Method of making a silane-modified thermoplastic composite resistance-welded heating element

Abstract

This invention belongs to the field of thermoplastic composite material joining technology, and specifically relates to a method for preparing a silane-modified thermoplastic composite resistance welding heating element. The invention first performs a cleaning pretreatment on a carbon nanotube film to remove surface contaminants. Then, plasma treatment technology is used to etch and activate the film surface, introducing oxygen-containing groups to provide active sites for silane grafting. Finally, silane is grafted onto the plasma-activated film using a silane coupling agent, utilizing its bifunctional properties to construct a molecular bridge between the carbon nanotubes and the resin, thereby improving the interfacial bonding force between the film heating element and the resin, and increasing the resistance welding strength. The heating element prepared by this invention has high specific strength, corrosion resistance, and better homogeneity with the base material, making it more suitable for complex welding joint service environments.

Description

Technical Field

[0001] This invention belongs to the field of thermoplastic composite material joining technology, and specifically relates to a method for preparing a silane-modified thermoplastic composite material resistance welding heating element. Background Technology

[0002] Continuous fiber-reinforced thermoplastic composites (CFRTPCs) have been widely used in aerospace, automotive, and transportation industries due to their superior specific strength, impact resistance, and recyclability, especially demonstrating great potential in the lightweight design of large and complex components. Because thermoplastic resins have the physical property of melting and curing, the connection between components can bypass traditional mechanical fastening or adhesive bonding processes, instead employing more efficient and continuous fusion welding technology. Among the many welding processes for thermoplastic composites, resistance welding, which generates heat by embedding a heating element at the welding interface, has significant advantages such as high energy utilization, low equipment requirements, and wide applicability, and has been successfully applied to structural connections in critical components such as aerospace parts.

[0003] In resistance welding systems, the properties of the heating element directly determine the weight reduction effect and service reliability of the weld joint. Compared to stainless steel mesh or carbon fiber fabric, carbon nanotube films (CNTFs), as a novel nanoscale heating element, possess high specific strength, corrosion resistance, and better homogeneity with composite material substrates, effectively reducing the added mass of the weld joint and avoiding heterogeneous implants at the weld interface. However, due to the highly chemically inert surface of carbon nanotubes, the interfacial wettability between CNTF and the resin matrix is ​​poor, making it difficult to form an effective bond. Therefore, this invention enhances the interfacial bonding force between carbon nanotubes and resin by modifying the surface of CNTF, thereby improving the welding strength.

[0004] The literature (DOI: 10.1016 / j.matlet.2022.133216) proposes a method for embedding carbon nanotubes into stainless steel wire mesh in the weld, achieving the welding of thermoplastic composites. However, the composite structure of this heating element is complex, and the weld strength is low, making it difficult to meet the strength requirements for industrial applications. Therefore, developing a silane modification method that can maintain the advantages of CNTF while effectively improving weld strength remains a pressing technical challenge in the field of thermoplastic composite joining. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a method for preparing a silane-modified thermoplastic composite resistance welding heating element. First, the carbon nanotube (CNTF) film undergoes plasma treatment. The plasma treatment utilizes surface etching and activation to improve the surface roughness of the CNTF while introducing oxygen-containing active groups. Then, the activated CNTF is modified with a silane coupling agent through silane grafting, enhancing the mechanical entanglement and chemical bonding between the carbon nanotubes and the resin, thereby improving the welding strength.

[0006] The technical solution of this invention: A method for preparing a silane-modified thermoplastic composite resistance welding heating element includes the following steps: Step 1) Pretreatment of heating elements: The carbon nanotube film (CNTF) is cut into heating elements of a set size, ultrasonically cleaned and dried at a constant temperature to remove surface contamination and any residual moisture inside the heating elements.

[0007] Step 2) Plasma Surface Treatment of Heating Element: In a constant temperature and humidity environment, dielectric barrier discharge (DBD) plasma treatment is selected. First, the power output is set to generate a large-area, high-density low-temperature plasma between the two insulating dielectrics. Then, the pre-treated heating element is fixed between the two insulating electrodes of the DBD. The reactive gas is introduced into the DBD cavity through a flow controller, and the power is connected to achieve rapid surface treatment while avoiding damage to the thin film. After one side of the modification is completed, the heating element is flipped over and fixed, and the above steps are repeated to ensure that both sides of the heating element are uniformly activated and modified by plasma.

