength and thermal stability. During the hot pressing process, the adhesive cures, enhancing the overall strength of the composite material. Through appropriate hot pressing conditions, the full combination of plant fibers and thermally conductive materials can be ensured, while ensuring the density and uniformity of the material. Description of the Drawings

[0031] Figure 1 Scanning electron microscope image of a single bamboo fiber without coated graphite sheets prepared in Example 1.

[0032] Figure 2 Scanning electron microscope image of a single fiber in the highly thermally conductive composite material of bamboo fiber and graphite sheet prepared in Example 1.

[0033] Figure 3 Optical microscope image of the bamboo fibers coated with graphite sheets and adhesives arranged directionally and parallelly in Example 1. Detailed Embodiments

[0034] The present invention will be specifically described below in conjunction with specific embodiments. It is necessary to point out here that the following embodiments are only for further illustration of the present invention and cannot be understood as limiting the protection scope of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the content of the present invention still fall within the protection scope of the present invention. The percentage "%" involved in the present invention, unless otherwise specified, refers to the mass percentage.

[0035] The testing instruments and testing conditions used in the examples are as follows:

[0036] Strength test

[0037] For the strength test, an Instron 3366 three-point bending test instrument was used. The composite material was hot-pressed into a long strip specimen with dimensions of length * width * height being 8 cm * 1 cm * 2 mm, placed on the bending device, the span was adjusted, and a load was applied to the specimen for the bending test until the specified bending degree was reached or fracture occurred.

[0038] Thermal diffusivity test

[0039] For the thermal diffusivity test, an LFA467 HyperFlash instrument was used. The thermally conductive composite material pressed into a film was cut into circular disc specimens with a diameter of 25.4 mm, and the thermal diffusivity in the direction parallel to the fibers and perpendicular to the fibers was measured at room temperature (25 °C).

[0040] The raw materials used in the examples and comparative examples, if not specifically limited, are all disclosed in the prior art. For example, they can be directly purchased or prepared according to the preparation methods disclosed in the prior art.

[0041] Example 1:

[0042] Step 1: Mix 1500 mL of formic acid and 300 mL of hydrogen peroxide, stir on a magnetic stirrer for half an hour, then place the bamboo tube in this mixed solution and heat it in a water bath at 100 °C for 12 hours. Repeat the above operations three times to obtain bamboo fibers. Wash them with anhydrous ethanol and dry them in the air. Cut the bamboo fibers into 8 cm and weigh 3 g, and then arrange them in the same direction.

[0043] Step 2: Prepare phenolic resin, graphite flakes, and bamboo fibers according to the contents of 10%, 60%, and 30% respectively. Dissolve 1 g of phenolic resin in 10 g of anhydrous ethanol, stir it on a magnetic stirrer until it becomes a transparent red solution, let it stand until the precipitation is complete, pour out the supernatant, and then weigh different sizes of graphite flakes (3.6 g, 1.2 g, 1.2 g of 200 μm, 25 μm, and 5 μm graphite flakes respectively) in a total of 6 g according to the ratio of 3:1:1. Pour the weighed graphite flakes into the above supernatant and stir for 30 minutes on a magnetic stirrer to make them mix evenly to obtain a thermally conductive fluid. Immerse the bamboo fibers in step 1 in this thermally conductive fluid for about 30 minutes, and place them in the air at room temperature for one day to evaporate most of the water.

[0044] Step 3: Place the above bamboo fibers in a press and hot-press them at 150 °C and 10 MPa for about 20 minutes to obtain a thermally conductive composite material of bamboo fibers and graphite flakes.

[0045] In this embodiment, the plant fiber is bamboo fiber, the adhesive is phenolic resin, and the thermally conductive two-dimensional material is graphite sheet. Using graphite sheets of different sizes helps improve the adhesion between the graphite sheet and the bamboo fiber, thereby enhancing its thermal conductivity. At the same time, the bamboo fibers are arranged in the same orientation to form continuous thermal conduction tracks, greatly improving the thermal conductivity of the composite material. By comparing the thermal conductivity coefficients of the composite materials made by directional arrangement and non-directional arrangement, it is found that for the composite material obtained by directional arrangement of the fibers, the thermal conductivity coefficient in the fiber growth direction is very superior, about twice that of the thermal conductivity coefficient in the vertical direction, and it also has good mechanical properties.

