Wood-plastic composite material with electromagnetic shielding performance and high strength and preparation method

Abstract

The invention discloses a wood-plastic composite material with electromagnetic shielding performance and high strength and a preparation method thereof, and belongs to the technical field of electromagnetic shielding materials, and the preparation method comprises the following steps: processing and drying raw materials; modifying the salix psammophila fibers; step-by-step mixing and paving assembly are conducted; the electromagnetic functional additive compound system is a mixture of multi-walled carbon nanotubes and expandable graphite, a mixture of nano conductive carbon black and expandable graphite, and a mixture of multi-walled carbon nanotubes and nano conductive carbon black; and hot press molding: carrying out hot press molding on the obtained plate blank to obtain the product. Maleic anhydride is adopted as an interfacial activator of salix psammophila fibers and a polylactic acid matrix, so that the interfacial compatibility of the composite material is improved, the mechanical strength is improved, and meanwhile, an electromagnetic functional auxiliary agent compounding system is constructed to provide excellent electromagnetic shielding effectiveness for the composite material. Test results show that the surface resistivity, the electromagnetic shielding effectiveness and the elastic modulus of the composite material all meet corresponding national / industrial standard requirements.

Description

Technical Field

[0001] The present application relates to the technical field of electromagnetic shielding materials, and specifically, to a wood-plastic composite material with both electromagnetic shielding performance and high strength and a preparation method thereof. Background Art

[0002] In recent years, with people's increasing attention to the ecological environment and the continuous in-depth research in the field of materials, natural plant fiber / bio-degradable plastic composite materials have gradually received extensive attention due to their excellent properties such as low cost, environmental friendliness, good mechanical properties, strong plasticity, and good chemical stability. They show good application prospects in the fields of building materials, gardening, vehicle and ship manufacturing, home decoration, etc.

[0003] However, since the main components of wood fibers, such as cellulose, hemicellulose, and lignin, contain a large number of hydroxyl groups, making it have strong chemical polarity, while most bio-degradable plastics are non-polar high molecular compounds, resulting in poor interfacial compatibility of the composite material, which greatly affects the comprehensive performance and application of the composite material. Therefore, improving the interfacial compatibility between wood fibers and bio-degradable plastics has always been a research problem.

[0004] In addition, electromagnetic interference generated by various electronic devices nowadays has seriously affected people's normal life. Long-term exposure to high-intensity electromagnetic radiation may have an impact on human health, such as headache, insomnia, memory loss, etc. And electromagnetic interference will affect the normal operation of electronic devices, resulting in a decline in device performance, misoperation, or even damage. This may include the normal operation of precision instruments such as computers, radios, and medical devices.

[0005] Therefore, how to develop a wood-plastic composite material with both electromagnetic shielding performance and high strength and a preparation method thereof is a technical problem that needs to be urgently solved by those skilled in the art. Summary of the Invention

[0006] In view of this, the present invention provides a wood-plastic composite material with both electromagnetic shielding performance and high strength and a preparation method thereof.

[0007] A preparation method of a wood-plastic composite material with both electromagnetic shielding performance and high strength includes the following steps:

[0008] (1) Processing and drying of raw materials: After peeling the Salix psammophila wood strips, they are crushed, screened, and dried to obtain Salix psammophila fibers;

[0009] (2) Modification of Salix psammophila fibers: The coupling agent is dissolved in an ethanol aqueous solution with a volume fraction of 95% to obtain a coupling agent solution, and the coupling agent solution is evenly sprayed on the Salix psammophila fibers obtained in step (1) and dried to obtain modified Salix psammophila fibers;

[0010] (3) Step-by-step mixing and paving for blank forming: Divide the compound system of modified Salix psammophila fiber, polylactic acid, and electromagnetic functional additive obtained in step (2) into the first and second parts of materials according to mass. Stir and mix the compound system of modified Salix psammophila fiber, polylactic acid, and electromagnetic functional additive in the first part, then continue to add the compound system of modified Salix psammophila fiber, polylactic acid, and electromagnetic functional additive in the remaining second part and stir and mix. Place the mixed material in a mold for paving, press the material, and let it stand for a period of time. After the material form is fixed, a board blank is obtained;

[0011] The above electromagnetic functional additive compound system is a mixture of multi-walled carbon nanotubes and expandable graphite, a mixture of nano-conductive carbon black and expandable graphite, or a mixture of multi-walled carbon nanotubes and nano-conductive carbon black;

[0012] (4) Hot pressing and forming: Hot press and form the board blank obtained in step (3) to obtain the above wood-plastic composite material with both electromagnetic shielding performance and high strength.

[0013] Further, in step (1), sieve and select Salix psammophila wood powder with a particle size of 40 - 60 mesh, and the drying temperature is 70 ± 2 °C. Dry it until the moisture content of the Salix psammophila fiber is below 2%.

[0014] Beneficial effects of adopting the above further technical solution: Research shows that Salix psammophila wood powder with a particle size of 40 - 60 mesh can form a good intertwined state inside the composite material, which is conducive to the uniform distribution of the matrix and the effective transfer of stress, further improving the mechanical properties. The drying temperature of 70 ± 2 °C can ensure the efficiency of achieving the drying effect of the modified wood powder, and at the same time, the wood powder will not become brittle due to too high a temperature. Drying to a moisture content below 2% not only ensures that the wood powder has a certain plasticity, which is convenient for hot pressing and forming; at the same time, it will not produce too much water vapor during hot pressing due to too high a moisture content, affecting the product quality.

