Preparation method of a phosphorus-based flame-retardant degradable epoxy resin and carbon fiber composite material thereof

Abstract

This invention discloses a method for preparing a flame-retardant and biodegradable epoxy resin based on diphenyl phosphate and its carbon fiber composite material, belonging to the field of polymer materials and composite materials technology. The method involves premixing tetrahydrophthalic acid diglycidyl ester with the flame retardant diphenyl phosphate, adding a curing agent 4,4′-diaminodiphenylmethane, and obtaining a flame-retardant epoxy resin cured product through degassing and staged curing. Further, carbon fiber fabric is impregnated with this resin mixture, and then cured under pressure to obtain a carbon fiber composite material sheet. This flame-retardant composite material can be mildly degraded in a mixed solvent of tetrahydrofuran and sodium hydroxide aqueous solution, achieving non-destructive recycling of carbon fibers. This invention abandons the cumbersome and complex monomer synthesis process, and the prepared resin possesses high intrinsic transparency, good mechanical properties, and efficient flame-retardant and smoke-suppressing characteristics (reaching UL-94 V-0 level). Furthermore, the composite material can degrade rapidly, exhibiting significant economic and environmental benefits.

Description

Technical Field

[0001] This invention relates to the field of polymer materials and composite materials technology. Specifically, it relates to a method for phosphorus-based flame-retardant modification of glycidyl ester type epoxy resin, and the application and recycling methods of this resin in carbon fiber composite materials. Background Technology

[0002] Glycidyl ester-based epoxy resins have potential applications in aerospace composites and electronic packaging due to their low viscosity and excellent weather resistance. However, the resin matrix and its carbon fiber composites are inherently flammable, and the insoluble and infusible three-dimensional cross-linked network formed after curing makes it difficult to degrade and recycle under mild conditions, thus posing a dual challenge of fire safety hazards and resource and environmental burden.

[0003] For flame retardant and degradation modification of epoxy resins, existing technologies typically rely on synthesizing complex reactive monomers or curing agents. For example, patent CN109608620B requires multi-step nucleophilic substitution and oxidation reactions of polyphosphazene to prepare epoxy-containing prepolymers, while patent CN118978669B requires multiple synthetic steps, including Mannich reaction and phosphorylation, to obtain benzoxazine phosphate curing agents. These synthetic routes are generally lengthy, involving toxic solvents and complex purification operations, limiting their feasibility for large-scale engineering. Furthermore, patents CN114409873B and CN119707877B, in order to balance flame retardancy and degradation, introduce rigid conjugated structures such as Schiff bases into the main chain. These chromophores cause the cured product to darken in color, sacrificing optical transparency. Regarding performance synergy, the introduction of conventional low-valent phosphorus-based flame retardants such as 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), while promoting char formation, severely disrupts crosslinking density, leading to a significant deterioration in glass transition temperature and tensile strength. Simultaneously, its smoke suppression effect is poor, resulting in persistently high total smoke emissions. In terms of degradation and recycling, patent CN116023761B relies on the reversible breakage of Schiff base bonds under acidic conditions, still posing a risk of potential chemical damage to the carbon fiber surface. Meanwhile, the transesterification-based degradation system of patent CN117820816B is time-consuming and exhibits relatively slow degradation kinetics.

[0004] This invention aims to overcome the aforementioned technical limitations, abandoning the cumbersome monomer synthesis method and instead using mature industrial products to directly construct a cross-linked network that combines flame retardancy, biodegradability, and high transparency in situ. By introducing a specific flame-retardant formulation into the matrix, it strives to maintain the resin's ultra-high intrinsic transparency while synergistically achieving UL-94 V-0 self-extinguishing rating and significant smoke suppression. This overcomes the problem of severe mechanical property degradation in conventional flame-retardant systems. Furthermore, by utilizing the multifunctional catalytic effect of this component, it enables rapid and non-destructive degradation of the resin matrix and efficient recycling of carbon fibers under extremely mild conditions. Summary of the Invention

[0005] The purpose of this invention is to provide a phosphorus-containing epoxy resin cured product that combines high transparency, good mechanical properties, and efficient flame retardant characteristics, and to apply it to carbon fiber reinforced composite materials that can be rapidly chemically degraded and recyclable without damage to the carbon fibers. By introducing reactive phosphate ester flame retardants into the epoxy resin matrix, a phosphorus-containing cross-linked network is constructed during the curing process, thereby significantly reducing heat and smoke release during combustion without significantly deteriorating mechanical properties and transparency, and endowing the composite material with controllable degradation capability under mild conditions.

