A self-resistance electrothermal assisted hot-press forming method for large-thickness CFRTs

Abstract

The application belongs to the technical field of composite materials, and relates to a self-resistance electric heating assisted hot pressing forming method for large-thickness CFRTs, which comprises the step of alternately stacking thermoplastic resin films and carbon fiber cloths together; a part of the carbon fiber cloths is used for being connected with electrodes, and is marked as carbon fiber cloth X; an isolation layer is arranged on the surface of the carbon fiber cloth X; during the stacking process, the two ends of the carbon fiber cloth X are connected with one electrode respectively; after the stacking, the carbon fiber cloth X and the adjacent carbon fiber cloth are separated through the isolation layer and the thermoplastic resin film; the base material of the isolation layer is an insulating polymer, and the insulating polymer is close to the melting point of the thermoplastic resin film and can resist high temperature; and the isolation layer is embedded with an electrically-conductive and thermally-conductive network. The application constructs an insulating barrier to block interlayer electric leakage through the isolation layer, avoids delamination by using the melting point matching characteristics, realizes uniform transmission of current and heat by means of the electrically-conductive and thermally-conductive network, and effectively inhibits the vicious cycle of 'current leakage-resin degradation-current leakage'.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of composite materials, and relates to a self-resistance electric heating assisted hot pressing forming method for large-thickness CFRTs. BACKGROUND

[0002] CFRTs (carbon fiber reinforced thermoplastic composite materials) have the advantages of high impact resistance, good moisture and heat resistance, high forming efficiency, weldability and good recycling performance, and have become an important direction for the development of low cost and high performance in the fields of aerospace, national defense industry and the like. At present, the application of thermoplastic composite materials is gradually developing from small-scale to large-scale and super-large-scale, and its function is developing from non-load-bearing structure to secondary load-bearing structure and then to main load-bearing structure with the characteristics of large size in plane and super-thickness in cross section.

[0003] The thermoplastic composite material forming large-thickness part does not change chemically, can greatly shorten the forming cycle, and requires lower forming conversion energy. However, the thermal conductivity of the fiber is much higher than that of the resin, the heat transfer in the laminate plane is more efficient, and the heat transfer between the layers more depends on the contact between the fibers. Especially for large-thickness thermoplastic composite plates, there is a difference in heat conduction rate between the surface and the internal area, resulting in uneven temperature distribution in the thickness direction, causing typical "skin-core" effect, and then affecting the forming quality and mechanical properties of the component.

[0004] The self-resistance assisted heating method refers to selecting a plurality of layers in the interior of the large-thickness carbon fiber composite part and introducing electrodes, using the large amount of Joule heat generated by the current through the fibers to heat and consolidate in sections, and realizing heat compensation for the "temperature valley" area. The heat transfer of the existing self-resistance assisted heating method depends on the electrical conductivity of the carbon fiber, and there are contact points between the carbon fibers, and the current can be directly transmitted in the carbon fiber network through these contact points and generate Joule heat.

[0005] For example, the self-resistance electric heating consolidation method for resin-based carbon fiber composite material disclosed in the patent with the authorization announcement number CN105479768B, the core of which is to expose the carbon fiber by washing the resin-based carbon fiber prepreg long edge two ends, to place electrodes between the layers in the middle of the preform during the laying process, to pass direct current through the preform, to use the Joule heat generated by the carbon fiber itself resistance to provide heat for the resin curing, and to complete the forming by combining with the pressure tank to apply pressure; the method controls the current size to regulate the preform temperature, shortens the heating time, improves the heating efficiency, and also can improve the temperature distribution, and exhibits certain advantages in the curing of thermosetting resin-based carbon fiber composite materials.

[0006] However, the above method is prone to cause "edge effect" in the molding of thermoplastic composite materials. In the process of electric heating, the heating element layer heats uniformly as a whole, but due to the fact that the specific heat capacity of air is much lower than that of the composite material plate, a large amount of heat will accumulate in the area without the plate covering, resulting in a temperature significantly higher than that of the plate area, which is the phenomenon of uneven temperature distribution, i.e. edge effect. Severe edge effect will bring a series of adverse consequences: in the welding of thermoplastic composite materials, the excessively high temperature in the edge area will cause the decomposition of the resin matrix, and the rapid expansion of the melt front, which will cause the direct contact between the conductive material in the heating element and the fibers in the laminate, and for CFRTs, this direct contact of fibers will cause current leakage at the welded joint, thereby interrupting the welding process.

[0007] In addition, the molding temperature of thermoplastic composite materials is usually high, and at this high temperature, the edge effect is more significant, which not only easily leads to the degradation of the edge resin, but also causes the interlaminar fiber lap, forming a vicious cycle of "current leakage-resin degradation-current leakage", which eventually leads to material damage and deformation. SUMMARY

[0008] The purpose of the present application is to solve the problems existing in the prior art and provide a self-resistance electrothermal auxiliary hot pressing molding method for large thickness CFRTs.

[0009] To achieve the above-mentioned purpose, the technical scheme adopted by the present application is as follows:

[0010] A self-resistance electrothermal auxiliary hot pressing molding method for large thickness CFRTs, comprising the step of alternately stacking a thermoplastic resin film and a carbon fiber cloth together;

[0011] A part of the carbon fiber cloth is used to connect with the electrode, denoted as carbon fiber cloth X, and the surface of the carbon fiber cloth X is provided with a separation layer;

[0012] During the stacking process, the two ends of the carbon fiber cloth X are connected with one electrode respectively;

[0013] After stacking, the carbon fiber cloth X and the adjacent carbon fiber cloth are separated by the separation layer and the thermoplastic resin film;

[0014] The base material of the separation layer is an insulating polymer, and the absolute value of the difference between the melting point of the insulating polymer and the melting point of the thermoplastic resin film is not more than 10℃, and the 5% thermal weight loss temperature of the insulating polymer is higher than the molding temperature of the thermoplastic resin film;

[0015] The separation layer is embedded with a conductive and heat-conductive network;

[0016] The thickness of the large thickness CFRTs is more than 25mm.