[0008] Step 3) Silane-modified plasma-activated heating element: After modification, the DBD plasma-activated heating element is removed and immediately subjected to silane modification to ensure that the surface active sites are in optimal condition before the experiment. First, anhydrous ethanol is used as the solvent to induce complete hydrolysis of the silane coupling agent molecules to form active silanols. Then, acetic acid is added dropwise to the anhydrous ethanol to create a stable acidic hydrolysis environment. Based on this, the plasma-treated heating element and silane coupling agent are added to the solvent. The uniformly mixed reaction system is stirred at a constant temperature to ensure a complete reaction. Continuous stirring ensures that the silane molecules undergo a condensation reaction with the active sites on the CNTF surface.

[0009] After the reaction, the silane-modified plasma-activated heating element was ultrasonically cleaned until the washing solution was clear and free of residue, in order to thoroughly remove silane molecules that remained on the film surface and in the nanopores due to physical adsorption. Then, it was dried at a constant temperature to remove residual solvent and promote further solidification of surface chemical bonds, finally obtaining a silane-modified thermoplastic composite resistance welding heating element.

[0010] Furthermore, the heating element described in step 1) has the following specifications: sheet resistance 2Ω / sq, thickness 8μm, and density 0.6g·cm³. -3 The dimensions are 75mm × 25.4mm.

[0011] Furthermore, the ultrasonic cleaning solutions mentioned in steps 1) and 3) are acetone and anhydrous ethanol, respectively, and the cleaning time is 10 min to 30 min, and the cleaning is repeated 2 to 3 times.

[0012] Furthermore, the constant temperature drying process described in steps 1) and 3) is 50℃~80℃, and the drying time is 6h~12h.

[0013] Furthermore, in step 2), the power output is set to 220W~550W, the electrode gap is adjusted to 1.5mm~2.5mm, and the power-on time is 1s~3s.

[0014] Furthermore, the constant temperature and humidity environment described in step 2) is a temperature of 23±3℃ and a humidity of 40±10%.

[0015] Furthermore, the reaction gas mentioned in step 2) is high-purity synthetic air, which is a mixture of nitrogen and oxygen in a volume ratio of 79:21, with a purity of ≥99.999% and a water content of ≤3ppm.

[0016] Furthermore, the silane coupling agent mentioned in step 3) is one or a mixture of two or more of 3-aminopropyltriethoxysilane (KH550), 3-glycidoxypropyltrimethoxysilane (KH560), and 3-(isobutenoyloxy)propyltrimethoxysilane (KH570).

[0017] Furthermore, the stable acidic hydrolysis environment described in step 3) is pH=3~5.

[0018] Furthermore, in step 3), the mass ratio of CNTF, silane coupling agent, and anhydrous ethanol in the reaction system is 1:10:80.

[0019] Furthermore, the reaction system described in step 3) has a reaction temperature of 70℃~80℃ and a reaction time of 1h~3h.

[0020] The beneficial effects of this invention are: This invention addresses the problem of interfacial debonding caused by the large polarity difference and weak interfacial affinity between carbon nanotube films and resin matrices. By modifying the silane graft, it effectively solves the structural defect that makes resistance welded joints prone to failure at the heating element under stress, thereby improving the strength of the welded joint.

[0021] Silane modification constructs a stable molecular bridge between the carbon nanotube network and the resin matrix, effectively improving the wettability of the carbon nanotube surface and enhancing the interfacial bonding force with the resin. The hydrolyzed groups of the silane molecule are anchored to the carbon nanotube surface through covalent bonding, while the organic functional groups at the other end are chemically bonded to the molten resin through van der Waals forces, molecular chain diffusion entanglement, or at the high welding temperatures. This microscopic bridging not only acts as a stress transfer medium but also ensures sufficient penetration and deep coating of the carbon nanotube film by the resin during welding. This transforms the weak interface, which originally relied solely on physical filling and mechanical interlocking, into a chemically strengthened interface with high load transfer efficiency, thereby preventing internal defects in the weld.

[0022] The carbon nanotube film selected in this invention possesses high specific strength and electrothermal conversion efficiency. Combined with silane modification, it ensures uniform heat generation and interfacial compatibility during welding. Simultaneously, the removal of dissimilar metal components eliminates residual stress and potential corrosion risks caused by mismatches in thermal expansion coefficients with the base material. While achieving extreme structural lightweighting, it ensures the homogeneity of the weld interface, improving the long-term reliability of the welded joint under harsh service environments.