[0006] The objective of this invention is achieved through the following technical solution: a method for preparing reactive flame-retardant epoxy resin and biodegradable carbon fiber composite materials, the method comprising the following steps: (1) Preparation of flame-retardant epoxy resin a Premixed flame retardant and epoxy monomer Diglycidyl tetrahydrophthalate and diphenyl phosphate flame retardant were mixed in a molar ratio and stirred at 60 °C until completely dissolved and a homogeneous transparent liquid was formed.

[0007] b. Add curing agent Add stoichiometric amounts of curing agent 4,4′-diaminodiphenylmethane to the above premixed liquid and stir at 60 °C until the system is homogeneous.

[0008] c. Degassing and Curing The mixture was degassed in a vacuum oven at 60 ℃ and poured into a preheated mold. A staged curing process was adopted: curing at 90 ℃ for 2 hours, then curing at 140 ℃ for 2 hours, and finally cooling and demolding to obtain flame-retardant epoxy resin cured product.

[0009] The flame retardant diphenyl phosphate has a mass fraction of 10% or 15%.

[0010] The amount of the curing agent 4,4′-diaminodiphenylmethane is based on maintaining the epoxy group to amine hydrogen equivalent ratio.

[0011] (2) Preparation of flame-retardant carbon fiber composite materials The degassed resin mixture from step (1) is used as an impregnation solution. Carbon fiber fabric is embedded in it using a manual lay-up process to form a resin-fiber-resin laminated structure. After the mold is closed, a curing process of 90 ℃ / 2 h and 140 ℃ / 2 h is performed at a pressure of 18 MPa. After cooling, the mold is opened to obtain a flame-retardant carbon fiber composite material sheet.

[0012] (3) Non-destructive recycling of carbon fiber Flame-retardant carbon fiber composite material was immersed in a mixed solvent of tetrahydrofuran and 20% sodium hydroxide aqueous solution, and stirred at 60°C until the resin matrix was completely dissolved. The carbon fibers were separated by filtration, washed, and dried to obtain clean recycled carbon fiber fabric; adding acid to the filtrate precipitated degradation products.

[0013] Due to the implementation of the above technical solutions, the advantages and effects of this invention are: (1) Synergistic flame retardant and smoke suppression effect: Diphenyl phosphate plays a flame retardant role in both the gas phase and the condensed phase, which increases the limiting oxygen index of epoxy resin to 29.0%, the vertical burning rating to V-0, the peak heat release rate and the total heat release are significantly lower than those of pure resin, and the total smoke release is reduced by more than 60%.

[0014] (2) Good mechanical and optical properties: The tensile strength and flexural strength of the modified resin decrease only slightly, while maintaining a high visible light transmittance, overcoming the weakness of traditional additive flame retardants that severely degrade mechanical and optical properties.

[0015] (3) Efficient degradation of composite materials and non-destructive recycling of fibers: The prepared carbon fiber composite material plates can achieve rapid and complete degradation of the resin matrix under mild conditions. There is no resin residue on the surface of the recycled carbon fibers and no damage to the fiber structure, which shows excellent recyclability. Attached Figure Description

[0016] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below, wherein: Figure 1 A schematic diagram of the cross-linking network of each component; Figure 2 Digital photographs showing the transparency of DCNC and its composite materials; Figure 3 Digital photograph of the DCNC / DPP15%-CF composite sample; Figure 4 Digital photograph of a DCNC / DPP15%-CF composite sample placed in a tetrahydrofuran / 20% sodium hydroxide mixed solution; Figure 5 Digital photograph of the DCNC / DPP15%-CF composite sample after being reacted in a tetrahydrofuran / 20% sodium hydroxide mixed solution at 60 °C for 2 h. Figure 6 Scanning electron microscope image for recovering carbon fibers. Detailed Implementation

[0017] The present invention will now be described in detail with reference to specific embodiments, but the present invention is not limited to these embodiments. Example 1

[0018] This embodiment prepares pure diglycidyl tetrahydrophthalate (DCNC) resin and its carbon fiber composite material without flame retardants, which serves as a reference for subsequent embodiments.