[0015] Further, in step (2), the above coupling agent is maleic anhydride.

[0016] Beneficial effects of adopting the above further technical solution: Maleic anhydride coupling agent realizes the interface strengthening between Salix psammophila and the polylactic acid matrix in the form of chemical bonding through esterification reaction and hydrogen bond action, which is beneficial to the improvement of mechanical strength.

[0017] Further, in step (2), the concentration of the above coupling agent solution is 20 wt%, and the dosage of the coupling agent is 2.5% of the mass of the Salix psammophila fiber.

[0018] Beneficial effects of adopting the above further technical solution: Adding the coupling agent in the form of a solution is conducive to the dispersion of the coupling agent between the matrices, achieving a better interface modification effect; research shows that when the addition amount of the coupling agent is 2.5%, the interface modification effect is the best.

[0019] Further, in step (2), the drying temperature is 70±2°C, and the modified Salix psammophila fiber is dried until the moisture content is below 2%.

[0020] Beneficial effects of adopting the above further technical solution: A drying temperature of 70±2°C can ensure the efficiency of achieving the drying effect of the modified wood powder, and at the same time, the wood powder will not become brittle due to excessive temperature. Drying to a moisture content below 2% not only ensures that the wood powder has a certain plasticity, facilitating hot pressing molding, but also will not generate excessive water vapor during hot pressing due to excessive moisture content, which affects the product quality.

[0021] Further, in step (3), the mass ratio of the above-mentioned modified Salix psammophila fiber to polylactic acid is 3:7, the addition amount of the electromagnetic functional additive compound system is 3-12% of the mass of the modified Salix psammophila fiber, the mass ratio of multi-walled carbon nanotubes to expandable graphite in the mixture of multi-walled carbon nanotubes and expandable graphite is 1:1, the mass ratio of nano-conductive carbon black to expandable graphite in the mixture of nano-conductive carbon black and expandable graphite is 1:1, and the mass ratio of multi-walled carbon nanotubes to nano-conductive carbon black in the mixture of multi-walled carbon nanotubes and nano-conductive carbon black is 1:1.

[0022] Beneficial effects of adopting the above further technical solution: Research shows that a mass ratio of 3:7 of Salix psammophila fiber to polylactic acid can ensure that the composite material has good mechanical toughness and fully exerts the reinforcing effect of Salix psammophila fiber. The addition amount of the electromagnetic functional additive compound system is 3-12% of the mass of the modified Salix psammophila fiber, which can ensure that the composite material has good electromagnetic properties. Under the condition that the mass ratio of multi-walled carbon nanotubes to expandable graphite in the mixture of multi-walled carbon nanotubes and expandable graphite is 1:1, the mass ratio of nano-conductive carbon black to expandable graphite in the mixture of nano-conductive carbon black and expandable graphite is 1:1, and the mass ratio of multi-walled carbon nanotubes to nano-conductive carbon black in the mixture of multi-walled carbon nanotubes and nano-conductive carbon black is 1:1, the synergistic effect of the two is optimized.

[0023] Further, in step (3), the mass ratio of the first part and the second part of the materials is 3:7. The modified Salix psammophila fiber, polylactic acid, and the electromagnetic functional additive compound system in the first part are stirred and mixed for 5-10 minutes at a stirring speed of 20-30 revolutions per minute, and then the remaining modified Salix psammophila fiber, polylactic acid, and the electromagnetic functional additive compound system in the second part are continuously added and stirred and mixed for 5-10 minutes at a stirring speed of 20-30 revolutions per minute.

[0024] Beneficial effects of adopting the above further technical solution: The "two-step" addition process can ensure more sufficient mixing of the materials and further improve the uniformity of the dispersion of the material system.

[0025] Further, in step (3), the pressure for pressing the material is 1 MPa, and the standing time is 10 minutes.

[0026] Beneficial effects of the above further technical solution: This process can achieve the preliminary shaping of the slab material, and the slab has sufficient initial strength to ensure that the slab does not collapse, meeting the convenience requirements of subsequent processes such as slab transportation and loading.

[0027] Further, in step (4), the hot pressing forming temperature of the slab obtained in step (3) is 185°C, the hot pressing pressure is 7 MPa, and the hot pressing time is 7 min.

[0028] Beneficial effects of the above further technical solution: The above process can achieve the full shaping of the material, achieving the dual effects of good sheet quality and high production efficiency.

[0029] The present invention also provides a wood-plastic composite material with both electromagnetic shielding performance and high strength prepared by the above method, with a density of 0.80 - 1.20 g / cm 3 。

[0030] The beneficial effects of the present invention are as follows: Aiming at the problems in the prior art that the mechanical strength is insufficient due to poor interfacial compatibility of the composite material and a single-component electromagnetic functional additive cannot provide excellent electromagnetic shielding performance, the present invention uses Salix psammophila, a characteristic psammophyte resource in Inner Mongolia Autonomous Region, and uses maleic anhydride as the interfacial activator between Salix psammophila fiber and polylactic acid matrix to improve the interfacial compatibility of the composite material and enhance the mechanical strength. At the same time, an "electromagnetic functional additive compounding system" is constructed to provide excellent electromagnetic shielding efficiency for the composite material. The test results show that the surface resistivity, electromagnetic shielding efficiency, and elastic modulus of the composite material meet the requirements of corresponding national / industry standards. Description of the Drawings

[0031] Figure 1 It is a line graph of the flexural strength and elastic modulus of the MWCNTs and EG compound composite material.