[0023] The heating element prepared by this invention has high specific strength, corrosion resistance and better homogeneity with the base material, making it more suitable for complex welding joint service environments. Attached Figure Description

[0024] Figure 1 The images show AFM images of the silane-modified CNTF heating element obtained in Example 1, where (a) is the surface morphology before modification and (b) is the surface morphology after modification.

[0025] Figure 2 The images show the water contact angles of the silane-modified CNTF heating element obtained in Example 1, where (a) is the water contact angle before modification (108.99°) and (b) is the water contact angle after modification (37.17°).

[0026] Figure 3 The image shows the planar damage morphology of the weld joint of the silane-modified CNTF heating element obtained in Example 1. Detailed Implementation

[0027] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings and technical solutions.

[0028] Example 1: Preparation of thermoplastic composite resistance welding heating element using KH550 grafted with CNTTF Step 1) Pretreatment of heating element: A carbon nanotube film with a sheet resistance of 2Ω / sq and a diameter of 75mm×25.4mm is used as the heating element. It is placed in acetone and cleaned with an ultrasonic cleaner for 30 minutes. After repeating twice, it is placed in an 80℃ constant temperature forced air drying oven and dried for 12 hours.

[0029] Step 2) Plasma treatment of the heating element: The test environment is adjusted to 23℃ and 40% humidity. After removing the CNTF from the oven, it is fixed between the two insulating media in the DBD plasma treatment. The output power is adjusted to 440W, and the gap between the insulating media is adjusted to 2mm. High-purity air is introduced into the DBD plasma treatment chamber through the flow controller. The power is turned on, generating a large-area, high-density, low-temperature plasma between the insulating media. The power is turned off after 2 seconds. The CNTF heating element is flipped over and fixed, and the above steps are repeated to ensure that both sides of the heating element are uniformly activated and modified by the plasma. Then, the CNTF heating element is removed.

[0030] Step 3) For the silane-grafted plasma-activated heating element, add 100 ml of anhydrous ethanol to a 250 ml single-necked flask equipped with a spherical condenser and a magnetic stirrer. Then, add a few drops of acetic acid to the ethanol to adjust the pH to 5. Weigh 0.8 g of the CNTF heating element and 8 g of KH550 coupling agent taken in Step 2) and add them to the single-necked flask. Maintain a constant temperature reflux at 75 °C for 2 hours, and continuously stir with a magnetic stirrer to ensure a complete reaction.

[0031] After the reaction was completed, the heating element was removed and placed in anhydrous ethanol, and cleaned with an ultrasonic cleaner for 30 minutes, repeated twice, to thoroughly remove silane molecules remaining on the film surface and in the nanopores that were physically adsorbed. Finally, the purified silane-modified CNTF was placed in an 80℃ constant temperature forced-air oven and dried for 12 hours to remove residual solvent and promote further solidification of surface chemical bonds, ultimately obtaining a silane-modified thermoplastic composite resistance welding heating element.

[0032] The prepared silane-modified CNTF heating element was subjected to surface roughness testing and water contact angle testing using atomic force microscopy, such as... Figure 1 and Figure 2Test results showed that silane modification effectively improved the surface roughness and wettability of the CNTF heating element, which helped to enhance its interfacial bonding with the resin. The modified CNTF heating element was then used for resistance welding of CF / PEEK composite laminates. The main welding parameters were: welding voltage 25V, welding time 45s, and welding pressure 1.8MPa. The welded samples were tested according to standard ASTM D5868, and the single lap shear strength of the joint was measured to be 40.63MPa, an increase of 41.1% compared to the welded sample with the unmodified heating element. Electron microscopy analysis of the fracture surface of the welded joint revealed sufficient wetting at the CNTF-resin interface, and pull-out of the parent material carbon fibers was observed at the weld interface (e.g., ...). Figure 3 This demonstrates that the strength of the weld interface has been effectively improved. This example illustrates that silane-modified thermoplastic composite resistance welding heating elements can enhance the strength of welded joints and improve the quality of the weld interface.