[0046] Example 2:

[0047] Step 1: Immerse sisal fibers in a 5% sodium hydroxide solution for 1 hour to roughen their surfaces, then wash them 3 times with absolute ethanol to obtain hemp fibers. Cut the hemp fibers into 8 cm and weigh 3 g, and then arrange them in the same orientation.

[0048] Step 2: Prepare PAE adhesive, MXene, and hemp fibers according to the contents of 10%, 60%, and 30% respectively. Dilute 1 g of PAE glue in 10 g of distilled water, stir it on a magnetic stirrer until it becomes a transparent light yellow solution, then prepare 6 g of MXene and pour it into the above clear solution, and stir it on a magnetic stirrer for 30 minutes to make it evenly mixed to obtain a thermally conductive fluid. Immerse the hemp fibers in Step 1 in this thermally conductive fluid for about 30 minutes, and place them in air at room temperature to dry for one day to evaporate most of the water.

[0049] Step 3: Place the above hemp fibers in a press and hot press them at 150 °C and 10 MPa for about 20 minutes to obtain a thermally conductive composite material of hemp fiber and MXene.

[0050] In this embodiment, the plant fiber is hemp fiber, the adhesive is PAE adhesive, and the thermally conductive two-dimensional material is MXene. The surface of the hemp fiber is treated to increase its surface roughness so that the surface energy can be better attached by the thermally conductive filler. By comparing the thermal conductivity coefficients of the hemp fiber composite materials with directional arrangement and non-directional arrangement, it is found that the thermal conductivity performance in the fiber growth direction is very superior, showing a metal-level thermal conductivity coefficient, and it also has good mechanical properties.

[0051] Example 3:

[0052] Step 1: Mix 1500 mL of formic acid and 300 mL of hydrogen peroxide, stir it on a magnetic stirrer for half an hour, then place the bamboo tube in this mixed solution and heat it in a water bath at 100 °C for 12 hours. Repeat the above operation three times to obtain bamboo fibers. Wash them with absolute ethanol and dry them. Cut the bamboo fibers into 8 cm and weigh 4 g, and then arrange them in the same direction.

[0053] Step 2: Prepare phenolic resin, graphite flakes, and bamboo fibers according to the contents of 10%, 50%, and 40% respectively. Take 1 g of phenolic resin and dissolve it in 10 g of absolute ethanol. Stir it under a magnetic stirrer until it becomes a transparent red solution. Let it stand still until the precipitation is complete, pour out the clear liquid, and then weigh 5 g of graphite flakes of different sizes (3 g, 1 g, and 1 g of 200 μm, 25 μm, and 5 μm graphite flakes respectively) according to the ratio of 3:1:1. Pour the weighed graphite flakes into the above-mentioned clear liquid and stir for 30 minutes under a magnetic stirrer to make them evenly mixed, obtaining a heat-conducting fluid. Immerse the bamboo fibers in step 1 in this heat-conducting fluid for about 30 minutes and place them in the air at room temperature to dry for one day to allow most of the moisture to evaporate.

[0054] Step 3: Place the above-mentioned bamboo fibers in a press and perform hot pressing at 150 °C and 10 MPa for about 20 minutes to obtain a heat-conducting composite material of bamboo fibers and graphite flakes.

[0055] In this embodiment, the plant fiber is bamboo fiber, the adhesive is phenolic resin, and the two-dimensional heat-conducting material is graphite flakes. Different from Example 1 is the ratio of the adhesive, heat-conducting material, and plant fiber, reducing the filling amount of the heat-conducting material, measuring its thermal conductivity and mechanical properties, and comparing with Example 1.

[0056] Example 4:

[0057] Step 1: Immerse sisal fiber in a 5% sodium hydroxide solution for 1 hour to make its surface rough, and then wash it 3 times with absolute ethanol to obtain sisal fiber. Cut the sisal fiber into 8 cm and weigh 4 g, and then arrange it in the same orientation.

[0058] Step 2: Prepare PAE adhesive, MXene, and sisal fiber according to the contents of 10%, 50%, and 40% respectively. Dilute 1 g of PAE glue in 10 g of distilled water and stir it under a magnetic stirrer until it becomes a transparent light yellow solution. Then prepare 5 g of MXene and pour it into the above-mentioned clear liquid, and stir for 30 minutes under a magnetic stirrer to make them evenly mixed, obtaining a heat-conducting fluid. Immerse the sisal fiber in step 1 in this heat-conducting fluid for about 30 minutes and place it in the air at room temperature to dry for one day to allow most of the moisture to evaporate.