[0017] The current leakage solving methods at the present stage are mostly concentrated in the field of composite resistance welding, and the commonly used solving methods are:

[0018] (1) Implanting an insulating interlayer: Fiberglass woven fabric is laid at the welding interface to block direct contact between the upper and lower carbon fiber layers using its rigid skeleton.

[0019] (2) Resin thickening and gap control: By using prepreg stacking design or injection process, the resin layer thickness in the welding area is >50μm, and the electron migration barrier is established by utilizing the resin insulation.

[0020] Traditional current leakage solutions (such as fiberglass insulation layers / resin thickening) suppress current leakage by passively mechanically isolating and blocking fiber contact, but at the cost of sacrificing thermal conductivity and increasing interface thickness, and face the risk of resin decomposition failure at high temperatures of 400°C.

[0021] like Figure 4 As shown in Figure b, in the prior art, the carbon fiber cloth X 20 and the adjacent carbon fiber cloth 3 are separated only by a thermoplastic resin film 17; as Figure 4 As shown in Figure a, the present invention provides an isolation layer 20-1 on the surface of carbon fiber cloth X 20, so that carbon fiber cloth X 20 and adjacent carbon fiber cloth 3 are separated by the isolation layer 20-1 and the thermoplastic resin film 17.

[0022] This invention designs a separation layer and a thermoplastic resin film between carbon fiber cloth X and adjacent carbon fiber cloths. Because the separation layer is heat-resistant, it does not degrade like the thermoplastic resin film. Even after the thermoplastic resin film degrades, the separation layer still effectively separates carbon fiber cloth X from its adjacent carbon fiber cloths. Since the substrate of the separation layer is an insulating polymer, it can construct a continuous encapsulation barrier between carbon fiber cloth X and adjacent carbon fiber cloths, blocking direct contact between exposed fibers and eliminating interlayer leakage, ensuring that current flows only along the intended path. Because the substrate of the separation layer and the thermoplastic resin film are insulating polymers, the separation layer effectively separates carbon fiber cloth X from its adjacent carbon fiber cloths. The absolute difference in melting points between the resin films does not exceed 10°C. Therefore, after being heated by electricity, the insulating layer can bond the two adjacent layers separated by it, avoiding delamination. Since the insulating layer has an embedded conductive and thermally conductive network, current and heat can be quickly transferred between the two layers separated by the insulating layer and evenly distributed to the entire cross-section of the preform (the whole composed of all the thermoplastic resin films and carbon fiber cloth layers). This reduces the edge current density and the probability of local heating and ablation, thereby effectively preventing unexpected and uncontrolled heat generation in the edge area caused by current leakage, and fundamentally suppressing the vicious cycle of edge effects.

[0023] This invention upgrades the compromise design of the prior art of "exchanging heat insulation for insulation" to an active regulation of "insulation-conductivity and heat conduction".

[0024] As a preferred technical solution:

[0025] The self-resistance electrothermal assisted hot-press forming method of the large-thickness CFRTs as described above, the material of the thermoplastic resin film is polyether ether ketone or polyphenylene sulfide, and the material of the insulating polymer is polyetherimide, polyether sulfone or polyamide.

[0026] The self-resistance electrothermal assisted hot-press forming method of the large-thickness CFRTs as described above, the thickness of the isolation layer is 75-85 nm, and the conductive and heat-conductive network is composed of carbon nanotubes.

[0027] The conductive and heat-conductive network composed of carbon nanotubes and the carbon fibers themselves jointly form a multi-scale heat-conductive system. Since the thermal conductivity of the carbon nanotubes is extremely high (about 3000 W / (m·K), which is much higher than that of the thermoplastic resin film (about 0.3 W / (m·K)), the system can quickly conduct the Joule heat generated in the electrode area along the thickness direction of the preform. This efficient heat diffusion effectively reduces the current density concentration near the electrode, thereby avoiding excessive Joule heat generated in the local area due to overheating.

[0028] The self-resistance electrothermal assisted hot-press forming method of the large-thickness CFRTs as described above, the process of arranging the isolation layer on the surface of the carbon fiber cloth X is as follows: after the carbon fiber cloth X is immersed in a finishing liquid composed of carbon nanotubes, an insulating polymer and a solvent, it is taken out and dried, and the process is completed; wherein the diameter of the carbon nanotubes is 10-20 nm, the length is 10-30 μm, the carbon nanotubes are industrial-grade MWCNTs, the content of the carbon nanotubes in the finishing liquid is 0.05 wt%, and the content of the insulating polymer in the finishing liquid is 1.5-3.0 wt%.

[0029] The self-resistance electrothermal assisted hot-press forming method of the large-thickness CFRTs as described above, the electrode is a microneedle electrode, which is composed of a metal base plate and a plurality of conical carbon steel needles fixed on the metal base plate in an array; when the carbon fiber cloth X is connected with the electrode, the needles penetrate through the isolation layer and contact with the carbon fibers. In this way, efficient electrical contact can be established, and the interface resistance can be significantly reduced; at the same time, the uniformly distributed needles form multi-channel current injection points, so that the current diffuses in the three-dimensional space inside the carbon fiber cloth X, effectively avoiding the edge effect of the traditional flat electrode, realizing uniform heating with a temperature difference of <5°C in the whole area; the mechanical anchoring effect of the needles can also compensate for the thermal expansion displacement, ensuring that the contact resistance fluctuation rate is <3% in the dynamic process, and finally improving the curing quality and efficiency of the composite material.

