Method for recovering waste polyimide for curing epoxy resin

Abstract

The invention discloses a method for recovering waste polyimide for curing epoxy resin, which comprises the following steps: mixing a waste polyimide material with organic diamine serving as an ammonolysis reagent, stirring to react, recovering excessive organic diamine through reduced pressure distillation after the reaction is finished, mixing a degradation product with pure water, and carrying out ultrasonic treatment to uniformly disperse the degradation product; monomer diamine and amide products (filtrate) are filtered and separated, the filtrate serves as an epoxy resin curing agent to be directly used for curing epoxy resin, high-value utilization of degradation products is achieved, and the prepared epoxy resin material has good thermal stability and reprocessability. A new raw material source is provided for production of epoxy resin, and the comprehensive utilization efficiency of resources can be improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of polymer materials, and particularly relates to a method for recycling waste polyimide for curing epoxy resin. Background Art

[0002] Polyimide (PI) is widely used in the fields of electronics and aerospace due to its excellent thermal stability and electrical insulation properties. However, the recycling and reuse of waste polyimide face many challenges, especially for thermosetting polyimide, which is insoluble and infusible and difficult to degrade, posing a potential threat to the environment. Therefore, how to effectively recycle and utilize these waste polyimides has become an important topic in materials science research.

[0003] Currently, several effective methods have been developed to recycle PI, such as hydrolysis and catalytic hydrogenation. Among them, the base-catalyzed hydrolysis method is one of the most common methods for recovering monomers from thermoplastic and thermosetting PI. Catalysts represented by NaOH or KOH can break the C-N bond of PI during the hydrolysis reaction to generate diamine and carboxylate products [Huang, F. et al. Ind. Eng. Chem. Res. 51, 7001–7006 (2012); Honma, T. et al. The Journal of Supercritical Fluids 166, 105037 (2020)]. However, the strong hydrolysis involving bases usually requires high temperature or additional pressure, which may damage the structure of diamine and reduce the monomer yield. Catalytic hydrogenation provides a green and economically effective solution for the cleavage of C-N bonds, but improving the cleavage efficiency of C-N bonds is a difficult problem. Even when heated at 150 °C for 15 h, the cleavage efficiency of the C-N bond of soluble aliphatic PI is still very low, resulting in a diamine yield of only 9% [Liu, X. et al. Org. Lett. 25, 3066–3071 (2023)]. So far, due to the chemical stability conferred by the aromatic heterocyclic structure, the recycling of PI is usually achieved under harsh reaction conditions or complex post-treatment, resulting in a significant decrease in monomer yield. In addition, most of the current recycling methods rely on high-temperature and high-pressure degradation, and usually require harsh reaction conditions, complex separation processes, and may cause secondary pollution, which poses great challenges to industrial applications and environmental protection. In addition, this also limits the application of the recycled products and makes it difficult to achieve efficient resource recycling. Summary of the Invention

[0004] In view of the deficiencies in the prior art, the present invention provides a method for recycling waste polyimide for curing epoxy resin. By means of chemical recycling technology, waste polyimide is converted into high-value-added materials, which not only helps to reduce environmental pollution, but also provides a new raw material source for the production of epoxy resin. This process conforms to the concept of sustainable development and can improve the comprehensive utilization efficiency of resources.

[0005] The technical solution of the present invention is as follows:

[0006] A method for recycling waste polyimide for curing epoxy resin, which comprises the following steps:

[0007] (1) Mix the waste polyimide material with an ammonolysis reagent (organic diamine), stir at 80 - 120 °C for 2 - 6 hours until complete degradation. During this process, the C─N bond is broken by the organic diamine to form an amide product and monomeric diamine, and the excess organic diamine and degradation products are separated by vacuum distillation;

[0008] (2) Mix the degradation product obtained in step (1) with pure water, ultrasonicate until it is evenly dispersed, and separate the monomeric diamine (solid product) and amide product (filtrate) by filtration. The filtrate is stored in a refrigerator and can be directly used as a curing agent for epoxy resin;

[0009] (3) Mix the above-mentioned filtrate with an epoxy monomer with an aldehyde group evenly, pour it into a mold, evaporate the solvent, and heat-cure to obtain an epoxy resin polymer.

[0010] In a preferred embodiment of the present invention, the structural formula of the waste polyimide is:

[0011]

[0012] In a preferred embodiment of the present invention, the mass ratio of the waste polyimide material to the diamine is 1:2 - 20.