[0019] 75 parts by weight of DCNC were placed in a container and preheated to 60°C for 10 minutes. Then, 25 parts by weight of curing agent DDM were added and stirred for 5 minutes until the system was completely homogeneous. The mixture was placed in a vacuum oven for degassing and then poured into a preheated PTFE mold. A staged curing process was used: curing at 90°C for 2 hours, followed by curing at 140°C for 2 hours. After cooling and demolding, pure DCNC resin samples were obtained, denoted as DCNC. The degassed DCNC and DDM mixture was used as an impregnation solution and manually laid into a stainless steel mold pre-lined with a layer of carbon fiber fabric. After closing the mold, the mixture was pressed under 18 MPa pressure using the same curing process (90°C / 2h + 140°C / 2h) to obtain a control group carbon fiber composite material, denoted as DCNC-CF.

[0020] The properties of the prepared DCNC resin strips and DCNC-CF composite plates were characterized. Tests included thermal properties (Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), Dynamic Thermomechanical Analysis (DMA), mechanical properties (tensile and flexural), flame retardant properties (Limiting Oxygen Index (LOI), Vertical Burning UL-94), and cone calorimetry (CCT). Detailed data are shown in Tables 1-4. Regarding the glass transition temperature (Tg), the Tg measured by DMA was 118.0 ℃, and the Tg measured by DSC was 108.3 ℃. Thermal stability was characterized by TGA; the initial decomposition temperature (T5%) was 285.7 ℃, and the half-decomposition temperature (T5%) was... 50 The percentage of the maximum thermal weight loss rate was 379.3 °C, and the temperature at which the maximum thermal weight loss rate (T) was [missing information]. max The residual carbon content (R) at 800℃ was 349.3℃. 800 The content of the flammability component was 16.6 wt%. In terms of mechanical properties, the tensile strength of DCNC resin was 82.8 MPa, the fracture strain was 22.3%, and the flexural strength was 142.5 MPa. Simultaneously, the resin exhibited high visible light transmittance (89.0%), indicating that it is intrinsically a transparent material. In the flame retardant performance evaluation, DCNC had a LOI of only 24.5%, received no rating (NR) in the UL-94 vertical burning test, and had a total burning time exceeding 30 s. Cone calorimetry testing showed a peak heat release rate (pHRR) as high as 1047.07 kW / m², a total heat release (THR) of 58.02 MJ / m², and a total smoke release (TSP) of 16.15 m², proving that pure DCNC resin is extremely flammable, with concentrated heat release and high smoke release during combustion, and therefore lacks intrinsic flame retardant capabilities.

[0021] The pHRR of DCNC-CF is 960.2 kW / m², THR is 15.3 MJ / m², and TSP is 2.6 m². Compared with pure resin, thanks to the non-combustible dilution effect and physical barrier function of carbon fiber, the heat and smoke release of the board are significantly reduced. However, the peak heat release is still at a high level, indicating that its fire risk is still significant. This set of data provides a reference benchmark for subsequent flame retardant modification. Example 2

[0022] This embodiment introduces the reactive flame retardant DOPO. The preparation method is similar to that of Example 1. The difference is that in the step of preheating DCNC at 60°C, 10 parts by weight and 15 parts by weight of DOPO are added respectively, and stirred until completely dissolved. Then, stoichiometric amounts of DDM (maintaining the equivalence ratio of epoxy groups to amine hydrogen) are added and stirred until homogeneous. After vacuum degassing, the mixture is poured into a mold and cured at 90°C / 2h + 140°C / 2h. The resulting samples are denoted as DCNC / DOPO10% and DCNC / DOPO15%, respectively. The crosslinking network diagram is shown below. Figure 1 As shown.

[0023] The DCNC / DOPO 10% system: the glass transition temperature measured by DMA was 95.3 ℃, and by DSC it was 96.5 ℃. Thermogravimetric analysis showed that T5% was 285.7 ℃, and T... 50 The % char content was 385.7 ℃, the Tmax was 344.7 ℃, and the char residue at 800 ℃ increased to 23.6 wt%. In terms of mechanical properties, the tensile strength was 66.2 MPa, the fracture strain was 14.1%, and the flexural strength was 87.6 MPa. The sample maintained high transparency, with a visible light transmittance of 91.2%. In the flame retardancy test, the LOI was 27.0%, there was no UL-94 rating, and the pHRR measured by cone calorimetry was 876.70 kW / m², THR was 39.73 MJ / m², and TSP was 15.40 m².