[0033] Example 2: Preparation of thermoplastic composite resistance welding heating elements using KH560 grafted with CNTTF Step 1) Pretreatment of heating element: A carbon nanotube film with a sheet resistance of 2Ω / sq and a diameter of 75mm×25.4mm is used as the heating element. It is placed in acetone and cleaned with an ultrasonic cleaner for 10 minutes. After repeating twice, it is placed in a 50℃ constant temperature forced air drying oven to dry for 8 hours.

[0034] Step 2) Plasma treatment of the heating element: The test environment is adjusted to 20℃ and 30% humidity. After removing the CNTF from the oven, it is fixed between the two insulating media in the DBD plasma treatment. The output power is adjusted to 550W, and the gap between the insulating media is adjusted to 1.5mm. High-purity air is introduced into the DBD plasma treatment chamber through the flow controller. The power is turned on, generating a large-area, high-density, low-temperature plasma between the insulating media. The power is turned off after 1 second. The CNTF heating element is flipped over and fixed, and the above steps are repeated to ensure that both sides of the heating element are uniformly activated and modified by the plasma. Then, the CNTF heating element is removed.

[0035] Step 3) For the silane-grafted plasma-activated heating element, add 100 ml of anhydrous ethanol to a 250 ml single-necked flask equipped with a spherical condenser and a magnetic stirrer. Then, add a few drops of acetic acid to the ethanol to adjust the pH to 3. Weigh 0.8 g of the CNTF heating element and 8 g of KH550 coupling agent taken in Step 2) and add them to the single-necked flask. Maintain a constant temperature reflux at 70 °C for 3 hours, and continuously stir with a magnetic stirrer to ensure complete reaction.

[0036] After the reaction, the heating element was removed and placed in anhydrous ethanol, then ultrasonically cleaned for 10 minutes, repeated twice, to thoroughly remove silane molecules remaining on the film surface and in the nanopores due to physical adsorption. Finally, the purified silane-modified CNTF was dried in a 50°C constant-temperature oven for 8 hours to remove residual solvent and promote further solidification of surface chemical bonds, ultimately obtaining a silane-modified thermoplastic composite resistance welding heating element. The prepared CNTF heating element was used for resistance welding of CF / PEEK thermoplastic composite materials, achieving a weld strength of 36.27 MPa, a 25.9% improvement compared to the unmodified heating element.

[0037] Example 3: Preparation of thermoplastic composite resistance welding heating elements using KH570 grafted with CNTF Step 1) Pretreatment of heating element: A carbon nanotube film with a sheet resistance of 2Ω / sq and a diameter of 75mm×25.4mm is used as the heating element. It is placed in acetone and cleaned with an ultrasonic cleaner for 20 minutes. After repeating twice, it is placed in a 60℃ constant temperature forced air drying oven and dried for 10 hours.

[0038] Step 2) Plasma treatment of the heating element: The test environment is adjusted to 26℃ and 50% humidity. After removing the CNTF from the oven, it is fixed between the two insulating media in the DBD plasma treatment. The output power is adjusted to 550W, and the gap between the insulating media is adjusted to 2.5mm. High-purity air is introduced into the DBD plasma treatment chamber through the flow controller. The power is turned on, generating a large-area, high-density, low-temperature plasma between the insulating media. The power is turned off after 3 seconds. The CNTF heating element is flipped over and fixed, and the above steps are repeated to ensure that both sides of the heating element are uniformly activated and modified by the plasma. Then, the CNTF heating element is removed.

[0039] Step 3) For the silane-grafted plasma-activated heating element, add 100 ml of anhydrous ethanol to a 250 ml single-necked flask equipped with a spherical condenser and a magnetic stirrer. Then, add a few drops of acetic acid to the ethanol to adjust the pH to 4. Weigh 0.8 g of the CNTF heating element and 8 g of KH550 coupling agent taken in Step 2) and add them to the single-necked flask. Maintain a constant temperature of reflux at 80 °C for 1 h, and continuously stir with a magnetic stirrer to ensure complete reaction.

[0040] After the reaction, the heating element was removed and placed in anhydrous ethanol, then ultrasonically cleaned for 20 minutes, repeated three times, to thoroughly remove silane molecules remaining on the film surface and in the nanopores due to physical adsorption. Finally, the purified silane-modified CNTF was dried in a 60℃ constant-temperature oven for 10 hours to remove residual solvent and promote further solidification of surface chemical bonds, ultimately obtaining a silane-modified thermoplastic composite resistance welding heating element. The prepared CNTF heating element was used for resistance welding of CF / PEEK thermoplastic composite materials, achieving a weld strength of 34.6 MPa, a 20.1% improvement compared to the unmodified heating element.

