A method for recycling carbon fiber by microwave spark discharge

Abstract

The present application relates to the technical field of recycling carbon fiber composite materials, and discloses a method for recycling carbon fiber by microwave spark discharge, including: step 1, performing microwave heating on the carbon fiber composite material in the same phase in a microwave heating device, and at the same time, performing uniform spark discharge in the same phase microwave field, and the position where the spark discharge occurs is controlled within the same horizontal plane height in the same phase microwave field, so that the carbon fiber composite material is quickly and evenly heated to 500°C within 1-5 minutes, and as the microwave and spark discharge time increase, the temperature can continue to rise rapidly to crack the resin in the composite material, complete the separation of carbon fiber and resin in the carbon fiber composite material, and obtain carbon fiber. The method for recycling carbon fiber by microwave spark discharge disclosed in the present application can improve the recycling capacity, and the performance retention rate of the recycled carbon fiber is stabilized at more than 90%, or even 95%.

Description

Technical Field

[0001] The invention belongs to the technical field of carbon fiber composite material recycling, and in particular relates to a method for recycling carbon fiber by microwave spark discharge. Background Art

[0002] Carbon fiber composite materials are a kind of composite materials with resin as the base material. Carbon fiber and resin are mixed and molded to obtain a light and high-strength composite material. Due to its advantages of high strength and light weight, it is widely used in the fields of automobiles, aerospace, wind power, sports equipment, etc. As the scope of application expands, a large amount of waste carbon fiber composite materials will be generated. These composite materials cannot be decomposed in the natural environment after being discarded, causing many environmental problems; in addition, carbon fiber has a high market value and has recycling value. Therefore, the recycling and reuse of waste carbon fiber composite materials is of great significance.

[0003] At present, the main carbon fiber recycling processes include mechanical method, chemical method and pyrolysis method. The mechanical recycling method is to shear, crush and other mechanical treatments on the discarded carbon fiber composite materials to make them into fragments, particles or powders. This method cannot obtain long fibers, and the carbon fiber structure is severely damaged. The recycled carbon fiber has low value and is usually used for fillers and other purposes; the chemical method refers to the use of chemical reagents to degrade the resin matrix in the discarded carbon fiber composite materials to separate the carbon fiber from the resin matrix. This recycling method is still limited to the laboratory stage, and it is time-consuming and will also produce waste solution. Subsequent treatment is required, which increases the overall process cost and is still far from industrial production; the pyrolysis method is to use electric heating or microwave heating to burn and crack the resin matrix, and remove the resin matrix by burning the composite material with electric heating. Since the heat conduction method is from the outside to the inside, to reach the internal pyrolysis temperature, it is necessary to continuously heat and keep the external environment warm, which requires a considerable amount of operation time, high process cost, uneven heating inside and outside, and easy to damage the performance of carbon fiber. The heat conduction method of microwave heating technology is from the inside to the outside, which has the advantages of rapid heating, energy saving, and suitability for high-temperature process applications. However, since the distribution of microwave energy is difficult to control, under ordinary microwave heating conditions, the electromagnetic wave signal has different phases, and there is local magnetic field enhancement or local magnetic field cancellation at each point in the heating area, which then causes problems such as uneven heating area and poor heating efficiency. At the same time, in a process environment above 600 degrees Celsius, as time goes by, carbon fiber is prone to react with oxygen and cause structural damage, resulting in uneven mechanical strength of the recycled carbon fiber and reduced performance, which does not meet the subsequent recycling application.