[0024] The DCNC / DOPO 15% system showed that the Tg measured by DMA and DSC decreased to 89.3 ℃ and 87.3 ℃, respectively. TGA characterization results showed a T5% of 253.6 ℃ and a Tg of [missing value]. 50The temperature was 392.3 ℃, Tmax was 334.3 ℃, and R800 increased to 24.5 wt%. Mechanical properties further deteriorated, with tensile strength decreasing to 47.2 MPa, strain decreasing to 9.0%, and flexural strength to 78.5 MPa. Transparency improved slightly, reaching 94.4%. Regarding flame retardancy, LOI only increased to 28.0%, UL-94 remained unrated, pHRR decreased to 774.14 kW / m² (a 26.1% decrease compared to pure DCNC), and THR decreased to 33.30 MJ / m², but TSP (16.01 m²) remained essentially the same as pure resin, indicating no significant contribution to smoke suppression. Overall, DOPO has limited ability to improve char formation and flame retardancy ratings and significantly damages mechanical properties. Example 3

[0025] This embodiment introduces the reactive flame retardant DPP. 68 parts by weight of DCNC epoxy resin and 10 or 15 parts by weight of the flame retardant diphenyl phosphate (DPP) are accurately weighed and added to a reaction vessel. The system is heated to 60°C and stirred continuously at this temperature for 10 minutes to ensure complete dissolution of DPP and the formation of a homogeneous, transparent liquid with DCNC. A stoichiometric amount of DDM (maintaining the equivalence ratio of epoxy groups to amine hydrogen) is added to the premixed liquid, and stirring is continued at 60°C for 5 minutes until the system becomes a homogeneous, transparent melt. The resulting mixture is quickly transferred to a preheated vacuum oven and degassed under vacuum conditions at 60°C. The degassed mixture is carefully poured into a polytetrafluoroethylene mold coated with a release agent. The mold is placed in a forced-air oven and a staged curing procedure is performed: first, curing at 90°C for 2 hours, followed by further curing at 140°C for 2 hours. After curing, allow it to cool naturally to room temperature, then demold to obtain the final flame-retardant epoxy resin, denoted as DCNC / DPP10% and DCNC / DPP15%. A schematic diagram of the crosslinking network is shown below. Figure 1 As shown.

[0026] The Tg measured by DMA and DSC for the 10% DCNC / DPP system were 86.4 ℃ and 88.6 ℃, respectively. Thermogravimetric analysis showed that T5% was 197.7 ℃. 50 The % is 346.7 ℃, Tmax is 333.7 ℃, and the char residue at 800 ℃ increases to 25.7 wt%. Mechanically, the tensile strength is 57.7 MPa, the fracture strain is 10.0%, the flexural strength is 112.8 MPa, and transparency remains good (transmittance 93.5%). In terms of flame retardancy, the LOI is 27.0%, there is no UL-94 rating, the pHRR decreases to 595.48 kW / m², the THR is 38.40 MJ / m², and the TSP decreases significantly to 6.45 m², a 60.1% reduction compared to pure resin.

[0027] The Tg of DCNC / DPP15% was 79.5 °C and 77.5 °C in DMA and DSC tests, respectively. TGA results showed a T5% Tg of 220.7 °C. 50 The % concentration was 341.0 ℃, the Tmax was 325.3 ℃, and the R800 was as high as 28.8 wt%, which are 1.7 times that of pure resin and 1.2 times that of the highest value of DOPO (24.5 wt%). In terms of mechanical properties, the tensile strength remained at 53.4 MPa, 13.1% higher than that of DCNC / DOPO 15%; the flexural strength was 97.9 MPa, 24.7% higher than that of DCNC / DOPO 15%. Transparency remained excellent, with a transmittance of 95.6%. Regarding flame retardancy, this sample showed a significant improvement. The LOI value reached 29.0%, passed the UL-94 V-0 rating in the vertical burning test, and the total afterflame time after two ignitions was only 4.3 s, achieving self-extinguishing upon removal of the flame. Cone calorimetry data further confirmed that the pHRR was 533.48 kW / m², a decrease of 49% compared to pure DCNC; the THR was 32.22 MJ / m², a decrease of 44.5%; and the TSP was 5.72 m², a decrease of 64.6%, demonstrating a significant smoke suppression effect.

