Composite heating element, preparation method of composite heating element and application of composite heating element in resistance welding of thermoplastic composite material

Abstract

The invention relates to a composite heating element, a preparation method of the composite heating element and application of the composite heating element in thermoplastic composite material resistance welding, and belongs to the technical field of thermoplastic composite material welding. In order to solve the problem that the strength of a welding joint is reduced due to the fact that the heating temperature distribution of a welding area of a traditional heating element is not uniform and heterogeneous inclusions exist, the invention provides a composite heating element which comprises two layers of GO / PA66 composite reinforced films and a graphite heating film arranged between the two layers of GO / PA66 composite reinforced films. Uniform heat distribution of a heating area is realized by optimizing the structural design of the composite heating element; and the longitudinal bonding force of welding is remarkably improved through introduction of the graphene oxide. The composite heating element has the thermal expansion coefficient similar to that of a thermoplastic composite material matrix, the stress distribution of a welding joint is remarkably improved, the higher-quality welding effect is provided, and the composite heating element has wide industrial application prospects in the field of thermoplastic composite material welding.

Description

Technical Field

[0001] The present invention belongs to the technical field of thermoplastic composite material welding, and particularly relates to a composite heating element, a preparation method thereof, and an application in resistance welding of thermoplastic composite materials. Background Art

[0002] With the rapid growth of the demand for lightweight and high-performance materials in the aerospace, rail transit, and automotive industries, carbon fiber reinforced thermoplastic composites (CFRTP) have gradually become the preferred materials for structural components due to their excellent properties such as high specific strength, high specific modulus, good fatigue resistance, and corrosion resistance. However, due to the certain complexity in the forming process of CFRTP, the manufacturing of large or complex structural components often requires the use of efficient and reliable connection technologies to assemble multiple components into a complete structure. Traditional connection methods, such as mechanical fastening and adhesive bonding, have obvious disadvantages. For mechanical fastening, the common challenges are stress concentration and possible galvanic corrosion. For adhesive bonding, a large amount of surface treatment and a long curing cycle are required.

[0003] Existing thermoplastic composite material welding technologies mainly include induction welding, ultrasonic welding, laser welding, and resistance welding, etc. Among them, resistance welding has become the focus of research and application due to its characteristics such as high efficiency, controllability, and wide application range. Resistance welding is based on the Joule effect, and uses the heat generated by the heating element to heat the resin material at the connection part to the molten state, and realizes the connection under the condition of applying pressure. Currently, traditional heating elements commonly used in resistance welding of thermoplastic composite materials include metal meshes mainly composed of stainless steel meshes, carbon fiber materials, and other special heating elements. However, the traditional heating elements have the following problems in practical applications:

[0004] 1. Heterogeneous inclusion problem: Traditional metal mesh heating elements are prone to introducing heterogeneous metal elements, resulting in a decrease in the corrosion resistance and fatigue resistance of the joints.

[0005] 2. Uneven heat distribution: The conductivity or thermal conductivity of traditional heating elements is uneven, which is likely to cause local overheating or cold spots at the welding interface, affecting the welding strength and quality.

[0006] 3. Poor material compatibility: The difference in the coefficient of thermal expansion between the metal heating element and the CFRTP matrix material is relatively large, which is likely to generate welding residual stress and joint failure.

[0007] In addition, when traditional heating elements are used for resistance welding of CFRTP, the most significant problem is the "edge effect". The so-called "edge effect" refers to the phenomenon that due to the low efficiency of natural convective heat transfer between the heating element and the air, the temperature of the part exposed to the air rises significantly, forming an obvious temperature gradient. During the welding process, the temperature in the interface edge region is often higher than that in the internal region, resulting in overheating degradation at the ends, increased joint deformation, and a decrease in welding strength. This phenomenon of edge overheating directly affects the uniformity and quality of the welded joint and becomes one of the key bottlenecks restricting the further development of resistance welding technology. Summary of the Invention

[0008] To solve the problems of uneven heating temperature distribution in the welding area and a decrease in the strength of the welded joint caused by heterogeneous inclusions in traditional heating elements, the present invention provides a composite heating element, a preparation method thereof, and an application in the resistance welding of thermoplastic composites.

[0009] Technical solution of the present invention:

[0010] A composite heating element, the composite heating element has a sandwich structure, including two layers of GO / PA66 composite strengthening films and a graphite heating film disposed between the two layers of GO / PA66 composite strengthening films. A plurality of parallel long grooves are provided on the graphite heating film, the depth of the long grooves penetrates through the graphite heating film, the long grooves are parallel to the axial direction of the graphite heating film, and the width of the long grooves is 1 - 1.5 mm.