[0004] CN115947976A discloses a system and method for rapidly recovering carbon fiber composite materials based on in-phase microwave heating. The system includes a first heating device, a second heating device and a conveying device. The first heating device includes a first heating cavity, a first conveyor and a microwave generator of in-phase microwaves. The first conveyor runs through the first heating cavity, and the microwave generator is connected to the first heating cavity; the second heating device includes a second heating cavity, a second conveyor and an electric heater. The electric heater is arranged in the second heating cavity, and the second conveyor is arranged in the second heating cavity. The second conveyor conveys the product heated by the first heating device to the second heating cavity. Based on this system, a two-step heating method is then adopted, especially the first step of heating adopts the in-phase microwave heating method, so that the heating energy distribution in the in-phase plane is more concentrated and uniform. However, since the carbon fiber composite material has a shielding type for microwaves, the penetration thickness of electromagnetic waves to the carbon fiber composite material is generally only a few centimeters. Therefore, when the thickness or stacking thickness of the carbon fiber composite material is large, the discarded carbon fiber composite material must be fragmented, the heating time is relatively extended, and the heating environment needs to be strictly controlled to be low oxygen. Summary of the invention

[0005] In order to overcome the deficiencies in the prior art, the present invention provides a method for recycling carbon fiber by microwave spark discharge, which can increase the cracking rate of the resin matrix in the microwave process, and the performance retention rate of the recovered carbon fiber is stabilized at more than 90%.

[0006] To achieve the above object, the present invention provides a method for recycling carbon fiber by microwave spark discharge, comprising:

[0007] Step 1: In a microwave heating device, the carbon fiber composite material is subjected to in-phase microwave heating, and at the same time, a uniform spark discharge is performed in the in-phase microwave field, and the position where the spark discharge occurs is controlled within the same horizontal plane height in the in-phase microwave field, so that the carbon fiber composite material is quickly and evenly heated to 500°C within 1-5 minutes, and as the microwave and spark discharge time increases, the temperature can continue to rise rapidly to crack the resin in the composite material, complete the separation of the carbon fiber and the resin in the carbon fiber composite material, and obtain carbon fiber. The carbon fiber composite material can be a small-sized recycled material after crushing, or a large-sized recycled material that has not been crushed, and the stacking height in the microwave heating device is controlled to be 10-150mm.

[0008] The spark discharge referred to in the present invention refers to the fact that after metal is placed in a microwave field, the free electrons of the metal itself will vibrate due to the microwave. When the placed metal has obvious edges or sharp points, such as the corners of aluminum foil or the sharp teeth of a metal fork, the charge will be concentrated at the obvious edges or sharp points, thereby generating a high-concentration voltage. When it reaches a certain level, gas breakdown will occur, causing a discharge reaction (spark) in the microwave oven. In the prior art, this situation is generally considered to cause equipment damage and needs to be avoided.

[0009] In the present invention, in-phase microwave heating can make the heating energy more concentrated and uniformly distributed in the in-phase plane, but because the conductor (carbon fiber composite material) has a shielding effect on electromagnetic waves, the penetration thickness of electromagnetic waves into carbon fiber composite materials is generally only a few centimeters. For the heating recovery of carbon fiber composite materials with a large stacking thickness, its thickness has a certain shielding effect on microwaves, resulting in uneven heating in the thickness direction. Therefore, spark discharge is performed while in-phase microwave heating, which can further induce heating uniformity in the thickness direction of the carbon fiber composite material.

[0010] First, the current intensity generated by spark discharge is usually large, which will release a lot of heat, which can directly increase the microwave environment temperature. The increase in microwave environment temperature can indirectly increase the heating rate of unit volume of material in the microwave field. The calculation formula is as follows:

[0011]

[0012] Where △T is the heating rate, is the microwave frequency (Hz), E is the electric field intensity, tanδ is the dielectric loss, is the relative dielectric constant, d is the material density, and C is the specific heat. In the above formula, the material is a carbon fiber composite material, and its heating rate is positively correlated with the dielectric constant and dielectric loss, while the dielectric constant and dielectric loss will increase with the increase of ambient temperature. Therefore, the rapid increase in ambient temperature accelerates the absorption effect of carbon fiber composite materials on microwaves, improves the energy utilization rate of microwaves, and brings about an increase in the heating rate of carbon fiber composite materials.