[0028] A horizontal comparison reveals that the DPP-modified system outperforms the DOPO system in both gas-phase flame retardancy and char formation. Regarding thermal stability, the introduction of DPP lowers the initial decomposition temperature but significantly increases the high-temperature char residue. In terms of mechanical property retention, the DPP system shows a smaller decrease in tensile and flexural strength compared to the DOPO system with the same addition amount. Regarding flame retardancy, the DOPO system failed to improve the UL-94 rating, while the DPP system achieved a V-0 rating at a 15% addition, with more significant reductions in pHRR, THR, and TSP. Example 4

[0029] This embodiment systematically describes the process of preparing carbon fiber reinforced composite material (CFRP) using the optimal flame retardant formulation described in Example 3 as the matrix, verifies its flame retardant advantages compared to pure resin-based composite materials, and demonstrates its rapid degradation under mild conditions and the ability to recycle carbon fibers without damage.

[0030] Following the proportions and procedures of Example 3, 68 parts by weight of DCNC and 15 parts by weight of DPP were weighed and premixed at 60°C for 10 minutes. 17 parts by weight of curing agent DDM were added, and stirring continued at 60°C for 5 minutes until homogeneous and transparent. Subsequently, the mixture was degassed in a 60°C vacuum oven to obtain a 15% DCNC / DPP resin premix. In a clean stainless steel lower mold, an appropriate amount of the degassed 15% DCNC / DPP resin mixture was poured in. A layer of carbon fiber fabric, precisely cut to the mold dimensions, was laid flat on the resin surface. The remaining resin mixture was poured in evenly, completely covering the carbon fiber fabric, forming a "resin-fiber-resin" sandwich layer structure. The upper mold was closed, and the mold was placed in a hot press. A pressure of 18 MPa was applied, and a staged heating and curing process was performed under this pressure: first, curing at 90°C for 2 hours, then at 140°C for 2 hours. After curing, the pressure was maintained, and the mixture was allowed to cool naturally to room temperature before the mold was opened and the composite material sheet was removed. The resulting flame-retardant carbon fiber composite material is designated as DCNC / DPP15%-CF. The control group DCNC-CF was prepared using pure DCNC resin with the same layup and compression molding process.

[0031] Compared with the control group DCNC-CF, the combustion parameters of the DCNC / DPP15%-CF composite material were significantly reduced: pHRR decreased by 52.8%, THR decreased by 52.9%, and TSP decreased by 26.9%. The data show that the introduction of DPP significantly suppressed the combustion intensity and smoke release of the composite material, and significantly improved its safety in fire scenarios.

[0032] As attached Figure 2-4 As shown, the above flame-retardant carbon fiber composite material DCNC / DPP15%-CF was cut into small pieces and immersed in a mixed solvent of tetrahydrofuran and 20% sodium hydroxide aqueous solution (THF / NaOH, volume ratio 8:2). After continuous stirring at 60°C for 2 hours, the cured resin matrix in the composite material was completely dissolved, and the carbon fiber layer was completely peeled off while maintaining the fabric shape. The mixture was filtered to obtain recycled carbon fiber, which was repeatedly washed with distilled water until neutral, and dried to obtain clean, flexible recycled carbon fiber fabric. Adding hydrochloric acid to the filtrate for neutralization precipitated resin degradation products. The recycled carbon fiber was observed by scanning electron microscopy (SEM). Figure 5 As shown, its surface is clean and smooth, with no resin residue. The inherent longitudinal groove structure of the fiber is clear and complete, with no damage such as etching or cracks, proving that the degradation process achieves non-destructive recycling of the carbon fiber itself.