[0011] A preparation method of a composite heating element, including the following steps:

[0012] Step 1: Prepare a graphite heating film:

[0013] Coat polyacrylic acid on a film-making substrate, and obtain a polyimide film after heating and curing. Raise the obtained polyimide film to the carbonization temperature in a gradient heating manner, keep it warm for a certain time to obtain a carbonized polyimide film, and then raise the obtained carbonized polyimide film to the pyrolysis temperature in a gradient heating manner, keep it warm for a certain time to obtain a pyrolyzed polyimide film. Remove the obtained pyrolyzed polyimide film from the film-making substrate, and obtain a graphite film after rolling. Ablate a plurality of long grooves parallel to the axial direction of the graphite heating film and penetrating through the graphite film on the obtained graphite film by nanosecond laser technology to obtain a graphite heating film;

[0014] Step 2: Prepare a GO / PA66 composite strengthening film:

[0015] Toluene, adipic acid and thionyl chloride were mixed and adipoyl chloride was prepared at a certain reaction temperature. After the reaction was completed, excessive thionyl chloride and toluene were removed by vacuum distillation to obtain an adipoyl chloride solution. Hexamethylenediamine, sodium hydroxide and graphene oxide suspension were mixed and ultrasonically dispersed to obtain a hexamethylenediamine-GO suspension. The obtained adipoyl chloride solution was added to the obtained hexamethylenediamine-GO suspension, and the flocculent PA66 / GO polymer formed at the two-phase interface was picked out, washed and dried to obtain a GO / PA66 composite material. The obtained GO / PA66 composite material was dissolved in formic acid, and fully stirred and dispersed to obtain a formic acid dispersion. The obtained formic acid dispersion was coated and air-dried to obtain a GO / PA66 composite reinforced film;

[0016] Step 3: Prepare a composite heating element:

[0017] One layer of the GO / PA66 composite reinforced film obtained in Step 2 was placed on each side of the graphite heating film obtained in Step 1, and a sandwich-structured composite heating element was obtained after hot pressing.

[0018] Further, the temperature of the heating and curing in Step 1 was 70-90 °C, and the curing time was 1-2 h; the polyimide film obtained was raised to the carbonization temperature in a gradient heating manner at a heating rate of 3-5 °C / min from room temperature to 400 °C, and then at a heating rate of 1-3 °C / min from 400 °C to 800 °C, and then at a heating rate of 0.5-1 °C / min from 800 °C to the carbonization temperature of 1500 °C, and the holding time at the carbonization temperature of 1500 °C was 30-40 min;

[0019] The carbonized polyimide film obtained was raised to the pyrolysis temperature in a gradient heating manner at a heating rate of 5-10 °C / min from room temperature to 1500 °C, and then at a heating rate of 3-5 °C / min from 1500 °C to 2400 °C, and then at a heating rate of 0.5-1 °C / min from 2400 °C to the pyrolysis temperature of 3100 °C, and the holding time at the pyrolysis temperature of 3100 °C was 30-40 min.

[0020] Further, the pressure of the calendering in Step 1 was 20-25 MPa, the calendering time was 2-3 h, and the thickness of the calendered graphite film was 35-50 μm.

[0021] Further, the frequency of the nanosecond laser in Step 1 was 120 Khz, the power was 60 W, the pulse width was 200 ns, and the processing speed was 50 mm / s; the distance between the long grooves was 0.5 mm.

[0022] Further, the volume molar ratio of toluene, adipic acid, thionyl chloride, ethylenediamine and sodium hydroxide described in Step 2 is 75 - 100 mL: 1 moL: 2 - 3 moL: 1 moL: 2 - 3 moL, the reaction temperature is 80 - 90 °C, the temperature of vacuum distillation is 75 - 80 °C, and the pressure of vacuum distillation is 0.5 - 0.8 atm.

[0023] Further, the concentration of graphene oxide in the hexamethylenediamine - GO suspension described in Step 2 is 0.2 wt%, the time of ultrasonic dispersion is 30 - 45 min, and the drying temperature is 80 - 100 °C for drying for 20 - 28 h.

[0024] Further, the sufficient stirring described in Step 2 is stirring at a speed of 250 - 300 rpm for 4 - 6 h; the concentration of the GO / PA66 composite material in the obtained formic acid dispersion is 20 - 30 wt%, the air drying is air drying at room temperature for 24 - 36 h; the thickness of the GO / PA66 composite reinforced film is 50 - 80 μm.

[0025] Further, the hot pressing pressure described in Step 3 is 15 - 30 MPa, the hot pressing temperature is 180 - 250 °C, and the hot pressing time is 12 - 20 min

[0026] Application of a composite heating element in the resistance welding of thermoplastic composites.

[0027] Further, the thermoplastic composite material is a carbon fiber reinforced thermoplastic composite material CFRTP.

[0028] Advantages of the present invention:

[0029] By optimizing the structural design of the composite heating element, the present invention provides a number of deep - penetrating long grooves parallel to the axial direction of the graphite heating film on the graphite heating film, which improves the flow of the melted thermoplastic composite material, realizes uniform heat distribution in the heating area, and avoids the local overheating phenomenon that easily occurs in traditional metal heating elements.

[0030] Through the in - situ composite technology, the present invention uniformly disperses graphene oxide in polyamide 66 (PA66). A variety of oxygen - containing functional groups on graphene oxide can form hydrogen bonds with the thermoplastic composite material. The high toughness of graphene oxide enables it to act as a "bridge" to provide mechanical support between the graphite heating film and the thermoplastic composite material. The introduction of graphene oxide generates non - covalent stacking interactions, namely π - π interactions, which significantly improves the longitudinal bonding force of welding and alleviates the problem of poor longitudinal thermal conductivity of the graphite heating film.