[0013] In addition to improving the heating uniformity in the thickness direction, the spark discharge in the microwave heating device can also promote the reaction of VOCs with oxygen. After the resin matrix is ​​cracked, it will be transformed into macromolecular gases, such as acetone, toluene, etc., and during the spark discharge, a large amount of plasma will be generated, which catalyzes the chemical reaction of acetone and toluene to generate water and carbon dioxide. On the one hand, this reaction reduces the cost of handling harmful substances, and on the other hand, it consumes oxygen in the microwave cavity, reduces the oxygen content in the microwave cavity, and is conducive to maintaining the performance of carbon fiber.

[0014] Therefore, the present invention can heat the carbon fiber composite material quickly and evenly in the microwave in-phase plane and in the microwave input direction, so that the carbon fiber composite material reaches the temperature required for resin disintegration and completes thermal cracking in a short time, and the oxygen content is simultaneously reduced during the cracking process, which greatly reduces the risk of thermal oxidation of the carbon fiber, so that the performance retention rate of the recovered carbon fiber is stabilized at more than 90%, and can even reach 95%, and the overall uniformity of the carbon fiber performance is increased; in addition, spark discharge in the microwave heating device can also degrade volatile organic compounds, which is more environmentally friendly.

[0015] In another embodiment, a spark discharge generating structure is arranged in the in-phase microwave heating device, wherein the spark discharge generating structure comprises a plurality of metal elements, wherein the metal elements have a tip or a protrusion, and the tip direction or the protrusion direction is arranged opposite to the microwave input direction.

[0016] In the above technical solution, when a metal element with a tip or protrusion is placed in the microwave field, microwave radiation will change the distribution of positive and negative charges inside the metal element, causing charged particles to gather at these tips or protrusions. At this time, the tips or protrusions of these metal elements act as electrodes under the action of the microwave electromagnetic field. When the radius of curvature of the electrode is very small, due to the extremely high electric field strength near it, gas breakdown will occur around the tip or protrusion of the metal element to induce discharge. The main forms of tip discharge are corona discharge and spark discharge. The current intensity of corona discharge is small and the heat generated is small. The current intensity of spark discharge is usually large and can instantly release a large amount of heat, which can achieve the coupling of microwave heating effect and microwave plasma effect.

[0017] In another embodiment, the tip or protrusion of the metal element is located no higher than the stacking height of the carbon fiber composite material in the microwave heating device.

[0018] In another embodiment, the spark discharge generating structure further comprises a carrier, and the metal element is arranged on the carrier. The carrier serves as a fixing seat for the metal element.

[0019] In another embodiment, the radius of curvature of the metal element is 1-50 μm.

[0020] In another embodiment, the metal element is made of any material selected from iron, steel, nickel, tantalum and platinum. As a preferred embodiment, a tungsten needle is preferred, and the tungsten needle has a slender structure.

[0021] In another embodiment, the overall length of the metal element is 10-150 mm.

[0022] In another embodiment, the plurality of metal elements may be arranged at equal intervals along the electric field direction; or arranged at equal intervals along the magnetic field direction; or arranged in a matrix.

[0023] In another embodiment, the surface density of the metal elements is controlled in the range of ≥30 pieces / 100 cm².

[0024] In another embodiment, the method for recycling carbon fiber by microwave spark discharge further comprises:

[0025] Step 2: In an environment with an oxygen concentration of 20%, the intermediate product obtained in step 1 is subjected to a second stage of heating at a temperature of 550° C. for 30 minutes.

[0026] The beneficial effects of the present invention are at least:

[0027] (1) In the present invention, on the basis of heating the carbon fiber composite material by microwaves in the same phase, spark discharge is performed at the same time. On the one hand, the current intensity of the spark discharge is usually large, which will release a large amount of heat, which can directly increase the microwave environment temperature, and then indirectly increase the heating rate of the carbon fiber composite material in the microwave field; on the other hand, after the microwave environment temperature increases, it will accelerate the absorption effect of the carbon fiber composite material on microwaves, improve the energy utilization rate of microwaves, and also quickly increase the heating rate of the carbon fiber composite material. Therefore, the carbon fiber composite material can reach the temperature required for the disintegration of the resin matrix and complete the cracking in a short time, improve the recycling capacity, and avoid the risk of oxidation of carbon fiber caused by long-term heating, improve the uniformity of the recycled carbon fiber, and the performance retention rate is stable at more than 90%.