[0030] The self-resistance electrothermal assisted hot-press forming method of the large-thickness CFRTs as described above, the specific steps are as follows:

[0031] (a) After laying an insulating film (such as a polyimide film) on the lower surface, the thermoplastic resin film and the carbon fiber cloth are alternately laid together on the insulating film, and all the thermoplastic resin films and the carbon fiber cloth jointly form a preform;

[0032] The preform includes at least one layer of carbon fiber cloth X; during the laying-up process, the two ends of the carbon fiber cloth X are connected with one electrode respectively;

[0033] The number and position of the carbon fiber cloth X are determined by a pre-test, in which the sample is heated and consolidated by an external heat source, the sample is pressed at the same time as the preform is heated and consolidated, and the maximum temperature difference between the surface and the core of the sample during the heating and consolidation process is monitored;

[0034] If the maximum temperature difference is ≥15℃, the preform is divided into upper, middle and lower parts along the thickness direction, the absolute value of the thickness difference between any two parts is not more than 5mm, a group of thermocouples are arranged in each part, the carbon fiber cloth X has three layers, the first layer is located at the middle position of the upper part, the second layer is located at the middle position of the middle part, and the third layer is located at the middle position of the lower part; the electrodes in the upper and lower parts are connected with one pulse power source a at the same time, the pulse power source a is connected with one temperature controller a, and the temperature controller a is connected with the thermocouples in the upper and lower parts at the same time; the electrode in the middle part is connected with one pulse power source b, the pulse power source b is connected with one temperature controller b, and the temperature controller b is connected with the thermocouples in the middle part; the electrodes between adjacent two parts are separated by an insulating film;

[0035] If the maximum temperature difference is <15℃, the carbon fiber cloth X has one layer, which is located at the middle position of the preform; the electrode is connected with one pulse power source c;

[0036] (b) After the insulating film is laid on the preform, the upper press is pressed thereon;

[0037] (c) Start the pulse power source and the external heat source (i.e. the heat source of the upper press) to heat and consolidate, and press the preform at the same time, the heating and consolidation process is divided into three stages, namely the heating-up stage, the holding stage and the cooling-down stage;

[0038] When the preform is divided into upper, middle and lower parts along the thickness direction, during the heating-up stage and the cooling-down stage, the temperature controller a adjusts the current output of the pulse power source a in real time according to the signal feedback by the thermocouples, and at the same time, the temperature controller b adjusts the current output of the pulse power source b in real time according to the signal feedback by the thermocouples, so that the absolute value of the temperature difference between the upper, middle and lower parts is ≤5℃;

[0039] (d) Turn off the pulse power source.

[0040] The corresponding relationship between the pulse current and the temperature inside the preform during the hot pressing process is shown in the figure Figure 3As shown. In the temperature rising stage, the pulse current presents corresponding dynamic adjustment change with the temperature rising from room temperature (25℃) to the target temperature, so as to realize stable temperature rising through the Joule heating effect; in the temperature maintaining stage, the temperature is maintained in the target interval, and the current also maintains relatively stable output, so as to guarantee continuous and uniform heat supply; in the temperature reducing stage, the temperature gradually reduces, and the current is adjusted accordingly, so as to assist in realizing the smooth temperature reducing process.

[0041] The self-resistance electrothermal auxiliary hot-press forming method of the large-thickness CFRTs as described above has a temperature rising rate of 6-8℃ / min in the temperature rising stage, a temperature maintaining duration of 20min in the temperature maintaining stage, and a temperature reducing speed of 5℃ / min in the temperature reducing stage.

[0042] The self-resistance electrothermal auxiliary hot-press forming method of the large-thickness CFRTs as described in any one of the above has an interlaminar shear strength of the large-thickness CFRTs of 55-75MPa, a porosity of ≤1.5%, a warpage of ≤1mm, and a current leakage rate of ≤2%.

[0043] Beneficial effects:

[0044] The application constructs a continuous encapsulation barrier by using the insulation polymer characteristics of the isolation layer substrate, blocks the direct contact of the interlaminar exposed fibers to eliminate interlaminar current leakage, and makes the current flow along the expected path; by means of the feature that the melting point of the isolation layer substrate and the thermoplastic resin film is close, it is ensured that the adjacent two layers can be bonded after being heated by electricity to avoid delamination; through the embedded conductive and heat-conductive network in the isolation layer, the current and heat are quickly transferred between the layers and evenly distributed to the entire cross section of the preform, reducing the edge current density and the local heating ablation probability, thereby effectively solving the edge effect and current leakage problem caused by high temperature in the existing self-resistance auxiliary heating technology in the forming of thermoplastic composites, fundamentally inhibiting the vicious cycle of "current leakage-resin degradation-current leakage", overcoming the defects of sacrificing thermal conductivity, increasing interface thickness and resin degradation failure at high temperature in the traditional solutions, realizing the active regulation of "insulation-conductive and heat-conductive", and guaranteeing the forming quality of the large-thickness CFRTs. BRIEF DESCRIPTION OF DRAWINGS

[0045] Figure 1 The flowchart for preparing the large-thickness CFRTs of embodiment 1 of the application is shown in the figure;

[0046] Figure 2 The electrode structure diagram of embodiment 1 is shown in the figure;

[0047] Figure 3 The corresponding relationship diagram between the pulse current and the internal temperature of the preform in the hot-press forming process is shown in the figure;

[0048] Figure 4a is the interlayer structure of the carbon fiber cloth X of the present application and the adjacent carbon fiber cloth, and b is the interlayer structure of the carbon fiber cloth X of the prior art and the adjacent carbon fiber cloth;