[0031] The composite heating element prepared by the present invention with a graphite heating film and a GO / PA66 composite strengthening film has a coefficient of thermal expansion similar to that of the thermoplastic composite matrix, significantly improves the stress distribution of the welded joint, is not prone to cracking of the welded joint, avoids joint failure caused by expansion differences, and provides a higher quality welding effect.

[0032] The preparation process of the composite heating element of the present invention is simple and easy to control, has low energy consumption, and the raw materials are non-toxic and environmentally friendly. Compared with the processing of traditional metal mesh heating elements, it does not require complex metal mesh weaving and surface treatment processes, and avoids the introduction of heterogeneous metal elements. The composite heating element provided by the present invention has broad industrial application prospects in the field of thermoplastic composite welding. Brief Description of the Drawings

[0033] Figure 1 Fourier infrared spectrogram of the GO / PA66 composite strengthening film and PA66 film prepared in Example 1;

[0034] Figure 2 Thermogravimetric curve of the GO / PA66 composite strengthening film and PA66 film prepared in Example 1;

[0035] Figure 3 Lap shear strength diagram of the GO / PA66 composite strengthening film and PA66 film prepared in Example 1;

[0036] Figure 4 Physical photograph of the composite heating element prepared in Example 1;

[0037] Figure 5 Physical photograph of the joint of the composite heating element prepared in Example 1 for resistance welding of thermoplastic composites;

[0038] Figure 6 Schematic diagram of the plate-to-plate lap method of the composite heating element provided by the present invention for resistance welding;

[0039] Figure 7 Schematic diagram of the circuit connection of the composite heating element provided by the present invention for resistance welding;

[0040] In the figure: 1. Graphite heating film; 2. GO / PA66 composite strengthening film; 3. Base material to be welded; 4. Composite heating element; 5. Ceramic fixture; 6. Copper electrode; 7. DC power supply; 8. Wire. Detailed Embodiments

[0041] The technical solutions of the present invention will be further described below in conjunction with embodiments, but are not limited thereto. Any modification or equivalent replacement of the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention shall be covered by the protection scope of the present invention. For the process equipment or devices not specifically noted in the following embodiments, conventional equipment or devices in the art are used. If not specifically specified, the raw materials used in the embodiments of the present invention can be obtained commercially; if not specifically specified, the technical means used in the embodiments of the present invention are all conventional means well known to those skilled in the art.

[0042] Example 1

[0043] This embodiment provides a composite heating element for resistance welding of thermoplastic composites and a preparation method thereof.

[0044] The preparation method of the composite heating element in this embodiment includes the following steps:

[0045] Step 1: Prepare a graphite heating film:

[0046] Using a graphite film as the film-making substrate, polyacrylic acid is coated on the graphite film, and it is placed in a muffle furnace and cured at 80 °C for 1 h to obtain a polyimide film. The obtained polyimide film is heated from room temperature to 400 °C at a heating rate of 3 °C / min, then from 400 °C to 800 °C at a heating rate of 1 °C / min, and then from 800 °C to 1500 °C at a heating rate of 0.5 °C / min, and finally kept at 1500 °C for 30 min to obtain a carbonized polyimide film;

[0047] The obtained carbonized polyimide film is heated from room temperature to 1500 °C at a heating rate of 5 °C / min, then from 1500 °C to 2400 °C at a heating rate of 3 °C / min, and then from 2400 °C to 3100 °C at a heating rate of 0.5 °C / min, and finally kept at 3100 °C for 30 min to realize the graphitization of carbon atoms in the polyimide film, obtaining a pyrolytic polyimide film;

[0048] The obtained pyrolytic polyimide film is peeled off from the graphite film substrate, calendered at 20 MPa for 2 h to obtain a graphite film with a thickness of 35 μm, a length of 50 mm, and a width of 12 mm, and then a hydrophobic PE film is coated on the surface of the graphite film by a coating machine to maintain the structural integrity of the graphite film;

[0049] Eight long grooves that penetrate the graphite film deeply and are parallel to the axial direction of the graphite heating film are ablated at the center position of the obtained graphite film by nanosecond laser technology to obtain a graphite heating film.

[0050] The frequency of the nanosecond laser is 120Khz, the power is 60W, the pulse width is 200ns, and the processing speed is 50mm / s. The width of the long grooves is all 1mm, the spacing between the long grooves is all 0.5mm, and the length of the long grooves is all 25mm.

[0051] Step 2: Prepare the GO / PA66 composite reinforced film:

[0052] Add 75mL of toluene to a round-bottom flask, add 1moL of adipic acid, and then slowly add 2moL of thionyl chloride. During the addition of thionyl chloride, install a straight condenser above the round-bottom flask, pass cold water and use an ice bath to control the temperature of the reaction system at 0 - 10°C. After complete feeding, raise the temperature of the reaction system to 80°C. A large amount of sulfur dioxide and hydrogen chloride gases will be generated during the reaction, so the reaction must be ensured to be carried out in a fume hood, and a nitrogen generator is connected to the reactor to continuously introduce nitrogen to ensure an anaerobic environment.

[0053] React at 80°C until no more gas escapes. After the reaction is completed, control the temperature of the reaction system at 75°C, and distill off the excess thionyl chloride and toluene under a reduced pressure of 0.5atm to obtain an adipoyl chloride solution.