[0028] (2) The spark discharge in the microwave heating device can also cause the VOCs produced during the cracking process to react with oxygen. On the one hand, it can reduce VOCs and reduce the cost of treating harmful substances. On the other hand, the reaction will consume oxygen in the microwave cavity and reduce the oxygen content in the microwave cavity. The continued reduction of oxygen content can further reduce the oxidative damage to carbon fiber, and the performance retention rate is stabilized at more than 90%. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Attached Figure 1 Schematic diagram of in-phase microwave spark discharge;

[0030] Attached Figure 2 It is the temperature rise curve of carbon fiber composite material with different heating methods;

[0031] Attached Figure 3 It is the resin removal rate curve of carbon fiber composite material under different heating methods;

[0032] Attached Figure 4 Schematic diagram of VOCs degradation in microwave spark discharge process;

[0033] Attached Figure 5 It is a schematic diagram of the spark discharge structure;

[0034] Attached Figure 6 A schematic diagram for arranging metal elements along the direction of the magnetic field;

[0035] Attached Figure 7 A schematic diagram showing the arrangement of metal elements along the direction of the electric field;

[0036] Attached Figure 8 The graph is a graph showing the relationship between different arrangement and setting modes of metal components and the removal rate of the resin matrix;

[0037] Attached Fig. 9 It is a curve diagram showing the influence of the tip or protrusion position of the metal element relative to the stacking height of the carbon fiber composite material on the resin matrix removal rate;

[0038] Attached Fig.10 This is a curve chart showing the effect of metal component surface density on resin removal rate.

[0039] Among them: 1-metal component; 2-carrier. DETAILED DESCRIPTION

[0040] The technical solution of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0041] In addition, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other. The present invention provides a method for recycling carbon fiber by microwave spark discharge, comprising:

[0042] Step 1: In a microwave heating device in the same phase, the carbon fiber composite material is subjected to microwave heating in the same phase. At the same time, a uniform spark discharge is performed in the microwave field in the same phase. The position where the spark discharge occurs is controlled within the same horizontal plane height in the microwave field in the same phase, so that the carbon fiber composite material is quickly and evenly heated to 500°C within 1-5 minutes, and as the microwave and spark discharge time increases, the temperature can continue to rise rapidly to crack the resin in the carbon fiber composite material and complete the separation of the carbon fiber and the resin in the carbon fiber composite material. The carbon fiber composite material can be a thermosetting resin carbon fiber composite material such as an epoxy resin carbon fiber composite material and a phenolic resin carbon fiber composite material, and the form can be a small-sized carbon fiber composite material that has been crushed, or a large-sized carbon fiber composite material that has not been crushed.

[0043] like Figure 1As shown, in the present application, in-phase microwave heating can concentrate the heating energy within the in-phase plane and distribute it more evenly. However, for the heating recovery of thick materials, their thickness has a certain shielding effect on microwaves, resulting in a temperature difference in the thickness direction. Therefore, spark discharge is performed while in-phase microwave heating is performed, which can further induce heating uniformity in the thickness direction of the material.

[0044] On the one hand, the current intensity of spark discharge is usually large, which will release a lot of heat, which can accelerate the temperature of the microwave environment, thereby accelerating the absorption effect of carbon fiber composite materials on microwaves, improving the energy utilization rate of microwaves, and greatly improving the heating rate of carbon fiber composite materials. Figure 2 As shown, it is a temperature rise curve of carbon fiber composite material under different heating methods and different heating times, wherein: the carbon fiber composite material is epoxy resin carbon fiber composite material, the accumulation per unit area is 10-15g / cm², the heating power used for electric heating is 100KW, the heating power used for the other three microwave heating methods is 20KW, and the microwave frequency is 915Hz. The figure shows that the heating rate of the carbon fiber composite material is the slowest in the electric heating method, which is about 10°C / minute. After 30 minutes of continuous heating at this heating power, the carbon fiber composite material is difficult to reach 400°C; when using general microwave heating, the heating rate is 15°C / minute, which is slightly higher than electric heating. After 30 minutes of continuous heating, the carbon fiber composite material can reach more than 450°C; the same-phase microwave heating has a faster heating rate than ordinary microwave heating due to the uniform and concentrated microwave energy. The heating rate is about 47.5°C / minute in the first 10 minutes, and the temperature of the carbon fiber composite material can reach 500°C after 10 minutes of heating; and the same-phase microwave spark discharge of the present application has a heating rate of 95°C / minute in the first 5 minutes due to the spark discharge effect. Compared with the first three heating methods, the temperature required for the resin matrix to crack can be reached in a very short time.