[0059] Step 3: Place the above-mentioned sisal fiber in a press and perform hot pressing at 150 °C and 10 MPa for about 20 minutes to obtain a heat-conducting composite material of sisal fiber and MXene.

[0060] In this embodiment, the plant fiber is sisal fiber, the adhesive is PAE adhesive, and the two-dimensional heat-conducting material is MXene. Different from Example 2 is the ratio of the adhesive, heat-conducting material, and plant fiber, reducing the filling amount of the heat-conducting material, measuring its thermal conductivity and mechanical properties, and comparing with Example 2.

[0061] Example 5:

[0062] Step 1: Mix 1500 mL of formic acid and 300 mL of hydrogen peroxide, stir on a magnetic stirrer for half an hour, then place the bamboo tube in the mixed solution and heat it in a water bath at 100 °C for 12 hours. Repeat the above operation three times to obtain bamboo fibers. Wash them with absolute ethanol and dry them in the air. Cut the bamboo fibers into 8 cm and weigh 3 g, and then arrange them in the same direction.

[0063] Step 2: Prepare phenolic resin, graphite flakes, and bamboo fibers according to the contents of 10%, 60%, and 30% respectively. Dissolve 1 g of phenolic resin in 10 g of absolute ethanol, stir it on a magnetic stirrer until it becomes a transparent red solution, let it stand until the precipitation is complete, pour out the supernatant, and then weigh different sizes of graphite flakes (180 μm, 20 μm, and 10 μm graphite flakes are 2.4 g, 2.4 g, and 1.2 g respectively) in a total of 6 g according to the ratio of 2:2:1. Pour the weighed graphite flakes into the above supernatant and stir them on a magnetic stirrer for 30 minutes to make them evenly mixed to obtain a heat-conducting fluid. Immerse the bamboo fibers in Step 1 in the heat-conducting fluid for about 30 minutes and let it air-dry at room temperature for one day to evaporate most of the moisture.

[0064] Step 3: Place the above bamboo fibers in a press and perform hot pressing at 150 °C and 10 MPa for about 20 minutes to obtain a heat-conducting composite material of bamboo fibers and graphite flakes.

[0065] The plant fiber in this example is bamboo fiber, the adhesive is phenolic resin, and the two-dimensional heat-conducting material is graphite flakes. Using graphite flakes of different sizes helps to improve the adhesion degree between the graphite flakes and bamboo fibers, thereby enhancing its heat-conducting performance. At the same time, the bamboo fibers are also arranged in the same orientation to form continuous heat-conducting tracks, greatly improving the heat-conducting performance of the composite material. The difference from Example 1 is the use of graphite flakes of different sizes and different mass ratios of graphite flakes. Measure its thermal conductivity and mechanical properties and compare them with Example 1.

[0066] Comparative Example 1:

[0067] Step 1: Mix 1500 ml of formic acid and 300 ml of hydrogen peroxide, stir on a magnetic stirrer for half an hour, then place the bamboo tube in the mixed solution and heat it in a water bath at 100 °C for 12 hours. Repeat the above operation three times to obtain bamboo fibers. Wash them with absolute ethanol and dry them in the air. Cut the bamboo fibers into 8 cm and weigh 3 g, and arrange them randomly and intertwined.

[0068] Step 2: Prepare phenolic resin, graphite flakes, and bamboo fibers according to the contents of 10%, 60%, and 30% respectively. Take 1 g of phenolic resin and dissolve it in 10 g of absolute ethanol, stir it under a magnetic stirrer until it becomes a transparent red solution, let it stand still until the precipitation is complete, pour out the supernatant, and then weigh different sizes of graphite flakes (3.6 g, 1.2 g, and 1.2 g of 200 μm, 25 μm, and 5 μm graphite flakes respectively) in a ratio of 3:1:1 for a total of 6 g. Pour the weighed graphite flakes into the above supernatant and stir it under a magnetic stirrer for 30 minutes to make it evenly mixed, obtaining a heat-conducting fluid. Immerse the bamboo fibers in Step 1 in this heat-conducting fluid for about 30 minutes, and place it in air at room temperature to dry for one day to allow most of the water to evaporate.