[0049] In the figure, 1 is an upper press, 2 is an upper insulating film, 3 is a carbon fiber cloth, 4 is an electrode, 4-1 is a needle, 4-2 is a metal base plate, 5 is a middle insulating film, 6 is a lower insulating film, 7 is a lead a, 8 is a lead b, 9 is a lower platform, 10 is a thermocouple, 11 is a pulse power a, 12 is a pulse power b, 15 is a temperature controller b, 16 is a temperature controller a, 17 is a thermoplastic resin film, 20 is a carbon fiber cloth X, and 20-1 is an isolation layer. DETAILED DESCRIPTION

[0050] The present application will be further described in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present application and not to limit the scope of the present application. Furthermore, it should be understood that after reading the content taught by the present application, those skilled in the art can make various modifications or changes to the present application, and these equivalent forms also fall within the scope defined by the appended claims of the present application.

[0051] In order to fully disclose the properties of the substances used in each example and comparative example, the manufacturer and brand of the substance are specified in the present application. In addition, products of other manufacturers and brands that meet the definition of the present application can also be applicable.

[0052] The following are the test methods for the relevant performance indicators in each example and comparative example. All index test processes are carried out in parallel for 6 times, and the average value is taken as the final test result:

[0053] Melting point: tested by differential scanning calorimetry (DSC), the standard referred to is GB / T 19466.3-2004;

[0054] 5% thermal weight loss temperature: tested in accordance with ASTM E1131-23 standard;

[0055] Interlaminar shear strength: tested in accordance with ASTM D2344-22 standard;

[0056] Porosity: tested in accordance with the ultrasonic method in HB7224-1995 standard;

[0057] Warping: First, use the level to calibrate the flat table top, ensure that its horizontal height error ≤0.5mm, as the measurement reference surface; for the measured large thickness CFRTs sample (square, side length 200mm), mark its four corners as A1, A2, A3, A4 respectively; use a precision of 0.01mm dial gauge to measure, the measurement area of each corner is determined within a vertical distance of 5mm from the corner vertex, and 3 points not on the same straight line are selected in the area, the vertical gap between the sample lower surface and the reference surface at each point is measured, and the average of the three measurement results of each corner is taken as the warping W of the corner (unit: mm); a ; finally, take the maximum value of the warping W a of the four corners as the warping of the measured sample (unit: mm);

[0058] Current leakage rate: test according to GB / T 12113-2023 standard.

[0059] Example 1

[0060] A self-resistance electric heating assisted hot pressing forming method of large thickness CFRTs, the specific steps are as follows:

[0061] (1) Preparation of main materials;

[0062] Carbon fiber cloth: the manufacturer is Weihai Guangwei Composite Material Co., Ltd., the brand is TZ300, and the thickness is 0.25mm, wherein a part of the carbon fiber cloth is used for connecting with the electrode, and is marked as carbon fiber cloth X;

[0063] Finishing liquid: composed of carbon nanotubes (manufacturer is Hefei Aikesiwei New Material Technology Co., Ltd., model is XJ-237, diameter is 10-20nm, length is 10-30μm), polyetherimide (melting point is 345℃, 5% thermal weight loss temperature is 523℃) and N,N-dimethylacetamide, the content of carbon nanotubes is 0.05wt%, and the content of polyetherimide is 3.0wt%;

[0064] Thermoplastic resin film: the material is polyether ether ketone (melting point is 340℃), and the thickness is 0.13mm;

[0065] (2) Setting an isolation layer on the surface of the carbon fiber cloth X;

[0066] After the carbon fiber cloth X is immersed in the finishing liquid, it is taken out and dried to form an isolation layer with a thickness of 75nm on the surface of the carbon fiber cloth X;

[0067] (3) Preparation of large thickness CFRTs;

[0068] (a) as Figure 1As shown, after laying the lower insulating film 6 on the lower platform 9, the thermoplastic resin film 17 and the carbon fiber cloth 3 are alternately laid on the lower platform 9, and all the thermoplastic resin films 17 and the carbon fiber cloth 3 together constitute a preform;

[0069] The preform includes at least one layer of carbon fiber cloth X; during the laying process, the two ends of the carbon fiber cloth X are connected with one electrode 4 respectively; as shown, Figure 2 The electrode 4 is a micro-needle electrode, which is composed of a metal base plate 4-2 and a plurality of needle pins 4-1 arranged in an array on the metal base plate 4-2; when the carbon fiber cloth X is connected with the electrode 4, the needle pins 4-1 contact the carbon fiber through the isolation layer;

[0070] The number and position of the carbon fiber cloth X are determined by a pre-test, in which the test sample is heated and consolidated by an external heat source (heating temperature: 340℃), and the test sample is pressed (2.5MPa) while being heated and consolidated, and the maximum temperature difference between the surface and the core of the test sample during the heating and consolidation process is monitored;

[0071] If the maximum temperature difference is ≥15℃, the preform is equally divided into upper, middle and lower parts along the thickness direction, as shown, Figure 1 As shown, a group of thermocouples 10 are arranged in each part, and the carbon fiber cloth X has three layers, the first layer is located at the middle position of the upper part, the second layer is located at the middle position of the middle part, and the third layer is located at the middle position of the lower part; the electrodes 4 in the upper part and the lower part are connected with a pulse power source a 11 through a wire a 7 at the same time, the pulse power source a 11 is connected with a temperature controller a 16, and the temperature controller a 16 is connected with the thermocouples 10 in the upper part and the lower part at the same time; the electrode 4 in the middle part is connected with a pulse power source b 12 through a wire b 8, the pulse power source b 12 is connected with a temperature controller b 15, and the temperature controller b 15 is connected with the thermocouples 10 in the middle part; the electrodes 4 in adjacent two parts are separated by the middle insulating film 5;