[0032] Figure 2 It is a line graph of the impact strength of the MWCNTs and EG compound composite material.

[0033] Figure 3 It is a schematic diagram of the synergistic mechanism of the MWCNTs and EG compound.

[0034] Figure 4 It is a line graph of the flexural strength and elastic modulus of the CCB and EG compound composite material.

[0035] Figure 5 It is a line graph of the impact strength of the CCB and EG compound composite material.

[0036] Figure 6 It is a schematic diagram of the synergistic mechanism of the CCB and EG compound.

[0037] Figure 7 It is a line chart of the flexural strength and elastic modulus of the MWCNTs and CCB composite material.

[0038] Figure 8 It is a line chart of the impact strength of the MWCNTs and CCB composite material.

[0039] Figure 9 It is a schematic diagram of the synergistic mechanism of the MWCNTs and CCB compound.

[0040] Figure 10 It is a bar chart of the logarithm of the surface resistivity of the MWCNTs and EG composite material.

[0041] Figure 11 It is a bar chart of the logarithm of the surface resistivity of the CCB and EG composite material.

[0042] Figure 12 It is a bar chart of the logarithm of the surface resistivity of the CCB and MWCNTs composite material.

[0043] Figure 13 It is a bar chart of the electromagnetic shielding effectiveness of the MWCNTs and EG composite material.

[0044] Figure 14 It is a bar chart of the electromagnetic shielding effectiveness of the CCB and EG composite material.

[0045] Figure 15 It is a bar chart of the electromagnetic shielding effectiveness of the MWCNTs and CCB composite material. Specific implementation mode

[0046] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0047] The following raw material parameters used in the present invention are:

[0048] Expandable graphite, diameter: 75 microns.

[0049] Multi-walled carbon nanotubes, inner diameter ID: 5 - 12 nm, outer diameter OD: 30 - 50 nm, length Length: 5 - 20 μm, specific surface area BET: > 300 m 2 / g.

[0050] Nano-conductive carbon black, diameter: 30 - 40 nm.

[0051] The polylactic acid used in the present invention is a powder, 100 mesh, with a molecular weight of 5×10 4 , and a density of 1.24 g / cm 3 .

[0052] Example 1

[0053] A preparation method of a wood-plastic composite material with both electromagnetic shielding performance and high strength, comprising the following steps:

[0054] (1) Processing and drying of raw materials: After removing the skin of the Salix psammophila wood strips, crushing them, and sieving to select Salix psammophila wood powder with a particle size of 40-60 mesh, drying at a temperature of 70±2°C until the moisture content of the Salix psammophila fiber is below 2%;

[0055] (2) Modification of Salix psammophila fiber: Dissolve the coupling agent maleic anhydride in an ethanol aqueous solution with a volume fraction of 95% to obtain a coupling agent solution with a concentration of 20 wt%. Spray the coupling agent solution evenly on the Salix psammophila fiber obtained in step (1). The dosage of the coupling agent is 2.5% of the mass of the Salix psammophila fiber. Dry at a temperature of 70±2°C until the moisture content of the modified Salix psammophila fiber is below 2% to obtain the modified Salix psammophila fiber;

[0056] (3) Step-by-step mixing and laying up: Divide the modified Salix psammophila fiber, polylactic acid, and electromagnetic functional additive compound system obtained in step (2) into two parts of materials by mass. The mass ratio of the first part and the second part of the materials is 3:7. Stir and mix the modified Salix psammophila fiber, polylactic acid, and electromagnetic functional additive compound system in the first part for 8 min at a stirring speed of 25 revolutions per minute, and then continue to add the remaining modified Salix psammophila fiber, polylactic acid, and electromagnetic functional additive compound system in the second part and stir and mix for 8 min at a stirring speed of 25 revolutions per minute. The mass ratio of the modified Salix psammophila fiber to polylactic acid is 3:7. The addition amount of the electromagnetic functional additive compound system is 9% of the mass of the Salix psammophila fiber. The electromagnetic functional additive compound system is a mixture of multi-walled carbon nanotubes (MWCNTs) and expandable graphite (EG). The mass ratio of the mixture of multi-walled carbon nanotubes and expandable graphite is 1:1. Place the mixed material in a mold of 200 mm×200 mm for laying up, press the material to a thickness of 15 mm, the pressure for pressing the material is 1 MPa, let it stand for 10 min, and after the material form is fixed, obtain a board blank;

[0057] (4) Hot pressing and forming: Hot press and form the board blank obtained in step (3) at a temperature of 185°C, a hot pressing pressure of 7 MPa, and a hot pressing time of 7 min. After trimming the sample, make a wood-plastic composite material with both electromagnetic shielding performance and high strength with a density of 1.0 g / cm 3 , and a thickness of 4 mm.