[0054] Add 1moL of hexamethylenediamine and 2moL of sodium hydroxide to a beaker, add a 1wt% GO (graphene oxide) suspension, and add deionized water to make up to 75mL, so that the content of GO in the resulting mixed system is 0.2wt%. Ultrasonically disperse for 30min to ensure uniform dispersion of GO, and obtain a hexamethylenediamine-GO suspension.

[0055] Slowly add the adipoyl chloride solution to the hexamethylenediamine-GO suspension to ensure the slow formation of PA66 / GO polymer at the two-phase interface. Use a spatula to pick out the generated flocculates at the interface, place the flocculates in an ultrasonic cleaner, ultrasonically wash the flocculates with deionized water as the cleaning solution, and then dry at 90°C for 24h to obtain the GO / PA66 composite material.

[0056] Dissolve the obtained GO / PA66 composite material in formic acid, mechanically stir at a speed of 300rpm for 5h to completely disperse the GO / PA66 composite material in formic acid, obtain a formic acid dispersion with a GO / PA66 composite material concentration of 25wt%, coat the formic acid dispersion on glass with a coating brush, the coating thickness is 50μm, and dry at room temperature for 24h to obtain a GO / PA66 composite reinforced film with a thickness of 50μm, a length of 25mm, and a width of 12mm.

[0057] Step 3: Prepare the composite heating element:

[0058] Remove the hydrophobic PE film covering the graphite heating film obtained in Step 1. Place a layer of the GO / PA66 composite reinforcement film obtained in Step 2 on each side of the graphite heating film obtained in Step 1. When placing, leave an electrode clamping area on both sides of the graphite heating film, and perform hot pressing with a hot press. The hot pressing pressure is 20 MPa, the hot pressing temperature is 200 °C, and the hot pressing time is 15 min to obtain a composite heating element with a sandwich structure.

[0059] Example 2

[0060] This example provides a composite heating element for resistance welding of thermoplastic composites and a preparation method thereof.

[0061] The preparation method of the composite heating element in this example includes the following steps:

[0062] Step 1. Prepare a graphite heating film:

[0063] Using a graphite film as the film-making substrate, coat polyacrylic acid on the graphite film, place it in a muffle furnace and cure it at 70 °C for 1 h to obtain a polyimide film. The obtained polyimide film is heated from room temperature to 400 °C at a heating rate of 4 °C / min, then heated from 400 °C to 800 °C at a heating rate of 2 °C / min, then heated from 800 °C to 1500 °C at a heating rate of 1 °C / min, and finally kept at 1500 °C for 30 min to obtain a carbonized polyimide film;

[0064] Heat the obtained carbonized polyimide film from room temperature to 1500 °C at a heating rate of 8 °C / min, then heat from 1500 °C to 2400 °C at a heating rate of 4 °C / min, then heat from 2400 °C to 3100 °C at a heating rate of 1 °C / min, and finally keep at 3100 °C for 30 min to realize the graphitization of carbon atoms in the polyimide film and obtain a pyrolytic polyimide film;

[0065] Peel the obtained pyrolytic polyimide film from the graphite film substrate, roll it at 25 MPa for 2 h to obtain a graphite film with a thickness of 40 μm, a length of 50 mm, and a width of 12 mm. Then use a coating machine to coat a layer of hydrophobic PE film on the surface of the graphite film to maintain the structural integrity of the graphite film;

[0066] Use nanosecond laser technology to ablate 7 long grooves parallel to the axial direction of the graphite heating film and penetrating the graphite film deeply at the center position of the obtained graphite film to obtain a graphite heating film.

[0067] The nanosecond laser frequency is 120 Khz, the power is 60 W, the pulse width is 200 ns, and the processing speed is 50 mm / s. The width of each long groove is 1.2 mm, the spacing between long grooves is 0.5 mm, and the length of each long groove is 25 mm.

[0068] Step 2: Preparation of GO / PA66 composite reinforced film:

[0069] Add 75 mL of toluene into a round-bottom flask, add 1 moL of adipic acid, and then slowly add 2 moL of thionyl chloride. During the addition of thionyl chloride, set a straight condenser above the round-bottom flask, pass cold water and use an ice bath to control the temperature of the reaction system at 0 - 10 °C. After complete feeding, raise the temperature of the reaction system to 90 °C. A large amount of sulfur dioxide and hydrogen chloride gases will be generated during the reaction, so the reaction should be ensured to be carried out in a fume hood, and a nitrogen generator should be connected to the reactor to continuously introduce nitrogen to ensure an anaerobic environment.

[0070] React at 90 °C for 5 h until no more gas escapes. After the reaction is completed, control the temperature of the reaction system at 80 °C, and distill off the excess thionyl chloride and toluene under a reduced pressure of 0.6 atm to obtain an adipoyl chloride solution.

[0071] Add 1 moL of hexamethylenediamine and 2 moL of sodium hydroxide into a beaker, add a 1 wt% GO (graphene oxide) suspension, and make up to 75 mL with deionized water so that the content of GO in the resulting mixed system is 0.2 wt%. Ultrasonically disperse for 40 min to ensure uniform dispersion of GO and obtain a hexamethylenediamine-GO suspension.