[0033] Table 1. Thermal transition temperature and thermal stability parameters of epoxy resin cured products DCNC 118.0 108.3 285.7 379.3 349.3 16.6 DCNC / DOPO10% 95.3 96.5 285.7 385.7 344.7 23.6 DCNC / DOPO15% 89.3 87.3 253.6 392.3 334.3 24.5 DCNC / DPP 10% 86.4 88.6 197.7 346.7 333.7 25.7 DCNC / DPP 15% 79.5 77.5 220.7 341.0 325.3 28.8 Table 2 Mechanical properties and optical transmittance of epoxy resin cured products DCNC 82.8 22.3 142.5 89.0 DCNC / DOPO10% 66.2 14.1 87.6 91.2 DCNC / DOPO15% 47.2 9.0 78.5 94.4 DCNC / DPP 10% 57.7 10.0 112.8 93.5 DCNC / DPP 15% 53.4 10.6 97.9 95.6 Table 3 Flame retardant properties and cone calorimetry test parameters of epoxy resin cured products DCNC 24.5 NR ＞30 1047.07 58.02 16.15 DCNC / DOPO10% 27.0 NR ＞30 876.70 39.73 15.40 DCNC / DOPO15% 28.0 NR ＞30 774.14 33.30 16.01 DCNC / DPP 10% 27.0 NR ＞30 595.48 38.40 6.45 DCNC / DPP 15% 29.0 V0 1.8+2.5 533.48 32.22 5.72 Table 4. Cone calorimetry test parameters for carbon fiber reinforced epoxy resin cured products DCNC-CF 960.2 15.3 2.6 DCNC / DPP15%-CF 453.2 7.2 1.9

Claims

1. A method for preparing a flame-retardant and biodegradable epoxy resin based on diphenyl phosphate, characterized in that, Includes the following steps: Step S1, Premixing: Diglycidyl tetrahydrophthalate and diphenyl phosphate flame retardant are mixed and stirred under heating until completely dissolved to form a homogeneous and transparent premixed liquid; Step S2, Adding curing agent: 4,4′-diaminodiphenylmethane curing agent is added to the premixed liquid obtained in Step S1 and stirred under heating until the system is homogeneous; Step S3, Degassing and curing: The mixture obtained in Step S2 is degassed under vacuum, then poured into a mold, cured using a staged curing process, cooled and demolded to obtain the flame-retardant and biodegradable epoxy resin.

2. The preparation method according to claim 1, characterized in that, In step S1, the amount of the flame retardant diphenyl phosphate added accounts for 10% or 15% of the total mass fraction.

3. The preparation method according to claim 1, characterized in that, In step S2, the amount of curing agent 4,4′-diaminodiphenylmethane is based on maintaining the equivalence ratio of epoxy groups to amine hydrogen; the heating temperature in both steps S1 and S2 is 60 °C.

4. The preparation method according to claim 1, characterized in that, In step S3, the temperature of vacuum degassing is 60°C; the staged curing process is as follows: first, cure at 90°C for 2 hours, then raise the temperature to 140°C and cure for 2 hours.

5. A method for preparing a flame-retardant and biodegradable carbon fiber composite material based on diphenyl phosphate, characterized in that, Using the mixture obtained after degassing in step S3 of the preparation method according to any one of claims 1-4 as an impregnation liquid, the method includes the following steps: combining the impregnation liquid with carbon fiber fabric through a layup process to form a resin-fiber-resin laminated structure. After the mold is closed, a staged heating and curing process is performed under pressure, followed by cooling and mold opening to obtain the flame-retardant and biodegradable carbon fiber composite material.

6. The preparation method according to claim 5, characterized in that, The pressure under the pressurization conditions is 18 MPa; the staged heating and curing procedure is as follows: curing at 90 ℃ for 2 hours, and then curing at 140 ℃ for 2 hours.

7. A non-destructive recycling method for the flame-retardant and biodegradable carbon fiber composite material obtained by the preparation method of claim 5 or 6, characterized in that, The process includes the following steps: immersing the flame-retardant and biodegradable carbon fiber composite material in a degradation mixed solvent, and reacting it under heating and stirring conditions until the resin matrix is ​​completely dissolved; The residue is filtered and separated, washed and dried to obtain regenerated carbon fiber fabric; acid is added dropwise to the filtrate for neutralization, and resin degradation products are precipitated.

8. The non-destructive recycling method according to claim 7, characterized in that, The degradation solvent is a mixture of tetrahydrofuran and a 20% sodium hydroxide aqueous solution.

9. The non-destructive recycling method according to claim 8, characterized in that, The volume ratio of tetrahydrofuran to a 20% sodium hydroxide aqueous solution is 8:2; the reaction temperature under the heating and stirring conditions is 60 °C, and the reaction time is 2 hours.

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