[0058] Example 2

[0059] Compared with Example 1, except that the electromagnetic functional additive compounding system is a mixture of nano-conductive carbon black (CCB) and expandable graphite (EG), and the mass ratio of nano-conductive carbon black to expandable graphite is 1:1, the other steps and parameters are the same as those in Example 1.

[0060] Example 3

[0061] Compared with Example 1, except that the electromagnetic functional additive compounding system is a mixture of multi-walled carbon nanotubes (MWCNTs) and nano-conductive carbon black (CCB), and the ratio of multi-walled carbon nanotubes to nano-conductive carbon black is 1:1, the other steps and parameters are the same as those in Example 1.

[0062] Performance test results of the wood-plastic composites with both electromagnetic shielding performance and high strength obtained in Examples 1-3:

[0063] The logarithmic values of the surface resistivity are 2, 2, and 1 respectively, meeting the electrical property requirements of Grade EP-2 (logarithmic value of surface resistivity ≤ 3) and Grade EP-1 (logarithmic value of surface resistivity ≤ 1) in the General Technical Requirements for Electromagnetic Shielding Plastics GB / T 32511-2016;

[0064] The electromagnetic shielding effectiveness values are 25 dB, 21 dB, and 48 dB respectively, meeting the electromagnetic shielding effectiveness requirements of Grade SE-3 (20 dB) and Grade SE-2 (40 dB) in the General Technical Requirements for Electromagnetic Shielding Plastics GB / T 32511-2016;

[0065] The flexural strength and elastic modulus are 25.23 MPa / 2733 MPa, 29.43 MPa / 3013 MPa, and 18.73 MPa / 2051 MPa respectively. The flexural strength of the composites under the multi-walled carbon nanotube / expandable graphite and nano-conductive carbon black / expandable graphite compounding systems meets the third-level index requirements for flexural strength (≥ 23 MPa) of plain unfoamed indoor wood-plastic composites in the Wood-Plastic Composites Classification LY / T 3274-2021. The elastic modulus of the composites all meets the third-level index requirements for elastic modulus (≥ 1800 MPa) of plain unfoamed indoor wood-plastic composites in the Wood-Plastic Composites Classification LY / T 3274-2021.

[0066] Example 4

[0067] Compared with Example 1, except that the mass ratio of the mixture of multi-walled carbon nanotubes and expandable graphite is 1:2, the other steps and parameters are the same as those in Example 1.

[0068] Example 5

[0069] Compared with Example 1, except that the mass ratio of the mixture of multi-walled carbon nanotubes and expandable graphite is 1:3, the other steps and parameters are the same as those in Example 1.

[0070] Example 6

[0071] Compared with Example 1, except that the mass ratio of the mixture of multi-walled carbon nanotubes and expandable graphite is 1:4, other steps and parameters are the same as those in Example 1.

[0072] Example 7

[0073] Compared with Example 2, except that the mass ratio of nano-conductive carbon black and expandable graphite is 1:2, other steps and parameters are the same as those in Example 2.

[0074] Example 8

[0075] Compared with Example 2, except that the mass ratio of nano-conductive carbon black and expandable graphite is 1:3, other steps and parameters are the same as those in Example 2.

[0076] Example 9

[0077] Compared with Example 2, except that the mass ratio of nano-conductive carbon black and expandable graphite is 1:4, other steps and parameters are the same as those in Example 2.

[0078] Example 10

[0079] Compared with Example 3, except that the ratio of multi-walled carbon nanotubes to nano-conductive carbon black is 1:2, other steps and parameters are the same as those in Example 3.

[0080] Example 11

[0081] Compared with Example 3, except that the ratio of multi-walled carbon nanotubes to nano-conductive carbon black is 1:3, other steps and parameters are the same as those in Example 3.

[0082] Example 12

[0083] Compared with Example 3, except that the ratio of multi-walled carbon nanotubes to nano-conductive carbon black is 1:4, other steps and parameters are the same as those in Example 3.

[0084] The following experiments were carried out using Examples 1-12 of the present invention.

[0085] 1. Influence of the compounding of electromagnetic function additives on the mechanical properties of the composite material

[0086] The influence of the compounding of MWCNTs and EG on the flexural strength, elastic modulus and impact strength of the composite material is as Figure 1 , Figure 2As shown in the figure. It can be seen that in the MWCNTs / EG composite system, the flexural strength and elastic modulus of the composite material increase with the increase of the proportion of EG. When the ratio of MWCNTs:EG is 1:4, the flexural strength and elastic modulus of the composite material reach the optimum, with the values of 32.12 MPa and 3754 MPa respectively, meeting the secondary indicators (flexural strength ≥ 30 MPa, elastic modulus ≥ 2500 MPa), and meeting the requirements of the flexural strength and elastic modulus for unfoamed indoor wood-plastic composites in LY / T 3274-2021 "Classification of Wood-Plastic Composites". Compared with the 1:1 composite ratio, it is increased by 27.30% and 37.35%. The impact strength of the composite material increases with the increase of the proportion of EG in the total amount of electromagnetic functional additives. When the ratio of MWCNTs:EG is 1:4, the impact strength of the composite material reaches the optimum, with the value of 3.012 kJ / m², which is increased by 22.01% compared with the 1:1 composite ratio.