[0069] Step 3: Place the above bamboo fibers in a press and perform hot pressing at 150 °C and 10 MPa for about 20 minutes to obtain a heat-conducting composite material of bamboo fibers and graphite flakes.

[0070] In this embodiment, the plant fiber is bamboo fiber, the adhesive is phenolic resin, and the two-dimensional heat-conducting material is graphite flakes. Using graphite flakes of different sizes helps to improve the adhesion degree between the graphite flakes and bamboo fibers, thereby enhancing its heat-conducting performance. However, the bamboo fibers are not arranged in the same orientation, but are randomly arranged and intertwined with each other. Measure its heat-conducting coefficient and mechanical properties and compare them with the bamboo fibers arranged in a directional manner.

[0071] Comparative Example 2:

[0072] Step 1: Immerse ramie fiber (sisal fiber) in a 5% sodium hydroxide solution for 1 hour to make its surface rough, and then wash it 3 times with absolute ethanol to obtain ramie fiber. Cut the ramie fiber into 8 cm and weigh 3 g, and arrange it randomly and intertwine it with each other.

[0073] Step 2: Prepare PAE adhesive, MXene, and ramie fiber according to the contents of 10%, 60%, and 30% respectively. Dilute 1 g of PAE glue in 10 g of distilled water, stir it under a magnetic stirrer until it becomes a transparent light yellow solution, then prepare 6 g of MXene and pour it into the above supernatant, and stir it under a magnetic stirrer for 30 minutes to make it evenly mixed, obtaining a heat-conducting fluid. Immerse the ramie fiber in Step 1 in this heat-conducting fluid for about 30 minutes, and place it in air at room temperature to dry for one day to allow most of the water to evaporate.

[0074] Step 3: Place the above ramie fiber in a press and perform hot pressing at 150 °C and 10 MPa for about 20 minutes to obtain a heat-conducting composite material of ramie fiber and MXene.

[0075] In this embodiment, the plant fiber is hemp fiber, the adhesive is PAE adhesive, and the thermally conductive two-dimensional material is MXene. The surface of the hemp fiber is treated to increase its surface roughness so that the surface energy can better adhere to the thermally conductive filler. However, the hemp fibers are not arranged in the same orientation, but are randomly intertwined with each other. Measure its thermal conductivity and mechanical properties and compare them with the hemp fibers arranged in an oriented manner.

[0076] Comparative Example 3:

[0077] Step 1: Prepare phenolic resin and graphite flakes according to a mass ratio of 1:6. Take 1 g of phenolic resin and dissolve it in 10 g of absolute ethanol. Stir it under a magnetic stirrer until it becomes a transparent red solution. Let it stand until the precipitation is complete, pour out the supernatant, and then weigh different sizes of graphite flakes (3.6 g, 1.2 g, and 1.2 g of 200 μm, 25 μm, and 5 μm graphite flakes respectively) in a ratio of 3:1:1 for a total of 6 g. Pour the weighed graphite flakes into the above supernatant and stir under a magnetic stirrer for 30 minutes to make them evenly mixed, obtaining a thermally conductive fluid. Place the thermally conductive fluid at room temperature to air-dry for one day to evaporate most of the water.

[0078] Step 2: Press the above thermally conductive fluid between polytetrafluoroethylene films and place it in a press for hot pressing at 150 °C and 10 MPa for about 20 minutes to obtain a pure graphite flake film.

[0079] The adhesive in this comparative example is phenolic resin, and the thermally conductive material is graphite flakes. By preparing a film containing only graphite flakes, measure its thermal conductivity and mechanical properties and compare them with the bamboo fiber composite material coated with graphite flakes.

[0080] Comparative Example 4:

[0081] Step 1: Prepare PAE adhesive and MXene according to a mass ratio of 1:6. Dilute 1 g of PAE glue in 10 g of distilled water. Stir it under a magnetic stirrer until it becomes a transparent light yellow solution. Then prepare 6 g of MXene and pour it into the above supernatant, and stir under a magnetic stirrer for 30 minutes to make them evenly mixed, obtaining a thermally conductive fluid. Place the thermally conductive fluid at room temperature to air-dry for one day to evaporate most of the water.