[0045] See attached Figure 3 The graph is a graph of the resin matrix removal rate of carbon fiber composite materials under four heating methods. It can be seen from the graph that the microwave spark discharge recycling of carbon fiber in the present invention can remove more than 90% of the resin within 10 minutes, and the resin removal rate is much higher than that of the other three heating methods.

[0046] In addition, the discharge can generate plasma, which can achieve the coupling of microwave heating effect and microwave plasma effect to degrade VOCs. Figure 4When carbon fiber composite materials are heated, the resin matrix will be cracked and transformed into large molecular gases, such as acetone, toluene, etc., and during spark discharge, a large amount of plasma will be generated, catalyzing the chemical reaction of acetone and toluene to generate water, carbon monoxide, and carbon dioxide. When microwave spark discharge produces high-energy ionization or plasma, a large number of high-energy electrons, ions, free radicals and other active substances (such as oxygen free radicals, hydrogen free radicals, hydroxyl free radicals, etc.) will be formed. These high-energy particles can collide and react with organic waste molecules to decompose them. For example, free radical reactions, the free radicals formed in the plasma (such as -OH, -O, etc.) have extremely high chemical activity and can oxidize or decompose organic pollutants and convert them into carbon dioxide, water or other harmless small molecules; and electron bombardment, high-energy electrons will break the bonds of organic molecules to form smaller fragments or free radicals, which can further undergo secondary reactions.

[0047] During the microwave spark discharge process, toluene is degraded under the action of spark discharge:

[0048] C7H8+9O2 7CO2 +4H2O...Toluene degradation

[0049] During the microwave spark discharge process, acetone degrades under the action of spark discharge:

[0050] C3H6O+4O2 3CO2+3H2O……Acetone degradation

[0051] The in-phase microwave heating device is provided with a spark discharge structure. Figure 5 , shows a spark discharge structure, the spark discharge structure includes a plurality of metal elements 1, the metal elements 1 have a tip or a protrusion, the curvature radius of the tip or the protrusion is 1-50 μm. Further, the spark discharge structure also includes a carrier 2, the carrier 2 is used to install the metal element 1.

[0052] See attached Figure 6 , which is one of the settings of the spark structure in the same-phase microwave heating device, the tip direction or protrusion direction of the metal element 1 is set relative to the microwave input direction, and a plurality of metal elements 1 are arranged in two rows with equal spacing along the magnetic field direction; see the attached Figure 7 , is another arrangement of the spark structure in the same phase microwave heating device, the tip or protrusion of the metal element 1 is opposite to the microwave input direction, and a plurality of metal elements 1 are arranged in two rows with equal spacing along the electric field direction. The effects of the two arrangements on the removal rate of carbon fiber composite material resin can be found in the attached Figure 8As can be seen from the figure, under the same heating time, when several metal components are arranged along the direction of the electric field, the resin removal rate of the carbon fiber composite material is close to 90% after 5 minutes of microwave heating spark discharge, and the resin cracking can be completed after 10 minutes.