[0087] The compounding of MWCNTs particles and EG particles can improve the interfacial bonding between the filler and the matrix. Due to its high aspect ratio and strong van der Waals force, MWCNTs particles can play a bridging role in the composite material, and can form a network structure with EG particles, reducing the generation of agglomeration phenomena. As the number of EG particles increases, because small-sized EG particles can form intercalation compounds in the matrix and are uniformly dispersed in the matrix, the dispersibility of the overall electromagnetic functional additives in the composite material is improved, and the mechanical properties of the composite material are improved. The synergistic mechanism of the compounding of MWCNTs and EG is as Figure 3 shown.

[0088] The effects of the compounding of CCB and EG on the flexural strength, elastic modulus and impact strength of the composite material are as Figure 4 、 Figure 5 shown. It can be seen that in the CCB / EG composite system, the flexural strength and elastic modulus of the composite material decrease with the increase of the proportion of EG. When the ratio of CCB:EG is 1:1, the flexural strength and elastic modulus of the composite material are 29.43 MPa and 3013 MPa respectively, meeting the tertiary indicators (flexural strength ≥ 23 MPa, elastic modulus ≥ 1800 MPa), and meeting the requirements of the flexural strength and elastic modulus for unfoamed indoor wood-plastic composites in LY / T 3274-2021 "Classification of Wood-Plastic Composites". When the ratio of CCB:EG is 1:4, the flexural strength and elastic modulus of the composite material are 22.13 MPa and 2267 MPa respectively, which are decreased by 24.80% and 24.75% compared with the 1:1 composite ratio. The impact strength of the composite material decreases with the increase of the proportion of EG in the total amount of electromagnetic functional additives. When the ratio of MWCNTs:EG is 1:4, the impact strength of the composite material is 2.194 kJ / m², which is decreased by 9.4% compared with the 1:1 composite ratio.

[0089] When CCB and EG are compounded, although the mechanical properties decrease with the increase in the proportion of expandable graphite, the mechanical properties are significantly improved compared to the composite material with only EG added. This is because CCB particles can play a role in bridging between EG particles, improving the overall dispersion of the electromagnetic functional additives, thus enhancing the mechanical properties. However, excessive EG particles may lead to self-aggregation between EG particles, resulting in a lack of interfacial adhesion between EG particles and the Salix psammophila / PLA matrix, affecting the mechanical properties of the composite material. The synergistic mechanism of CCB and EG compounding is as Figure 6 shown.

[0090] The effects of the compounding of MWCNTs and CCB on the flexural strength, elastic modulus and impact strength of the composite material are as Figure 7 , Figure 8 shown. It can be seen that in the CCB / MWCNTs compounding system, the flexural strength and elastic modulus of the composite material increase with the increase in the proportion of CCB. When MWCNTs:CCB is 1:4, the flexural strength and elastic modulus of the composite material are 27.05 MPa and 2721 MPa respectively, reaching the third-level indicators (flexural strength ≥ 23 MPa, elastic modulus ≥ 1800 MPa), meeting the requirements for flexural strength and elastic modulus of unfoamed indoor wood-plastic composites in LY / T 3274-2021 "Classification of Wood-Plastic Composites". Compared with the compounding ratio of 1:1, it increases by 44.42% and 232.66%. The impact strength of the composite material increases with the increase in the proportion of CCB in the total amount of electromagnetic functional additives added. When MWCNTs:CCB is 1:4, the impact strength of the composite material is 2.104 kJ / m², which is 41.80% higher than that of the compounding ratio of 1:1.

[0091] The compounding of MWCNTs and CCB can improve the mechanical properties of the composite material mainly because there is a synergistic reinforcement mechanism between MWCNTs and CCB. MWCNTs particles play the roles of "skeleton" and "beam" in the CCB reinforcement system, forming a unified and integral reinforcement structure, thus improving the mechanical properties of the composite material and achieving an overall reinforcement effect. Since both CCB and MWCNTs are electromagnetic functional additives with nano-scale sizes, and there is a nano-scale MWCNTs / CCB nano-scale electromagnetic functional additive network structure jointly constructed by MWCNTs and CCB in the composite system, with the increase in the proportion of CCB in the total amount of electromagnetic functional additives added, this nano-structure is further improved, and the nano-network structure of MWCNTs particles and CCB particles in the composite system is further improved. Therefore, the mechanical properties of the composite material are improved. The synergistic mechanism of MWCNTs and CCB compounding is as Figure 9 shown.

[0092] 2. Influence of the compounding of electromagnetic functional additives on the logarithm value of the surface resistivity of the composite material

[0093] The influence of the compounding of MWCNTs and EG on lg(ps) of the composite material is as Figure 10 shown. It can be seen that the lg(ps) of the MWCNTs / EG composite material shows an upward trend with the increase of the compounding ratio. When the compounding ratio of MWCNTs is 1:1 and 1:2, lg(ps) is 2, belonging to superconducting materials. Compared with the composite material filled with 9% unmodified EG, lg(ps) has decreased by 11 orders of magnitude. Compared with the composite material filled with 9% unmodified MWCNTs, it has only increased by 1 - 2 orders of magnitude, and the change of the lg(ps) order of magnitude is small.