[0082] Step 2: Press the above thermally conductive fluid between polytetrafluoroethylene films and place it in a press for hot pressing at 150 °C and 10 MPa for about 20 minutes to obtain a pure MXene film.

[0083] The adhesive in this comparative example is PAE adhesive, and the thermally conductive material is MXene. By preparing a film containing only MXene, measure its thermal conductivity and mechanical properties and compare them with the hemp fiber composite material coated with MXene.

[0084] Comparative Example 5:

[0085] Step 1: Without surface treatment of sisal fibers, cut the sisal fibers into 8 cm and weigh 3 g, and then arrange them in the same orientation.

[0086] Step 2: Prepare PAE adhesive, MXene, and sisal fibers according to the contents of 10%, 60%, and 30% respectively. Dilute 1 g of PAE glue in 10 g of distilled water, stir it under a magnetic stirrer until it becomes a transparent light yellow solution, then prepare 6 g of MXene and pour it into the above clear solution, and stir it under a magnetic stirrer for 30 minutes to make it evenly mixed to obtain a thermal conductive fluid. Immerse the sisal fibers in Step 1 in this thermal conductive fluid for about 30 minutes, and place it in air at room temperature to dry for one day to evaporate most of the water.

[0087] Step 3: Place the above sisal fibers in a press and hot press them at 150 °C and 10 MPa for about 20 minutes to obtain a thermal conductive composite material of sisal fibers and MXene.

[0088] The plant fiber in this example is sisal fiber, the adhesive is PAE adhesive, and the two-dimensional thermal conductive material is MXene. Different from Example 2, the sisal fibers are not surface-treated, the surface is smooth, and the thermal conductive filler cannot adhere well. Measure its thermal conductivity and mechanical properties, and compare them with Example 2 with surface treatment.

[0089] Comparative Example 6:

[0090] Step 1: Mix 1500 mL of formic acid and 300 mL of hydrogen peroxide, stir it on a magnetic stirrer for half an hour, then place the bamboo tube in this mixed solution, and heat it in a water bath at 100 °C for 12 hours. Repeat the above operation three times to obtain bamboo fibers. Wash them with anhydrous ethanol and dry them. Cut the bamboo fibers into 8 cm and weigh 3 g, and then arrange them in the same direction.

[0091] Step 2: Arrange the bamboo fibers just delignified in the same orientation and place them in a press and hot press them at 150 °C and 10 MPa for about 20 minutes to obtain a pure bamboo fiber material.

[0092] This comparative example will hot press only the delignified bamboo fibers into samples, measure their thermal conductivity and mechanical properties, and compare them with Examples 1 and 2 with added thermal conductive materials and adhesives.

[0093] Test Example:

[0094] Thermal Conductivity and Strength Test

[0095] This test example conducted thermal conductivity and strength tests on the thermal conductive composite materials obtained in the above Examples 1-5 and Comparative Examples 1-6 (the thermal conductivity is divided into the direction parallel to the fiber and the direction perpendicular to the fiber). All tests were repeated three times and the average value was taken. The test results are shown in Table 1.

[0096] Table 1

[0097]

[0098] The experimental results show that after the bamboo fibers and hemp fibers are arranged directionally (Examples 1 and 2), their thermal conductivities can reach 151 W / mK and 141 W / mK, which are higher than those of the currently existing bio-based thermal conductive composite materials. This is mainly demonstrated in the direction parallel to the fibers, while the thermal conductivity in the direction perpendicular to the fibers (Comparative Examples 1 and 2) is about half of that in the parallel direction; the thermal conductivity of the samples containing only the thermal conductive material (Comparative Examples 3 and 4) is about 80 W / mK, indicating that the addition of the thermal conductive material is the main source of the thermal conductivity of the composite material, and the fibers can provide good thermal conduction paths, and the adhesive can improve the adhesion of the thermal conductive material. By comparing Example 2 with Comparative Example 5, it can be found that surface treatment of the hemp fibers is more conducive to the adhesion of the thermal conductive material, thereby improving the thermal conductivity of the composite material. From Comparative Example 6, it can be seen that the pure bamboo fiber material without the addition of the thermal conductive material and the adhesive has almost no thermal conductivity, but has excellent mechanical properties, indicating that the addition of the thermal conductive material will affect the mechanical properties. Therefore, the proportioning among the three is particularly important. From the strength, it can be seen that the strength of the composite material is inversely proportional to the filling amount of the thermal conductive material. The less the thermal conductive material is filled, the higher the strength, but the corresponding thermal conductivity will decrease. However, compared with the existing biomass-based thermal conductive composite materials, the composite material of the present invention still has a very high thermal conductivity and mechanical strength.