[0072] If the maximum temperature difference is <15℃, the carbon fiber cloth X has one layer, which is located at the middle position of the preform; the electrode 4 is connected with a pulse power source c;

[0073] (b) After laying the upper insulating film 2 on the preform, the upper press 1 is pressed on it;

[0074] (c) start the pulse power supply and the external heat source to perform the heat consolidation, the heat consolidation is performed while applying pressure (the pressure is 2.5 MPa) to the preform, the heat consolidation is divided into three stages, which are a temperature rising stage, a temperature holding stage and a temperature falling stage; in the temperature rising stage and the temperature falling stage, the temperature controller a 16 adjusts the current size output by the pulse power supply a 11 in real time according to the signal fed back by the thermocouple 10, and at the same time, the temperature controller b 15 adjusts the current size output by the pulse power supply b 12 in real time according to the signal fed back by the thermocouple 10, so that the absolute value of the temperature difference of the upper, middle and lower three parts is ≤5℃; the temperature rising rate of the temperature rising stage is 8℃ / min (the temperature starting point is room temperature), the temperature holding time of the temperature holding stage is 20 min (the temperature holding temperature is 350℃), and the temperature falling rate of the temperature falling stage is 5℃ / min (the temperature ending point is room temperature);

[0075] (d) turn off the pulse power supply.

[0076] The thickness of the finally prepared large-thickness CFRTs is 25 mm, the interlaminar shear strength is 75 MPa, the porosity is 1%, the warpage is 0.5 mm, and the current leakage rate is 2%.

[0077] Example 2

[0078] A self-resistance electric heating assisted hot pressing forming method of large-thickness CFRTs, the specific steps are as follows:

[0079] (1) Preparation of main materials;

[0080] Carbon fiber cloth: the manufacturer is Weihai Guangwei Composite Material Co., Ltd., the brand is TZ300, and the thickness is 0.25 mm, wherein a part of the carbon fiber cloth is used for connection with the electrode, and is denoted as carbon fiber cloth X;

[0081] Finishing liquid: composed of carbon nanotubes (manufacturer is Hefei Aikesixiwei New Material Technology Co., Ltd., model is XJ-237, diameter is 10-20 nm, length is 10-30 μm), polyether sulfone (melting point is 270℃, 5% thermal weight loss temperature is 385℃) and N,N-dimethylformamide, the content of the carbon nanotubes is 0.05wt%, and the content of the polyether sulfone is 2.5wt%;

[0082] Thermoplastic resin film: the material is polyphenylene sulfide (melting point is 280℃), and the thickness is 0.13 mm;

[0083] (2) Setting an isolation layer on the surface of the carbon fiber cloth X;

[0084] After the carbon fiber cloth X is immersed in the finishing liquid, it is taken out and dried to form an isolation layer with a thickness of 75 nm on the surface of the carbon fiber cloth X;

[0085] (3) Preparing large-thickness CFRTs;

[0086] (a) after laying the insulating film on the lower platform, lay the thermoplastic resin film and the carbon fiber cloth alternately on it, and all the thermoplastic resin films and the carbon fiber cloth together constitute a preform;

[0087] The preform includes at least one layer of carbon fiber cloth X; during the laying process, the two ends of the carbon fiber cloth X are connected with one electrode respectively; the electrode is a microneedle electrode, which is composed of a metal base plate and a plurality of needles fixed on the metal base plate in an array; when the carbon fiber cloth X is connected with the electrode, the needles penetrate through the isolation layer and contact the carbon fiber;

[0088] The number and position of the layers of the carbon fiber cloth X are determined through a pre-test, in which the test sample is heated and consolidated by an external heat source (heating temperature: 253℃), and the test sample is pressed (2MPa) while being heated and consolidated; the maximum temperature difference between the surface and the core of the test sample during the heating and consolidation process is monitored;

[0089] If the maximum temperature difference is ≥15℃, the preform is equally divided into upper, middle and lower parts along the thickness direction, and each part is provided with a group of thermocouples; the carbon fiber cloth X has three layers, the first layer is located at the middle position of the upper part, the second layer is located at the middle position of the middle part, and the third layer is located at the middle position of the lower part; the electrodes in the upper and lower parts are connected with a pulse power source a at the same time, the pulse power source a is connected with a temperature controller a, and the temperature controller a is connected with the thermocouples in the upper and lower parts at the same time; the electrodes in the middle part are connected with a pulse power source b, the pulse power source b is connected with a temperature controller b, and the temperature controller b is connected with the thermocouples in the middle part; the electrodes between adjacent two parts are separated by an insulating film;

[0090] If the maximum temperature difference is <15℃, the carbon fiber cloth X has one layer, which is located at the middle position of the preform; the electrodes are connected with a pulse power source c;

[0091] (b) after laying the insulating film on the preform, press the upper press on it;

[0092] (c) start the pulse power source and the external heat source to heat and consolidate, and press the preform at the same time (pressure: 2MPa); the heating and consolidation process is divided into three stages, namely, the heating-up stage, the holding stage and the cooling-down stage; during the heating-up stage and the cooling-down stage, the temperature controller a adjusts the current output of the pulse power source a in real time according to the signal feedback of the thermocouples, and at the same time, the temperature controller b adjusts the current output of the pulse power source b in real time according to the signal feedback of the thermocouples, so that the absolute value of the temperature difference between the upper, middle and lower parts is ≤5℃; the heating-up rate of the heating-up stage is 6℃ / min (temperature starting point: room temperature), the holding time of the holding stage is 20min (holding temperature: 300℃), and the cooling-down rate of the cooling-down stage is 5℃ / min (temperature ending point: room temperature);

[0093] (d) turning off the pulse power supply.