[0072] Slowly add the adipoyl chloride solution into the hexamethylenediamine-GO suspension to ensure the slow formation of PA66 / GO polymer at the two-phase interface. Use a spatula to pick out the generated floccules at the interface, place the floccules in an ultrasonic cleaner, and ultrasonically wash the floccules with deionized water as the cleaning solution, and then dry at 90 °C for 25 h to obtain GO / PA66 composite material.

[0073] Dissolve the obtained GO / PA66 composite material in formic acid, and mechanically stir at a speed of 250 rpm for 6 h to completely disperse the GO / PA66 composite material in formic acid, obtaining a formic acid dispersion with a GO / PA66 composite material concentration of 20 wt%. Apply the formic acid dispersion on the glass with a brush, with a coating thickness of 50 μm, and air-dry at room temperature for 36 h to obtain a GO / PA66 composite reinforced film with a thickness of 50 μm, a length of 25 mm, and a width of 12 mm.

[0074] Step 3: Preparation of composite heating element:

[0075] Remove the hydrophobic PE film covering the graphite heating film obtained in Step 1. Place a layer of the GO / PA66 composite reinforced film obtained in Step 2 on each side of the graphite heating film obtained in Step 1, leaving an electrode clamping area on both sides of the graphite heating film during placement. Perform hot pressing with a hot press, with a hot pressing pressure of 15 MPa, a hot pressing temperature of 250 °C, and a hot pressing time of 20 min to obtain a composite heating element with a sandwich structure.

[0076] Example 3

[0077] This example provides a composite heating element for resistance welding of thermoplastic composites and a preparation method thereof.

[0078] The preparation method of the composite heating element in this example includes the following steps:

[0079] Step 1: Prepare a graphite heating film:

[0080] Using a graphite film as the film-making substrate, polyacrylic acid is coated on the graphite film, and it is placed in a muffle furnace and cured at 90 °C for 1 h to obtain a polyimide film. The obtained polyimide film is heated from room temperature to 400 °C at a heating rate of 5 °C / min, then from 400 °C to 800 °C at a heating rate of 3 °C / min, and then from 800 °C to 1500 °C at a heating rate of 1 °C / min. Finally, it is kept at 1500 °C for 30 min to obtain a carbonized polyimide film;

[0081] The obtained carbonized polyimide film is heated from room temperature to 1500 °C at a heating rate of 10 °C / min, then from 1500 °C to 2400 °C at a heating rate of 5 °C / min, and then from 2400 °C to 3100 °C at a heating rate of 1 °C / min. Finally, it is kept at 3100 °C for 30 min to realize the graphitization of carbon atoms in the polyimide film, and a pyrolytic polyimide film is obtained;

[0082] The obtained pyrolytic polyimide film is peeled off from the graphite film substrate and calendered at 20 MPa for 3 h to obtain a graphite film with a thickness of 50 μm, a length of 50 mm, and a width of 12 mm. Then, a hydrophobic PE film is coated on the surface of the graphite film by a coating machine to maintain the structural integrity of the graphite film;

[0083] Six long grooves that penetrate the graphite film deeply and are parallel to the axial direction of the graphite heating film are ablated at the center position of the obtained graphite film by nanosecond laser technology to obtain a graphite heating film.

[0084] The nanosecond laser frequency is 120 Khz, the power is 60 W, the pulse width is 200 ns, and the processing speed is 50 mm / s. The width of each long groove is 1.5 mm, the spacing between long grooves is 0.5 mm, and the length of each long groove is 25 mm.

[0085] Step 2: Prepare a GO / PA66 composite strengthening film:

[0086] Add 75 mL of toluene to a round-bottom flask, add 1 moL of adipic acid, and then slowly add 2 moL of thionyl chloride. During the addition of thionyl chloride, set up a straight condenser above the round-bottom flask, pass cold water, and use an ice bath to control the temperature of the reaction system at 0 - 10 °C. After complete feeding, raise the temperature of the reaction system to 90 °C. The reaction will produce a large amount of sulfur dioxide and hydrogen chloride gases, so the reaction must be carried out in a fume hood, and a nitrogen generator should be connected to the reactor to continuously introduce nitrogen to ensure an anaerobic environment.

[0087] React at 90 °C for 5 h until no more gas escapes. After the reaction is completed, control the temperature of the reaction system at 80 °C, and distill off the excess thionyl chloride and toluene under a reduced pressure of 0.8 atm to obtain an adipoyl chloride solution.

[0088] Add 1 moL of hexamethylenediamine and 2 moL of sodium hydroxide to a beaker, add a 1 wt% GO (graphene oxide) suspension, and add deionized water to make up to 75 mL so that the content of GO in the resulting mixed system is 0.2 wt%. Ultrasonically disperse for 45 min to ensure uniform dispersion of GO and obtain a hexamethylenediamine-GO suspension.

[0089] Slowly add the adipoyl chloride solution to the hexamethylenediamine-GO suspension to ensure the slow formation of PA66 / GO polymer at the two-phase interface. Use a spatula to pick out the generated floccules at the interface, place the floccules in an ultrasonic cleaner, ultrasonically wash the floccules with deionized water as the cleaning solution, and then dry at 100 °C for 20 h to obtain the GO / PA66 composite material.