[0013] In a preferred embodiment of the present invention, the organic diamine is at least one of ethylenediamine, 1,3-propanediamine or 1,4-butanediamine.

[0014] In a preferred embodiment of the present invention, after filtration, a solid product and a filtrate are obtained. The solid product is 4,4'-diaminodiphenyl ether, and the filtrate contains an amide product, and the amide product contains an amide bond and an amino group.

[0015] In a preferred embodiment of the present invention, the preparation steps of the epoxy monomer are as follows:

[0016] A. Add vanillin, epichlorohydrin and a phase transfer catalyst to a single-necked flask, and stir and react at 60 °C - 110 °C for 2 - 10 h;

[0017] B. Cool the reaction solution of step A to room temperature, dropwise add an aqueous sodium hydroxide solution, and stir the reaction at room temperature for 2 - 5 h;

[0018] C. Dilute the reaction solution of step B with dichloromethane, extract the organic phase three times with water, dry the organic phase with anhydrous magnesium sulfate, filter, concentrate the organic phase, drop it into absolute ethanol, precipitate the product, and wash and purify the crude product with absolute ethanol; Dry under vacuum to obtain; The reaction equation is:

[0019]

[0020] In a preferred embodiment of the present invention, the phase transfer catalyst is tetrabutylammonium bromide or benzyltriethylammonium chloride.

[0021] In a preferred embodiment of the present invention, the molar ratio of vanillin, epichlorohydrin, and the catalyst is 1∶1 - 20∶0.1 - 1.

[0022] More preferably, the concentration of the sodium hydroxide solution is 20 - 50%.

[0023] In a preferred embodiment of the present invention, step (3) is specifically as follows:

[0024] 3.1) Dissolve the epoxy monomer with an aldehyde group in an organic solvent, drop it into the filtrate, stir evenly, pour it into a mold, react at room temperature for 2 - 8 h, and slowly evaporate the solvent at 40 - 60 °C;

[0025] 3.2) Carry out programmed temperature curing, and the curing program is: cure at 70 - 90 °C for 1 - 3 h, cure at 100 - 120 °C for 4 - 8 h, and cure at 140 - 150 °C for 1 - 3 h.

[0026] In a preferred embodiment of the present invention, the organic solvent is absolute ethanol, methanol, acetonitrile, N,N - dimethylacetamide, or N,N - dimethylformamide.

[0027] In a preferred embodiment of the present invention, the molar ratio of the curing group in the filtrate to the epoxy monomer with an aldehyde group is 3∶2, where the curing group refers to an amino group.

[0028] In a preferred embodiment of the present invention, the temperature - rising program is: cure at 80 °C for 2 h, cure at 100 °C for 4 h, and cure at 140 °C for 1 h.

[0029] The beneficial effects brought by the technical solution of the present invention:

[0030] 1. Through a mild ammonolysis reaction, PI is degraded into its monomers and amide products at low temperature and atmospheric pressure. In addition to efficiently recovering the ammonolysis reagent and monomer diamine through a simple method, the remaining amide products can be used to cure epoxy resin.

[0031] 2. The waste polyimide does not produce small molecule waste during the recycling process, and the organic diamine used as the ammonolysis reagent can be recycled and reused. The yield of the recovered diamine monomer is as high as 99%, realizing the high-value utilization of all degradation products.

[0032] 3. The amide products obtained by ammonolysis recovery are a mixture of polymers containing a large number of amide bonds and amino groups, which can be used to cure epoxy resin. The prepared epoxy resin system has good thermal stability, and the high char residue rate shows high fire safety.

[0033] 4. From the perspective of environmental protection, bio-based raw materials are selected to replace petroleum-based raw materials to synthesize epoxy monomers with aldehyde groups, and a dynamic Schiff base structure is introduced into the epoxy resin, so that the epoxy resin prepared by this method has good reprocessability.

[0034] Other features and beneficial effects of the present invention will be described in the subsequent specification, and, in part, will be obvious from the specification, or will be understood by implementing the present invention. Brief Description of the Drawings

[0035] Figure 1 is a schematic diagram of the polyimide ammonolysis mechanism in Example 1 of the present invention.

[0036] Figure 2 is a schematic diagram of the structure of the amide product (RDPI) in Example 1 of the present invention.

[0037] Figure 3 is the FTIR spectrum of the polyimide and the ammonolysis mixture.

[0038] Figure 4 is the nuclear magnetic resonance 1H NMR spectrum (a) and 13C NMR (b) spectrum of the ammonolysis mixture.