[0094] Combined with Figure 3 it can be seen that MWCNTs particles have a high aspect ratio and excellent electrical conductivity and can be used as the skeleton of the conductive network. Due to its layered structure and high specific surface area, EG particles can be used as the connection points of the conductive paths, increasing the contact opportunities between MWCNTs particles, thus forming more conductive paths, promoting the formation of the conductive network, further improving the conductive network, and enhancing the electrical conductivity of the composite material.

[0095] The influence of the compounding of CCB and EG on lg(ps) of the composite material is as Figure 11 shown. It can be seen that the lg(ps) of the CCB / EG composite material shows an upward trend with the increase of the compounding ratio. When the compounding ratio of CCB / EG is 1:1, lg(ps) is 2, belonging to superconducting materials. At other ratios, lg(ps) is 3. Compared with the composite material filled with 9% unmodified CCB, lg(ps) has decreased by 11 orders of magnitude. Compared with the composite material filled with 9% unmodified CCB, the change of the lg(ps) order of magnitude is not large.

[0096] Combined with Figure 6 it can be seen that the synergistic effect of CCB particles and EG particles can improve the electrical conductivity of the composite material. Among them, CCB particles act as a "bridge" between EG particles, playing a role in lapping, improving the dispersion state of EG particles in the composite material, and enhancing the electrical conductivity of the composite material.

[0097] The influence of the compounding of CCB and MWCNTs on lg(ps) of the composite material is as Figure 12 shown. It can be seen that the lg(ps) of the MWCNTs / CCB composite material changes little with the increase of the compounding ratio, and lg(ps) is 1. Compared with the composite materials filled with 9% unmodified CCB and MWCNTs, lg(ps) has decreased by 1 order of magnitude, and the change of the lg(ps) order of magnitude is small, and they all belong to superconducting materials.

[0098] Combined withFigure 9 It can be seen that fibrous MWCNTs particles can be dispersed among granular CCB particles. Among them, the fibrous MWCNTs particles provide long-range conductivity, and the granular CCB provides both short-range conductivity. The MWCNTs particles play a "bridge" role. Through the synergistic effect of two fillers with different geometries, a more perfect nano-scale conductive network structure is formed in the system, improving the conductive performance of the composite material.

[0099] 3. Influence of the compounding of electromagnetic functional additives on the electromagnetic shielding performance of the composite material

[0100] The influence of the compounding of MWCNTs and EG on the electromagnetic shielding effectiveness of the composite material is as Figure 13 shown. It can be seen that the electromagnetic shielding performance of the MWCNTs / EG composite material in the X-band (8.2 - 12.4 GHz) decreases with the increase of the compounding ratio. When the MWCNTs / EG compounding ratio is 1:1, the electromagnetic shielding effectiveness of the composite material is the largest, reaching 25 dB, meeting the electromagnetic shielding effectiveness requirement of the SE-3 level (20 dB) and satisfying the electromagnetic shielding effectiveness requirements of GB / T 32511-2016 "General Technical Requirements for Electromagnetic Shielding Plastics". When the MWCNTs / EG compounding ratio is 1:4, the electromagnetic shielding effectiveness of the composite material drops to the lowest, being 10 dB, a 60% decrease compared with when the MWCNTs / EG compounding ratio is 1:1.

[0101] During the high-temperature heat treatment process, the tubular structure of the MWCNTs particles is changed, and the molten MWCNTs particles are distributed in the form of a film on the surface layer and the interlayer pores of the EG. This structure increases the surface conductivity of the EG particles and the ability to attenuate electromagnetic waves, but does not change the diamagnetic characteristics of the EG particles. Therefore, the electromagnetic shielding performance of the composite material is improved compared with that of the single EG composite material. However, with the increase of the compounding ratio, since the electromagnetic shielding performance of EG itself is not as excellent as that of MWCNTs, the electromagnetic shielding effectiveness will decrease with the increase of the compounding ratio.

[0102] The influence of the compounding of CCB and EG on the electromagnetic shielding effectiveness of the composite material is as Figure 14 shown. It can be seen that the electromagnetic shielding performance of the CCB / EG composite material in the X-band (8.2 - 12.4 GHz) decreases with the increase of the compounding ratio. When the CCB / EG compounding ratio is 1:1, the electromagnetic shielding effectiveness of the composite material is the largest, reaching 21 dB, meeting the electromagnetic shielding effectiveness requirement of the SE-3 level (20 dB) and satisfying the electromagnetic shielding effectiveness requirements of GB / T 32511-2016 "General Technical Requirements for Electromagnetic Shielding Plastics". When the CCB / EG compounding ratio is 1:4, the electromagnetic shielding effectiveness of the composite material drops to the lowest, being 9 dB, a 57.14% decrease compared with when the CCB / EG compounding ratio is 1:1.

[0103] The combination of CCB particles and EG particles can form a more perfect conductive network. Due to its conductive properties, CCB particles can be used as components of the conductive network, while the layered structure and high specific surface area of EG particles can serve as connection points for conductive paths, increasing the contact opportunities between CCB particles, thus forming more conductive paths, reducing the resistivity, and enhancing the electromagnetic shielding effectiveness.