[0090] Dissolve the obtained GO / PA66 composite material in formic acid, mechanically stir at a speed of 300 rpm for 4 h to completely disperse the GO / PA66 composite material in formic acid, obtain a formic acid dispersion with a GO / PA66 composite material concentration of 30 wt%, apply the formic acid dispersion on the glass with a brush, with a coating thickness of 50 μm, and dry at room temperature for 24 h to obtain a GO / PA66 composite reinforced film with a thickness of 80 μm, a length of 25 mm, and a width of 12 mm.

[0091] Step 3: Prepare a composite heating element:

[0092] Remove the hydrophobic PE film covering the graphite heating film obtained in Step 1. Place a layer of the GO / PA66 composite reinforced film obtained in Step 2 on each side of the graphite heating film obtained in Step 1. When placing, leave an electrode clamping area on both sides of the graphite heating film, and perform hot pressing with a hot press. The hot pressing pressure is 30 MPa, the hot pressing temperature is 180 °C, and the hot pressing time is 12 min to obtain a composite heating element with a sandwich structure.

[0093] Comparative Example 1

[0094] This comparative example provides a pure PA66 film, and the specific preparation method is as follows:

[0095] Add 75 mL of toluene into a round-bottom flask, add 1 moL of adipic acid, and then slowly add 2 moL of thionyl chloride. During the addition of thionyl chloride, install a straight condenser above the round-bottom flask, pass cold water, and use an ice bath to control the temperature of the reaction system at 0-10 °C. After complete feeding, raise the temperature of the reaction system to 80 °C. A large amount of sulfur dioxide and hydrogen chloride gases will be generated during the reaction, so the reaction should be ensured to be carried out in a fume hood, and a nitrogen generator should be connected to the reactor to continuously introduce nitrogen to ensure an anaerobic environment.

[0096] React at 80 °C until no more gas escapes. After the reaction is completed, control the temperature of the reaction system at 75 °C, and distill off the excess thionyl chloride and toluene under a reduced pressure of 0.5 atm to obtain an adipoyl chloride solution.

[0097] Add 1 moL of hexamethylenediamine and 2 moL of sodium hydroxide into a beaker, and make up to 75 mL with deionized water to obtain a hexamethylenediamine solution.

[0098] Slowly add the adipoyl chloride solution into the hexamethylenediamine solution to ensure that PA66 flocs are slowly formed at the two-phase interface. Place the flocs in an ultrasonic cleaner, and ultrasonically wash the flocs with deionized water as the cleaning solution, and then dry at 90 °C for 24 h to obtain PA66.

[0099] Dissolve the obtained PA66 in formic acid, and mechanically stir at a speed of 300 rpm for 5 h to completely disperse PA66 in formic acid to obtain a formic acid dispersion with a PA66 concentration of 25 wt%. Coat the formic acid dispersion on glass with a coating brush, with a coating thickness of 50 μm, and dry at room temperature for 24 h to obtain a PA66 film with a thickness of 50 μm, a length of 25 mm, and a width of 12 mm. Hot-press two layers of PA66 films with a hot-press pressure of 20 MPa, a hot-press temperature of 200 °C, and a hot-press time of 15 min to obtain a pure PA66 film.

[0100] Investigate the performance of the graphite heating film, GO / PA66 composite strengthening film, and composite heating element prepared in Example 1, and the results are as follows:

[0101] I. Thermoelectric properties of the graphite heating film:

[0102] Use an LFA447 laser thermal conductivity tester to measure the transverse and longitudinal thermal conductivities of the graphite heating film prepared in Example 1, and characterize the thermal performance of the graphite heating film. In addition, use an RTS-8 four-probe square resistance tester to evaluate the resistivity of the graphite heating film to provide the key thermoelectric parameters of the composite heating element. The results are shown in Table 1.

[0103] Table 1

[0104]

[0105] Through the analysis of the thermal conductivity shown in Table 1, it is found that the transverse thermal conductivity of the graphite heating film is much higher than the longitudinal thermal conductivity, indicating that the heat conduction of the graphite film is mainly concentrated in the plane direction and is suitable for use in planar heating elements. The in-plane thermal conductivity of the heating element measured at 594.34 W / m·K allows for rapid heat transfer along the plane of the heating element. This high thermal conductivity ensures uniform heating of the entire welding interface and promotes consistent melting of the thermoplastic composite layer. In addition, the resistivity of the graphite heating film is 31 mΩ·cm, which contributes to efficient Joule heating and enables rapid and precise heat generation. This low resistivity enables the welding interface to quickly reach the melting point of the thermoplastic composite, improving the welding efficiency. This ability ensures uniform material fusion, improves joint quality, and minimizes potential defects caused by non-uniform heat distribution.

[0106] II. Fourier Transform Infrared Spectroscopy Analysis of GO / PA66 Composite Reinforced Film

[0107] The Fourier transform infrared spectra of the GO / PA66 composite reinforced film prepared in Example 1 and the pure PA66 film prepared in Comparative Example 1 were measured using a Nicolet 380 Fourier transform infrared spectrometer; to analyze the role of in-situ polymerized graphene oxide in PA66.