[0094] The final large-thickness CFRTs has a thickness of 25 mm, an interlaminar shear strength of 65 MPa, a porosity of 1.5%, a warpage of 1 mm, and a current leakage rate of 2%.

[0095] Example 3

[0096] A self-resistance electric heating assisted hot-press forming method for large-thickness CFRTs, the specific steps are as follows:

[0097] (1) Preparation of main materials;

[0098] Carbon fiber cloth: the manufacturer is Jiangsu Hengshen Co., Ltd., the model is HFW200P, and the thickness is 0.24 mm. Part of the carbon fiber cloth is used for connection with the electrode, and is recorded as carbon fiber cloth X;

[0099] Finishing liquid: composed of carbon nanotubes (manufacturer is Hefei Aikesixiwei New Material Technology Co., Ltd., model is XJ-237, diameter is 10-20 nm, length is 10-30 μm), polyetherimide (melting point is 345℃, 5% thermal weight loss temperature is 523℃), and N,N-dimethylacetamide. The content of carbon nanotubes is 0.05wt%, and the content of polyetherimide is 2.5wt%;

[0100] Thermoplastic resin film: the material is polyether ether ketone (melting point is 340℃), and the thickness is 0.12 mm;

[0101] (2) Setting an isolation layer on the surface of the carbon fiber cloth X;

[0102] After the carbon fiber cloth X is immersed in the finishing liquid, it is taken out and dried to form an isolation layer with a thickness of 75 nm on the surface of the carbon fiber cloth X;

[0103] (3) Preparing large-thickness CFRTs;

[0104] (a) After laying the insulating film on the lower surface, the thermoplastic resin film and the carbon fiber cloth are alternately laid together on it, and all the thermoplastic resin films and the carbon fiber cloth together constitute a preform;

[0105] The preform includes at least one layer of carbon fiber cloth X; during the laying process, the two ends of the carbon fiber cloth X are connected with an electrode respectively; the electrode is a microneedle electrode, which is composed of a metal base plate and a plurality of needles fixed on the metal base plate in an array; when the carbon fiber cloth X is connected with the electrode, the needles penetrate through the isolation layer and contact with the carbon fiber;

[0106] The number of layers and the position of the carbon fiber cloth X are determined by a pre-test, in which the sample is heated and consolidated by an external heat source (heating temperature: 340℃), the sample is pressed (2.5MPa) at the same time as the preform is heated and consolidated, and the maximum temperature difference between the surface and the core of the sample during the heating and consolidation process is monitored;

[0107] If the maximum temperature difference is ≥15℃, the preform is equally divided into upper, middle and lower parts along the thickness direction, and each part is provided with a group of thermocouples. The carbon fiber cloth X has three layers, the first layer is located at the middle position of the upper part, the second layer is located at the middle position of the middle part, and the third layer is located at the middle position of the lower part. The electrodes in the upper and lower parts are connected to a pulse power source a at the same time, the pulse power source a is connected to a temperature controller a, and the temperature controller a is connected to the thermocouples in the upper and lower parts at the same time. The electrodes in the middle part are connected to a pulse power source b, the pulse power source b is connected to a temperature controller b, and the temperature controller b is connected to the thermocouples in the middle part. The electrodes between adjacent two parts are separated by an insulating film.

[0108] If the maximum temperature difference is <15℃, the carbon fiber cloth X has one layer, which is located at the middle position of the preform; the electrode is connected to a pulse power source c.

[0109] (b) After laying the insulating film on the preform, the upper press is pressed thereon;

[0110] (c) Start the pulse power source and the external heat source to heat and consolidate, and press the preform at the same time (pressure: 2.5MPa). The heating and consolidation process is divided into three stages, namely, the heating stage, the holding stage and the cooling stage. In the heating stage and the cooling stage, the temperature controller a adjusts the current output of the pulse power source a in real time according to the signal feedback of the thermocouple, and at the same time, the temperature controller b adjusts the current output of the pulse power source b in real time according to the signal feedback of the thermocouple, so that the absolute value of the temperature difference between the upper, middle and lower three parts is ≤5℃. The heating rate of the heating stage is 8℃ / min (temperature starting point: room temperature), the holding time of the holding stage is 20min (holding temperature: 360℃), and the cooling rate of the cooling stage is 5℃ / min (temperature end point: room temperature);

[0111] (d) Turn off the pulse power source.

[0112] The final large-thickness CFRTs has a thickness of 25mm, an interlaminar shear strength of 75MPa, a porosity of 1%, a warpage of 0.5mm, and a current leakage rate of 2%.

[0113] Example 4

[0114] A self-resistance electric heating assisted hot pressing forming method of large-thickness CFRTs, the specific steps are as follows:

[0115] (1) Preparation of main materials;

[0116] Carbon fiber cloth: the manufacturer is Weihai Guangwei Composite Material Co., Ltd., the brand is TZ300, and the thickness is 0.25 mm, wherein a part of the carbon fiber cloth is used for connection with the electrode, and is recorded as carbon fiber cloth X;

[0117] Finishing liquid: composed of carbon nanotubes (the manufacturer is Hefei Aikesiwei New Material Technology Co., Ltd., the model is XJ-237, the diameter is 10-20 nm, and the length is 10-30 μm), polyamide (the melting point is 270 °C, and the 5% thermal weight loss temperature is 450 °C), and N,N-dimethylacetamide, the content of the carbon nanotubes is 0.05 wt%, and the content of the polyamide is 1.5 wt%;

[0118] Thermoplastic resin film: the material is polyphenylene sulfide (the melting point is 280 °C), and the thickness is 0.13 mm;