[0104] The influence of the compounding of MWCNTs and CCB on the electromagnetic shielding effectiveness of the composite material is as Figure 15 shown. It can be seen that the electromagnetic shielding performance of the MWCNTs / CCB composite material in the X-band (8.2 - 12.4 GHz) decreases with the increase of the compounding ratio. When the MWCNTs / CCB compounding ratio is 1:1, the electromagnetic shielding effectiveness of the composite material is the largest, reaching 48 dB. When the MWCNTs / CCB compounding ratio is 1:4, the electromagnetic shielding effectiveness of the composite material drops to the lowest, which is 32 dB, a decrease of 33.33% compared with when the MWCNTs / CCB compounding ratio is 1:1, reaching the SE-2 level (40 dB) and SE-3 level (20 dB) respectively, meeting the electromagnetic shielding effectiveness requirements of GB / T 32511-2016 "General Technical Requirements for Electromagnetic Shielding Plastics".

[0105] The combination of MWCNTs particles and CCB particles can form a more perfect conductive network. Due to its high aspect ratio and excellent conductive properties, MWCNTs particles can be used as the backbone of the conductive network, while the high specific surface area and conductive properties of CCB particles can serve as connection points for conductive paths, increasing the contact opportunities between MWCNTs particles, thus forming more conductive paths and enhancing the electromagnetic shielding effectiveness. However, too many CCB particles may lead to the destruction of the conductive network because CCB particles may aggregate to form larger agglomerates, resulting in an increase in the transmission of electromagnetic waves because the aggregation of CCB particles may form a transmission channel for electromagnetic waves, reducing the reflection and absorption ability of the composite material to electromagnetic waves.

[0106] The conclusions are as follows:

[0107] (1)Construct the MWCNTs / EG, CCB / EG, and CCB / MWCNTs composite systems. The optimal compounding ratios of the MWCNTs / EG and CCB / MWCNTs composites for mechanical properties are both 1:4. Their static bending strength, elastic modulus, and impact strength are increased by 27.30%, 37.35%, and 22.01% respectively compared with the compounding ratio of 1:1. The optimal compounding ratio of the CCB / EG composite for mechanical properties is 1:1. The static bending strength, elastic modulus, and impact strength of the composite decrease with the increase in the proportion of EG in the total amount of electromagnetic functional additives, with the lowest decreases being 24.80%, 24.75%, and 9.4% respectively. The MWCNTs / EG, CCB / EG, and CCB / MWCNTs composites meet the secondary index, tertiary index, and tertiary index respectively, and meet the requirements for static bending strength and elastic modulus of unfoamed indoor wood-plastic composites in LY / T 3274-2021 "Classification of Wood-Plastic Composites".

[0108] (2)Construct the MWCNTs / EG, CCB / EG, and CCB / MWCNTs composite systems. The optimal compounding ratios of the electromagnetic properties of the MWCNTs / EG, CCB / EG, and CCB / MWCNTs composites are all 1:1. When the compounding ratios of the three composites are all 1:1, the logarithmic values of the surface resistivity of the three materials meet the requirements of superconducting materials. The electromagnetic shielding effectiveness of the MWCNTs / EG, CCB / EG, and CCB / MWCNTs composites reaches the SE-3 level (20 dB), SE-3 level (20 dB), and SE-2 level (40 dB) respectively, meeting the requirements of the electromagnetic shielding effectiveness in GB / T 32511-2016 "General Technical Requirements for Electromagnetic Shielding Plastics".

[0109] (3)The mechanism for enhancing the electromagnetic properties of the composites by constructing the MWCNTs / EG, CCB / EG, and CCB / MWCNTs composite systems is as follows: In the MWCNTs / EG composite system, MWCNTs can serve as the backbone of the conductive network. Due to its layered structure, EG can serve as the connection point of the conductive path, increasing the contact opportunities between MWCNTs, thus forming more conductive paths and promoting the formation of the conductive network. In the CCB / EG composite system, CCB acts as a "bridge" between EG particles, playing a role in bridging, improving the dispersion state of expandable graphite in the composite, and enhancing the electromagnetic properties of the composite. In the CCB / MWCNTs composite system, MWCNTs can be dispersed between granular CCB particles. Among them, MWCNTs provide long-range conductivity, and granular CCB provides short-range conductivity. MWCNTs also play a "bridge" role. Through the synergistic effect of two different geometric-shaped fillers, a more perfect nano-scale conductive network structure is formed in the system.

[0110] (4)Comprehensively considering the mechanical properties, surface resistivity, and electromagnetic shielding effectiveness of the composite materials under the three composite systems of MWCNTs / EG, CCB / EG, and CCB / MWCNTs, the optimal composite ratio with both mechanical and electromagnetic properties is selected as 1:1. Among them, the flexural strength and elastic modulus of the composite materials under the MWCNTs / EG and CCB / EG composite systems reach the third-level indicators, meeting the requirements for flexural strength and elastic modulus of unfoamed indoor wood-plastic composite materials in LY / T 3274-2021 "Classification of Wood-Plastic Composites". The logarithmic value of the surface resistivity reaches that of superconducting materials, and the electromagnetic shielding effectiveness reaches the SE-3 level, meeting the electromagnetic shielding effectiveness requirements of GB / T 32511-2016 "General Technical Requirements for Electromagnetic Shielding Plastics". Under the CCB / MWCNTs composite system, the logarithmic value of the surface resistivity of the composite material reaches that of superconducting materials, and the electromagnetic shielding effectiveness reaches the SE-2 level, meeting the electromagnetic shielding effectiveness requirements of GB / T 32511-2016 "General Technical Requirements for Electromagnetic Shielding Plastics".