[0108] The results are as Figure 1 shown. After the formation of the GO / PA66 composite, the bands at 1638 cm -1 and 1534 cm -1 correspond to the C=O stretching vibration of amide I and the C-N stretching and N-H bending vibrations of amide II, respectively. The CH2-NH bond and N-H in-plane bending vibrations of amide III appear at 1261 cm -1 The bands for the O=C-N bending vibration of amide IV and the out-of-plane bending vibration of amide V overlap in the wavenumber range from 723 cm -1 to 687 cm -1 The amide VI is related to the out-of-plane bending vibration of C=O at 573 cm -1 The chemical structure of PA66 also appears as other bands in the spectrum. The bands at 3385 cm -1 and 3305 cm -1 are both attributed to the N–H stretching vibration, and the lower wavenumber indicates the influence of intermolecular or intramolecular hydrogen bonds in PA66 molecules. Since the addition of GO or has little effect on the N–H stretching vibration of PA66, and this effect is not caused by amide bonds, therefore, based on 3385 cm -1The band intensity at [specific location] was compared with the band intensity at 1534 cm of amide II -1 to estimate the amide bond content in PA66 and the GO / PA66 composite reinforced film. The results showed that the amide bond content in the GO / PA66 composite reinforced film was more than that in PA66. This indicated that the formation of amide bonds was promoted during the in-situ polymerization of the GO / PA66 composite reinforced film.

[0109] In addition, through the study of the Fourier transform infrared spectroscopy peak positions of the PA66 film and the GO / PA66 composite reinforced film, it was found that some characteristic peaks of PA66 underwent red shifts, which were 3301, 1640, and 1540 cm -1 , respectively. This indicated that the chemical environment of the amide group was affected by the graphene oxide filler, and covalent bonds were formed between the carboxylic acid groups of graphene oxide and the amide groups of the PA66 matrix, proving that the binding force of the GO / PA66 composite reinforced film was enhanced.

[0110] III. Thermal Property Analysis of GO / PA66 Composite Reinforced Film

[0111] The thermogravimetric curves of the GO / PA66 composite reinforced film prepared in Example 1 and the pure PA66 film prepared in Comparative Example 1 were measured using an FTA449F5 synchronous thermal analyzer to characterize the thermal properties of the GO / PA66 composite reinforced film for evaluating the thermal properties of the composite heating element.

[0112] The results were as Figure 2 shown. When thermogravimetric analysis was performed on the GO / PA66 composite reinforced film, it was significantly found that after compounding with graphene oxide, the thermal decomposition temperature increased. The highest thermal decomposition temperature of the reinforced film was 393 °C, which was 24 °C higher than that of the pure PA66 film at 368 °C. This indicated that the enhancement effect of the graphene oxide-based GO / PA66 composite reinforced film not only depended on high dispersibility and strong interfacial interactions but also on other factors. The oriented two-dimensional filler effectively restricted the molecular motion of PA66 through interfacial interactions of amide bonds and effectively hindered the mass transfer of volatile decomposition products. In addition, when the 2D filler was aligned with the plane direction, a continuous layer with carbon generated by the pyrolysis of PA66 could be rapidly constructed.

[0113] IV. Analysis of Resistance Welding Effect of Composite Heating Element

[0114] Using carbon fiber reinforced thermoplastic composite CFRTP as the material to be welded, the material to be welded and the composite heating element prepared in Example 1 were installed on the Figure 6 plate-to-plate lap joint mode of Figure 7 the thermoplastic composite resistance welding fixture, and the pressure and fixation were provided by a cylinder. After checking that all parts were normal, welding work was carried out separately, adjusting the voltage and current of the power supply to control the power density at 133,000 W / m2 , adjust the cylinder pressure to 3 MPa, control the welding time at 50 s, and cool for 10 s after welding to obtain the welded product.

[0115] Using carbon fiber reinforced thermoplastic composite CFRTP as the material to be welded, install the material to be welded and the graphite heating film prepared in Example 1 according to Figure 6 the plate-to-plate lap joint mode on Figure 7 the thermoplastic composite resistance welding fixture, and use the cylinder to provide pressure and fixation. After checking that all parts are normal, carry out the welding work separately, adjust the voltage and current of the power supply, and control the power density to be 133,000 W / m 2 , adjust the cylinder pressure to 3 MPa, control the welding time at 50 s, and cool for 10 s after welding to obtain the comparative welded product.

[0116] After the specimen machining is completed, conduct a single-lap shear test according to the ASTM D5868 standard. The welding test method is to conduct a tensile shear strength test using an Instron 5967 30KN universal material testing machine. This test measures the strength of the material by applying a unidirectional tensile force. During the test, 3 parallel splines are selected for each group of tests for tensile testing. The tensile rate is 200 mm / min. The samples are clamped in the fixture and stretched along the long axis direction of the testing machine. The strength of the joint is calculated by processing the mechanical data, and the results are as Figure 3 shown.

[0117] It can be seen from the comparison that the tensile shear strength of the welded joint without the GO / PA66 composite strengthening film is 18.1 MPa. The tensile shear strength of the welded joint with the GO / PA66 composite strengthening film is increased to 19.8 MPa, showing a good strengthening effect. The increase in this strength of about 9.4% can be attributed to the additional functional groups provided by graphene oxide in the GO / PA66 composite strengthening film, which enhances the chemical and physical interactions between the CF / PA66 layers.