[0053] See also Figure 1 The stacking height of the carbon fiber composite material in the microwave oven is recorded as H, and the tip or protrusion of the metal element 1 should not be higher than the stacking height H of the carbon fiber composite material. Fig. 9 , shows the resin removal rate curves when the tip or protrusion of the metal element 1 is located at 50%, 100%, and 120% of the stacking height of the carbon fiber composite material (other conditions are the same). It can be seen from the figure that under the same heating time, when the tip or protrusion of the metal element 1 is located at 50% of the stacking height of the carbon fiber composite material, the resin removal rate is the highest, and when it is located at 100% of the stacking height of the carbon fiber composite material, the resin removal rate is second, and when the tip or protrusion of the metal element 1 is higher than the stacking height of the carbon fiber composite material, the resin matrix removal rate effect is average.

[0054] The metal element 1 is made of any material selected from iron, steel, nickel, tantalum and platinum. The metal element is preferably a tungsten needle.

[0055] The overall length of the metal element 1 is 10-150 mm.

[0056] In the present application, in the spark discharge structure, the surface density of the metal element 1 also affects the removal rate of the resin matrix. When a plurality of metal elements 1 are arranged in two rows along the electric field direction, the relationship between the surface density and the removal rate of the resin matrix is ​​shown in the attached figure. Fig.10 As shown in the curve graph, it can be seen that the greater the surface density of the metal component 1, the better the removal rate of the resin matrix. When the surface density reaches 300 pieces / 100cm², the resin removal rate can reach 90%.

[0057] The present invention provides a method for recycling carbon fiber by microwave spark discharge, which also includes:

[0058] Step 2: In an environment with an oxygen concentration of 20%, the carbon fiber obtained in step 1 is heated for a second time at a heating temperature of 550° C. for 30 minutes.

[0059] Through this step, cleaner and softer carbon fiber can be obtained.

[0060] Obviously, the above embodiments are only examples for clearly explaining the present invention, and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all implementation methods here. The obvious changes or modifications derived from these are still within the protection scope of the present invention.

Claims

1. A method for recycling carbon fiber by microwave spark discharge, characterized in that: include: Step 1, in a co-phase microwave heating device, the carbon fiber composite material is subjected to in-phase microwave heating, and at the same time, uniform spark discharge is performed in the in-phase microwave field, and the position where the spark discharge occurs is controlled within the same horizontal plane height in the in-phase microwave field, so that the carbon fiber composite material is quickly and evenly heated to 500°C within 1-5 minutes, and as the microwave and spark discharge time increase, the temperature can continue to and quickly rise to crack the resin in the carbon fiber composite material, complete the separation of the carbon fiber and the resin in the carbon fiber composite material, and obtain the carbon fiber; the in-phase microwave heating device is provided with a spark discharge generating structure; the spark discharge generating structure includes a plurality of metal elements, the metal element has a tip or a protrusion, and the tip direction or the protrusion direction is arranged relative to the microwave input direction.

2. The method for recycling carbon fiber by microwave spark discharge according to claim 1, characterized in that: The tip or protrusion of the metal element is not higher than the stacking height of the carbon fiber composite material in the microwave heating device.

3. The method for recycling carbon fiber by microwave spark discharge according to claim 1, characterized in that: The spark discharge generating structure further comprises a carrier, and the plurality of metal elements are arranged on the carrier.

4. The method for recycling carbon fiber by microwave spark discharge according to claim 1, characterized in that: The tip or protrusion of the metal element has a curvature radius of 1-50 μm.

5. The method for recycling carbon fiber by microwave spark discharge according to claim 1, characterized in that: The metal element is made of any one of tungsten, iron, steel, nickel, tantalum and platinum.

6. The method for recycling carbon fiber by microwave spark discharge according to claim 1, characterized in that: The overall length of the metal element is 10-150 mm.

7. The method for recycling carbon fiber by microwave spark discharge according to claim 1, characterized in that: The plurality of metal elements may be arranged at equal intervals along the direction of the microwave electric field; or arranged at equal intervals along the direction of the magnetic field; or arranged in a matrix.

8. The method for recycling carbon fiber by microwave spark discharge according to claim 1, characterized in that: Also includes: Step 2: In an environment where the oxygen concentration is not higher than 20%, the carbon fiber obtained in step 1 is subjected to a second heating process at a temperature of 550° C. for a heating time of 15-30 minutes.

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