[0119] (2) Setting an isolation layer on the surface of the carbon fiber cloth X;

[0120] After the carbon fiber cloth X is immersed in the finishing liquid, it is taken out and dried to form an isolation layer with a thickness of 85 nm on the surface of the carbon fiber cloth X;

[0121] (3) Preparing a large-thickness CFRTs;

[0122] (a) After laying the insulating film on the lower platform, the thermoplastic resin film and the carbon fiber cloth are alternately laid together on the lower platform, and all the thermoplastic resin films and the carbon fiber cloth together constitute a preform;

[0123] The preform includes at least one layer of the carbon fiber cloth X; during the laying process, the two ends of the carbon fiber cloth X are connected with an electrode respectively; the electrode is a microneedle electrode, which is composed of a metal base plate and a plurality of needles fixed on the metal base plate in an array; when the carbon fiber cloth X is connected with the electrode, the needles penetrate through the isolation layer and contact the carbon fiber;

[0124] The number of layers and the position of the carbon fiber cloth X are determined through a pre-test; during the test, the test sample is heated and consolidated by an external heat source (the heating temperature is 253 °C); the test sample is the same as the preform, and pressure (2 MPa) is applied to the test sample during the heating and consolidation; the maximum temperature difference between the surface and the core of the test sample during the heating and consolidation process is monitored;

[0125] If the maximum temperature difference is ≥ 15℃, the preform is equally divided into upper, middle and lower parts along the thickness direction, and each part is provided with a group of thermocouples. The carbon fiber cloth X has three layers, the first layer is located at the middle position of the upper part, the second layer is located at the middle position of the middle part, and the third layer is located at the middle position of the lower part. The electrodes in the upper part and the lower part are connected with a pulse power source a at the same time, the pulse power source a is connected with a temperature controller a, and the temperature controller a is connected with the thermocouples in the upper part and the lower part at the same time. The electrode in the middle part is connected with a pulse power source b, the pulse power source b is connected with a temperature controller b, and the temperature controller b is connected with the thermocouple in the middle part. The electrodes of adjacent two parts are separated by an insulating film.

[0126] If the maximum temperature difference is < 15℃, the carbon fiber cloth X has one layer, which is located at the middle position of the preform; the electrode is connected with a pulse power source c.

[0127] (b) After laying the insulating film on the preform, the upper press is pressed thereon;

[0128] (c) Start the pulse power source and the external heat source to heat and consolidate, and at the same time, apply pressure (pressure is 2MPa) to the preform. The heating and consolidation is divided into three stages, which are respectively the heating stage, the holding stage and the cooling stage. In the heating stage and the cooling stage, the temperature controller a adjusts the current output of the pulse power source a in real time according to the signal feedback of the thermocouple, and at the same time, the temperature controller b adjusts the current output of the pulse power source b in real time according to the signal feedback of the thermocouple, so that the absolute value of the temperature difference of the upper, middle and lower three parts is ≤ 5℃. The heating rate of the heating stage is 6℃ / min (the temperature starting point is room temperature), the holding time of the holding stage is 20min (the holding temperature is 300℃), and the cooling rate of the cooling stage is 5℃ / min (the temperature ending point is room temperature).

[0129] (d) Turn off the pulse power source.

[0130] The final large-thickness CFRTs has a thickness of 25mm, an interlaminar shear strength of 65MPa, a porosity of 1.5%, a warpage of 1mm and a current leakage rate of 2%.

[0131] Example 5

[0132] A self-resistance electric heating assisted hot pressing forming method of large-thickness CFRTs, and the difference from example 1 is only that the electrode is a flat plate electrode.

[0133] The final large-thickness CFRTs has a thickness of 25mm, an interlaminar shear strength of 55MPa, a porosity of 1.5%, a warpage of 0.5mm and a current leakage rate of 2%.

[0134] Compared with Example 1, the interlaminar shear strength of the large-thickness CFRTs of Example 5 is reduced because the flat electrodes cannot form multi-channel current injection points, and the current is difficult to diffuse three-dimensionally inside the carbon fiber cloth X. Instead, the current is randomly concentrated in the in-plane direction, strengthening the lateral random conduction path of the carbon fiber network, resulting in random diffusion of the current in the in-plane direction. At the same time, the flat electrodes have no mechanical anchoring effect and cannot compensate for thermal expansion displacement. The contact resistance fluctuation rate increases during the dynamic process, and the insulation performance of the resin decays further at high temperatures, causing interlaminar leakage, abnormal heating and crystallinity gradient in non-target areas, and affecting the interlaminar bonding quality.

[0135] Comparative Example 1

[0136] A hot-press forming method of a composite material, and the difference from Example 1 is that step (2) is not performed, and the carbon fiber cloth used in step (3) is a carbon fiber cloth without a separation layer.

[0137] The interlaminar shear strength of the large-thickness CFRTs finally obtained is 45 MPa, the porosity is 3%, the warpage is 3 mm, and the current leakage rate is 15%.

[0138] Compared with Example 1, the interlaminar shear strength of the large-thickness CFRTs of Comparative Example 1 is significantly reduced, and the porosity and current leakage rate are significantly deteriorated, because the separation layer is not provided, and a continuous encapsulation barrier cannot be built between the carbon fiber cloth X and the adjacent carbon fiber cloth. The exposed carbon fibers between the layers are easily directly contacted to form a "minimum resistance path". After the pulse current is passed, the current is easily randomly flowed along these contact points, resulting in excessively high local current density, causing unexpected and uncontrolled heat generation. Not only does it cause excessive degradation of the resin, but also makes the heat distribution inside the preform uneven, ultimately leading to poor interlaminar bonding, increased porosity, and increased current leakage rate.