Claims

1. A composite heating element, characterized in that: The composite heating element is a sandwich structure, including two layers of GO / PA66 composite strengthening films and a graphite heating film arranged between the two layers of GO / PA66 composite strengthening films. The graphite heating film is provided with a plurality of parallel long grooves, the depth of the long grooves penetrates the graphite heating film, the long grooves are parallel to the axial direction of the graphite heating film, and the width of the long grooves is 1 to 1.5 mm.

2. A method for preparing the composite heating element according to claim 1, characterized in that: The steps include: Step 1: Prepare graphite heating film: Coating polyacrylic acid on a film-making substrate, heating and curing to obtain a polyimide film, heating the obtained polyimide film to a carbonization temperature in a gradient heating manner, keeping the temperature for a certain period of time to obtain a carbonized polyimide film, then heating the obtained carbonized polyimide film to a pyrolysis temperature in a gradient heating manner, keeping the temperature for a certain period of time to obtain a pyrolyzed polyimide film, removing the obtained pyrolyzed polyimide film from the film-making substrate, rolling to obtain a graphite film, and ablating a plurality of long grooves parallel to the axial direction of the graphite heating film and penetrating the graphite film in depth on the obtained graphite film by nanosecond laser technology to obtain a graphite heating film; Step 2: Preparation of GO / PA66 composite reinforced film: Toluene, adipic acid and thionyl chloride are mixed to prepare adipoyl chloride at a certain reaction temperature, and after the reaction is completed, excess thionyl chloride and toluene are removed by reduced pressure distillation to obtain an adipoyl chloride solution, hexamethylenediamine, sodium hydroxide and graphene oxide suspension are mixed and ultrasonically dispersed to obtain a hexamethylenediamine-GO suspension, the obtained adipoyl chloride solution is added to the obtained hexamethylenediamine-GO suspension, flocculent PA66 / GO polymer generated at the interface of the two phases is picked, washed and dried to obtain a GO / PA66 composite material, the obtained GO / PA66 composite material is dissolved in formic acid, fully stirred and dispersed to obtain a formic acid dispersion, the obtained formic acid dispersion is coated and then dried to obtain a GO / PA66 composite reinforcement membrane; Step 3: Preparation of composite heating element: A layer of the GO / PA66 composite reinforcement film obtained in step 2 is placed on both sides of the graphite heating film obtained in step 1, and a composite heating element with a sandwich structure is obtained after hot pressing.

3. The method for preparing the composite heating element according to claim 2, characterized in that: The temperature of the heating curing in step 1 is 70-90°C, and the curing time is 1-2h; the temperature of the obtained polyimide film is raised to the carbonization temperature by the gradient heating method, which is from room temperature to 400°C at a heating rate of 3-5°C / min, then from 400°C to 800°C at a heating rate of 1-3°C / min, and then from 800°C to the carbonization temperature of 1500°C at a heating rate of 0.5-1°C / min, and the heat preservation time at the carbonization temperature of 1500°C is 30-40min; The obtained carbonized polyimide film is raised to the pyrolysis temperature in a gradient heating manner, wherein the temperature is raised from room temperature to 1500°C at a heating rate of 5 to 10°C / min, then from 1500°C to 2400°C at a heating rate of 3 to 5°C / min, and then from 2400°C to the pyrolysis temperature of 3100°C at a heating rate of 0.5 to 1°C / min. The insulation time at the pyrolysis temperature of 3100°C is 30 to 40 minutes.

4. The method for preparing the composite heating element according to claim 2 or 3, characterized in that: The calendering pressure in step 1 is 20-25 MPa, the calendering time is 2-3 h, and the thickness of the graphite film obtained by calendering is 35-50 μm.

5. The method for preparing the composite heating element according to claim 4, characterized in that: In step 1, the frequency of the nanosecond laser is 120Khz, the power is 60W, the pulse width is 200ns, and the processing speed is 50mm / s; the spacing between the long grooves is 0.5mm.

6. The method for preparing the composite heating element according to claim 5, characterized in that: In step 2, the volume molar ratio of toluene, adipic acid, thionyl chloride, ethylenediamine and sodium hydroxide is 75-100mL:1moL:2-3moL:1moL:2-3moL, the reaction temperature is 80-90°C, the temperature of the reduced pressure distillation is 75-80°C, and the pressure of the reduced pressure distillation is 0.5-0.8atm.

7. The method for preparing the composite heating element according to claim 6, characterized in that: Step 2: The concentration of graphene oxide in the hexamethylenediamine-GO suspension is 0.2wt%, the ultrasonic dispersion time is 30-45min, and the drying temperature is 80-100°C for 20-28h.

8. The method for preparing the composite heating element according to claim 7, characterized in that: The sufficient stirring in step 2 is stirring at a speed of 250-300 rpm for 4-6 hours; the concentration of the GO / PA66 composite material in the obtained formic acid dispersion is 20-30wt%; the drying is drying at room temperature for 24-36 hours; the thickness of the GO / PA66 composite reinforcement film is 50-80μm.

9. The method for preparing the composite heating element according to claim 8, characterized in that: In step 3, the hot pressing pressure is 15-30 MPa, the hot pressing temperature is 180-250° C., and the hot pressing time is 12-20 min.

10. Use of the composite heating element according to claim 1 in resistance welding of thermoplastic composite materials.

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