[0099] Although the present invention has been described in detail above with general descriptions and specific embodiments, based on the present invention, some modifications or improvements can be made, which are obvious to those skilled in the art. Therefore, these modifications or improvements made without departing from the spirit of the present invention all fall within the scope of protection required by the present invention.

Claims

1. A directional heat-conducting plant fiber composite material, comprising parallel arranged plant fibers, and a two-dimensional heat-conducting material and an adhesive coated on the surface of the plant fibers.

2. The directional heat-conducting plant fiber composite material according to claim 1, characterized in that: The plant fiber is selected from at least one of bamboo fiber, hemp fiber, coconut shell fiber and cotton fiber; and / or, The length of the plant fiber is greater than 8 cm, preferably 8 to 15 cm; and / or, The plant fiber is an acid-treated or alkali-treated plant fiber; and / or, The two-dimensional thermal conductive material is selected from at least one of graphite sheets, boron nitride, and Mxene. Preferably, the graphite sheets include graphite sheets of different particle sizes; and / or, The adhesive is selected from at least one of phenolic resin, PAE adhesive, urea-formaldehyde resin and melamine; and / or, In the directional heat-conducting plant fiber composite material, the mass percentage of the two-dimensional heat-conducting material is 10-70wt%, preferably 40-60wt%; the mass percentage of the adhesive is 5-20wt%, preferably 5-15wt%; the mass percentage of the plant fiber is 10-70wt%, preferably 25-55wt%.

3. The directional heat-conducting plant fiber composite material according to claim 2, characterized in that: The graphite flakes include graphite flakes A with a particle size range of 180 to 220 μm, graphite flakes B with a particle size range of 20 to 30 μm, and graphite flakes C with a particle size of 5 to 10 μm. Preferably, the mass ratio of graphite flakes A, B, and C is (1 to 6): (1 to 3): 1, preferably (2 to 5): (1 to 2):

1.

4. A method for preparing the directional heat-conducting plant fiber composite material according to any one of claims 1 to 3, comprising the step of coating the two-dimensional heat-conducting material on the surface of parallel arranged plant fibers, preferably, the plant fibers are pre-treated with acid or alkali.

5. The preparation method according to claim 4, characterized in that: The specific steps include: Step 1: aligning the plant fibers treated with acid or alkali to obtain parallel-arranged plant fibers; Step 2: adding an adhesive and a two-dimensional heat-conducting material into a solvent to obtain a heat-conducting fluid, immersing the plant fibers arranged in parallel in step 1 into the heat-conducting fluid, taking them out and placing them to an air-dried state; Step 3: hot-pressing the air-dried plant fibers in step 2 to obtain the directional heat-conducting plant fiber composite material.

6. The preparation method according to claim 5, characterized in that: The adhesive is selected from at least one of phenolic resin, PAE adhesive, urea-formaldehyde resin and melamine; and / or, The solvent is selected from at least one of ethanol, methanol, acetone and deionized water.

7. The preparation method according to claim 5, characterized in that: In the step 1: The acid used in the acid treatment is at least one selected from formic acid, acetic acid, hydrochloric acid, sulfuric acid, and oxalic acid; and / or, The alkali used in the alkali treatment is selected from at least one of sodium hydroxide, potassium hydroxide and ammonia water.

8. The preparation method according to claim 5, characterized in that: In the step 2: The mass ratio of the adhesive to the two-dimensional thermal conductive material is 1:(1-7), preferably 1:(4-6); and / or, The mass ratio of the two-dimensional thermal conductive material to the solvent is 1:(1-10), preferably 1:(1-3); and / or, The soaking time is 10 to 60 minutes, preferably 30 to 60 minutes.

9. The preparation method according to claim 5, characterized in that: In the step three: The conditions for hot pressing are: temperature 120-150°C, pressure 5-10 MPa, and time 10-30 min.

10. Use of the directional heat-conducting plant fiber composite material according to any one of claims 1 to 3 or the directional heat-conducting plant fiber composite material obtained by the preparation method according to any one of claims 4 to 9 in heat dissipation materials or packaging materials.

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