[0111] In summary, it can be concluded that the electromagnetic properties of the composite materials under the CCB / MWCNTs composite system reach the optimum. The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be obvious to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to these embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A preparation method of a wood-plastic composite material with both electromagnetic shielding performance and high strength, characterized in that, It includes the following steps: (1) Processing and drying of raw materials: After peeling the Salix psammophila wooden strips, they are crushed, sieved, and dried to obtain Salix psammophila fibers. (2) Modification of Salix psammophila fibers: The coupling agent is dissolved in an ethanol aqueous solution with a volume fraction of 95% to obtain a coupling agent solution. The coupling agent solution is evenly sprayed on the Salix psammophila fibers obtained in step (1) and dried to obtain modified Salix psammophila fibers. (3) Step-by-step mixing and laying up for blanking: The modified Salix psammophila fibers, polylactic acid, and electromagnetic functional additive compound system obtained in step (2) are divided into the first part and the second part of the materials according to mass. The modified Salix psammophila fibers, polylactic acid, and electromagnetic functional additive compound system in the first part are stirred and mixed, and then the remaining second part of the modified Salix psammophila fibers, polylactic acid, and electromagnetic functional additive compound system is continuously added and stirred and mixed. The mixed material is placed in a mold for laying up, the material is pressed, and left standing for a period of time. After the material form is fixed, a blank is obtained. The electromagnetic functional additive compound system is a mixture of multi-walled carbon nanotubes and expandable graphite, a mixture of nano-conductive carbon black and expandable graphite, or a mixture of multi-walled carbon nanotubes and nano-conductive carbon black. (4) Hot pressing and forming: The blank obtained in step (3) is hot pressed and formed to obtain the wood-plastic composite material with both electromagnetic shielding performance and high strength.

2. The preparation method of a wood-plastic composite material with both electromagnetic shielding performance and high strength according to claim 1, characterized in that, In step (1), the sieved Salix psammophila wood powder has a particle size of 40 - 60 mesh, the drying temperature is 70 ± 2 °C, and it is dried until the moisture content of the Salix psammophila fibers is below 2%.

3. The preparation method of a wood-plastic composite material with both electromagnetic shielding performance and high strength according to claim 1, characterized in that, In step (2), the coupling agent is maleic anhydride.

4. The preparation method of a wood-plastic composite material with both electromagnetic shielding performance and high strength according to claim 1, characterized in that, In step (2), the concentration of the coupling agent solution is 20 wt%, and the dosage of the coupling agent is 2.5% of the mass of the Salix psammophila fibers.

5. The preparation method of a wood-plastic composite material with both electromagnetic shielding performance and high strength according to claim 1, characterized in that In step (2), the drying temperature is 70 ± 2 °C, and it is dried until the moisture content of the modified Salix psammophila fibers is below 2%.

6. The preparation method of a wood-plastic composite material with both electromagnetic shielding performance and high strength according to claim 1, characterized in that, In step (3), the mass ratio of the modified Salix psammophila fibers to polylactic acid is 3:7, the addition amount of the electromagnetic functional additive compound system is 3 - 12% of the mass of the modified Salix psammophila fibers, the mass ratio of multi-walled carbon nanotubes to expandable graphite in the mixture of multi-walled carbon nanotubes and expandable graphite is 1:(1 - 4), the mass ratio of nano-conductive carbon black to expandable graphite in the mixture of nano-conductive carbon black and expandable graphite is 1:(1 - 4), and the mass ratio of multi-walled carbon nanotubes to nano-conductive carbon black in the mixture of multi-walled carbon nanotubes and nano-conductive carbon black is 1:(1 - 4).

7. The preparation method of a wood-plastic composite material with both electromagnetic shielding performance and high strength according to claim 1, characterized in that, In step (3), the mass ratio of the first part to the second part of the materials is 3:

7. The modified Salix psammophila fibers, polylactic acid, and electromagnetic functional additive compound system in the first part are stirred and mixed for 5 - 10 min at a stirring speed of 20 - 30 revolutions per minute, and then the remaining second part of the modified Salix psammophila fibers, polylactic acid, and electromagnetic functional additive compound system is continuously added and stirred and mixed for 5 - 10 min at a stirring speed of 20 - 30 revolutions per minute.

8. The preparation method of a wood-plastic composite material with both electromagnetic shielding performance and high strength according to claim 1, characterized in that, In step (3), the pressure for pressing the material is 1 MPa, and the standing time is 10 min.

9. The preparation method of a wood-plastic composite material with both electromagnetic shielding performance and high strength according to claim 1, characterized in that, In step (4), the hot pressing and forming temperature of the blank obtained in step (3) is 185 °C, the hot pressing pressure is 7 MPa, and the hot pressing time is 7 min.

10. A wood-plastic composite material with both electromagnetic shielding performance and high strength prepared by the method according to any one of claims 1-9, characterized in that, The density is 0.80 - 1.20 g / cm 3 .

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