Claims

1. A method for preparing a resistance welding heating element of a silane-modified thermoplastic composite material, characterized in that, Includes the following steps: Step 1) Pretreatment of heating elements: The carbon nanotube film (CNTF) is cut into heating elements of a set size, ultrasonically cleaned and dried at a constant temperature to remove surface contamination and any residual moisture inside the heating elements; Step 2) Plasma surface treatment of heating element: In a constant temperature and humidity environment, dielectric barrier discharge (DBD) plasma treatment is selected; first, the power output power is set, and then the pre-treated heating element is fixed between the two insulated electrodes of the DBD. The reaction gas is introduced into the DBD cavity through the flow controller, the power is connected, and plasma discharge is performed; after one side is modified, the heating element is flipped over and fixed, and the above steps are repeated to ensure that both sides of the heating element are uniformly activated and modified by plasma; Step 3) Silane-modified plasma-activated heating element: After modification, the DBD plasma-activated heating element is removed and immediately modified with silane. First, anhydrous ethanol is selected as the solvent, and then acetic acid is added dropwise to the anhydrous ethanol to construct a stable acidic hydrolysis environment. Then, the plasma-treated heating element and silane coupling agent are added to the solvent. The uniformly mixed reaction system is stirred at a constant temperature to ensure a complete reaction. After the reaction is complete, the silane-modified plasma-activated heating element is ultrasonically cleaned until the washing solution is clear and free of residue; then it is dried at a constant temperature to remove residual solvent and promote further solidification of surface chemical bonds, finally obtaining a silane-modified thermoplastic composite resistance welding heating element.

2. The method for preparing a silane-modified thermoplastic composite resistance welding heating element according to claim 1, characterized in that, In step 2), the power output is set to 220W~550W, the electrode gap is adjusted to 1.5mm~2.5mm, and the power-on time is 1s~3s.

3. The method for preparing a silane-modified thermoplastic composite resistance welding heating element according to claim 1, characterized in that, The heating element described in step 1) has a sheet resistance of 2 Ω / sq, a thickness of 8 μm, and a density of 0.6 g·cm³. -3 The dimensions are 75mm × 25.4mm.

4. The method for preparing a silane-modified thermoplastic composite resistance welding heating element according to claim 1, characterized in that, The ultrasonic cleaning solutions mentioned in steps 1) and 3) are acetone and anhydrous ethanol, respectively. The cleaning time is 10 min to 30 min, and the cleaning is repeated 2 to 3 times.

5. The method for preparing a silane-modified thermoplastic composite resistance welding heating element according to claim 1, characterized in that, The constant temperature drying process described in steps 1) and 3) is 50℃~80℃, and the drying time is 6h~12h.

6. The method for preparing a silane-modified thermoplastic composite resistance welding heating element according to claim 1, characterized in that, The constant temperature and humidity environment mentioned in step 2) is a temperature of 23±3℃ and a humidity of 40±10%; the reaction gas is high-purity synthetic air, which is a mixture of nitrogen and oxygen in a volume ratio of 79:21, with a purity of ≥99.999% and a water content of ≤3ppm.

7. The method for preparing a silane-modified thermoplastic composite resistance welding heating element according to claim 1, characterized in that, The silane coupling agent mentioned in step 3) is one or a mixture of two or more of 3-aminopropyltriethoxysilane, 3-glycidoxypropyltrimethoxysilane, and 3-(isobutenoyloxy)propyltrimethoxysilane.

8. The method for preparing a silane-modified thermoplastic composite resistance welding heating element according to claim 1, characterized in that, The stable acidic hydrolysis environment described in step 3) is pH=3~5.

9. The method for preparing a silane-modified thermoplastic composite resistance welding heating element according to claim 1, characterized in that, In step 3), the mass ratio of CNTF, silane coupling agent and anhydrous ethanol in the reaction system is 1:10:

80.

10. The method for preparing a silane-modified thermoplastic composite resistance welding heating element according to claim 1, characterized in that, The reaction system described in step 3) has a reaction temperature of 70℃~80℃ and a reaction time of 1h~3h.

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