Claims

1. A self-resistive electrothermal assisted hot-press forming method of large-thickness CFRTs, characterized in that, The method comprises the step of alternately laying the thermoplastic resin film and the carbon fiber cloth together; A part of the carbon fiber cloth is used for connecting with the electrode, denoted as carbon fiber cloth X, and a surface of the carbon fiber cloth X is provided with an isolation layer; During the laying process, two ends of the carbon fiber cloth X are connected with one electrode respectively; After the laying, the carbon fiber cloth X is separated from the adjacent carbon fiber cloth through the isolation layer and the thermoplastic resin film; The base material of the isolation layer is an insulating polymer, and an absolute value of a difference between a melting point of the insulating polymer and a melting point of the thermoplastic resin film is not more than 10℃, and a 5% thermal weight loss temperature of the insulating polymer is higher than a molding temperature of the thermoplastic resin film; The isolation layer is embedded with a conductive and heat-conductive network; The thickness of the large-thickness CFRTs is more than 25mm.

2. A self-resistive electrothermal assisted hot pressing forming method of large thickness CFRTs according to claim 1, characterized in that, The material of the thermoplastic resin film is polyether ether ketone or polyphenylene sulfide, and the material of the insulating polymer is polyetherimide, polyether sulfone or polyamide.

3. A self-resistive electrothermal assisted hot press forming method of large thickness CFRTs according to claim 1, characterized in that, The thickness of the isolation layer is 75-85nm, and the conductive and heat-conductive network is composed of carbon nanotubes.

4. A self-resistive electrothermal assisted hot pressing forming method of large thickness CFRTs according to claim 3, characterized in that, The process of providing the isolation layer on the surface of the carbon fiber cloth X is as follows: after the carbon fiber cloth X is immersed in a finishing liquid composed of carbon nanotubes, an insulating polymer and a solvent, the carbon fiber cloth X is taken out and dried, and the process is completed; wherein the diameter of the carbon nanotubes is 10-20nm, the length of the carbon nanotubes is 10-30μm, the content of the carbon nanotubes in the finishing liquid is 0.05wt%, and the content of the insulating polymer in the finishing liquid is 1.5-3.0wt%.

5. A self-resistive electrothermal assisted hot pressing forming method of large thickness CFRTs according to claim 1, characterized in that, The electrode is a microneedle electrode, which is composed of a metal base plate (4-2) and a plurality of needle prongs (4-1) arranged in an array and fixed on the metal base plate (4-2); when the carbon fiber cloth X is connected with the electrode, the needle prongs (4-1) penetrate through the isolation layer and contact with the carbon fiber.

6. A self-resistive electrothermal assisted hot pressing forming method of large thickness CFRTs according to claim 1, characterized in that, The specific steps are as follows: (a) after laying the insulating film on the lower platform, alternately lay the thermoplastic resin film and the carbon fiber cloth together on the insulating film, and all the thermoplastic resin films and the carbon fiber cloth together constitute a preform; The preform includes at least one layer of carbon fiber cloth X; during the laying process, two ends of the carbon fiber cloth X are connected with one electrode respectively; The number and position of the carbon fiber cloth X are determined through a pre-test; during the test, the sample is heated and solidified by an external heat source, the sample is the same as the preform, and pressure is applied to the sample during the heating and solidification, and the maximum temperature difference between the surface and the core of the sample during the heating and solidification is monitored; If the maximum temperature difference is greater than or equal to 15℃, the preform is divided into upper, middle and lower parts along the thickness direction, the absolute value of the thickness difference between any two parts is not more than 5mm, a group of thermocouples are arranged in each part, and the carbon fiber cloth X has three layers, the first layer is located at the middle position of the upper part, the second layer is located at the middle position of the middle part, and the third layer is located at the middle position of the lower part; the electrodes in the upper part and the lower part are connected with one pulse power source a at the same time, the pulse power source a is connected with one temperature controller a, and the temperature controller a is connected with the thermocouples in the upper part and the lower part at the same time; the electrodes in the middle part are connected with one pulse power source b, the pulse power source b is connected with one temperature controller b, and the temperature controller b is connected with the thermocouples in the middle part; the electrodes between the adjacent two parts are separated by the insulating film; If the maximum temperature difference is less than 15℃, the carbon fiber cloth X is one layer and is located in the middle of the preform; the electrode is connected with a pulse power supply c; (b) After laying the insulating film on the preform, the upper press is pressed thereon; (c) Start the pulse power supply and the external heat source to perform the heating and consolidation, and apply pressure to the preform during the heating and consolidation, which is divided into three stages, i.e., a temperature rising stage, a temperature maintaining stage and a temperature falling stage; When the preform is divided into upper, middle and lower parts along the thickness direction, during the temperature rising stage and the temperature falling stage, the temperature controller a adjusts the current output by the pulse power supply a in real time according to the signal fed back by the thermocouple, and the temperature controller b adjusts the current output by the pulse power supply b in real time according to the signal fed back by the thermocouple, so that the absolute value of the temperature difference of the upper, middle and lower parts is ≤5℃; (d) Turn off the pulse power supply.

7. A self-resistive electrothermal assisted hot pressing forming method of large thickness CFRTs according to claim 6, characterized in that, The temperature rising rate of the temperature rising stage is 6-8℃ / min, the temperature maintaining time of the temperature maintaining stage is 20min, and the temperature falling rate of the temperature falling stage is 5℃ / min.

8. A self-resistive electrothermal assisted hot pressing forming method of thick- gauge CFRTs according to any one of claims 1 to 7, characterized in that, The interlaminar shear strength of the large-thickness CFRTs is 55-75MPa, the porosity is ≤1.5%, the warpage is ≤1mm, and the current leakage rate is ≤2%.

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