[0039] Figure 5 is the nuclear magnetic resonance hydrogen spectrum of the epoxy monomer synthesized in Example 1 of the present invention.

[0040] Figure 6 is the DSC curve (a) and thermogravimetric curve (b) of the epoxy resin prepared in Examples 1-5 of the present invention.

[0041] Figure 7 is a photo of the reprocessing process of the epoxy resin prepared in Example 1 of the present invention.

[0042] Figure 8It is the tensile stress-strain curve of the original epoxy resin and the reprocessed epoxy resin in Example 1 of the present invention.

[0043] Figure 9 It is the nuclear magnetic resonance hydrogen spectrum of the 4,4'-diaminodiphenyl ether monomer recovered in Example 1 of the present invention.

[0044] Figure 10 It is the thermogravimetric curve of the PI film prepared using the recycled 4,4'-diaminodiphenyl ether in Example 1 of the present invention. Detailed implementation manners

[0045] The present invention will be further explained below in conjunction with the accompanying drawings and specific embodiments.

[0046] The structural formula of the polyimide in the following examples is

[0047]

[0048] Example 1

[0049] Add 5 g of polyimide (PI-1) to 15 mL of ethylenediamine and stir at 90 °C for 2 h. After the polyimide is fully degraded, the degradation solution (ammonolysis mixture) is distilled under reduced pressure to recover ethylenediamine and the degradation product, and the recovered ethylenediamine can be reused;

[0050] Add distilled water to the degradation product and ultrasonically disperse it. 4,4'-diaminodiphenyl ether precipitates and is insoluble in water. Filter to collect the precipitate and the filtrate (amide product RDPI). The obtained precipitate is washed and dried under vacuum to obtain the recovered 4,4'-diaminodiphenyl ether; the collected filtrate is stored in the refrigerator and used as an epoxy resin curing agent.

[0051] As Figure 1 and Figure 2 shown, when the ammonolysis reagent reacts with PI, two amino groups can react with different parts of the PI molecular chain respectively to form multiple amide bonds. Therefore, there will be the following three situations in the amide product RDPI: one is that only one amino group in the ammonolysis reagent participates in the reaction, causing the imide to break, thereby generating a tetra-arm amine; the second is that the two amino groups of the ammonolysis reagent react with different molecular chains of PI respectively, thereby forming a cross-linked structure and forming a polymer. The third is that the two amino groups of the ammonolysis reagent form intramolecular cross-linking on the same PI molecular chain, thereby generating a cyclic structure or a polymer. Refer to Figure 3 the FTIR spectrum of polyimide and the ammonolysis mixture. It can be seen from the figure that the characteristic peaks (1775 and 1710 cm -1 ) of PI in the ammonolysis mixture disappear significantly. At the same time, a carbonyl stretch of about 1615 cm -1 and a benzene ring skeletal vibration of about 1496 cm -1, the C-O-C stretching vibration is about 1212 cm -1 , indicating that PI is completely degraded into amide products. Refer to Figure 4 the nuclear magnetic resonance of the ammonolysis mixture 1 HNMR spectrum (a) and 13 C NMR (b) spectrum. As can be seen from the figure, only a small amount of amide products are observed, which is due to the formation of amide polymers caused by the two amino-capped groups in the ammonolysis reagent.

[0052] Preparation of bio-based epoxy monomer: Weigh 7.6 g (0.05 mol) of vanillin, 18.5 g (0.2 mol) of epichlorohydrin, and 0.4 g (1 mmol) of tetrabutylammonium bromide and add them to a single-neck flask. After reacting at 80 °C for 2 h, cool to room temperature; slowly drip 50 wt% aqueous sodium hydroxide solution into the above reaction solution through a constant-pressure dropping funnel, and continue to stir and react at room temperature for 3 h; after the reaction is completed, add dichloromethane for dilution, extract the organic phase with water three times, dry the organic phase with anhydrous magnesium sulfate, filter, concentrate the organic phase, drop it into excess absolute ethanol, precipitate the product, wash and purify the crude product with absolute ethanol, and dry it under vacuum to obtain the epoxy monomer with an aldehyde group. As Figure 5 For the nuclear magnetic resonance hydrogen spectrum of the prepared epoxy monomer, it can be seen from the figure that the target epoxy monomer is successfully synthesized.

[0053] Dissolve 2 g of epoxy monomer in 5 mL of acetonitrile, add the filtrate as the curing agent according to the molar ratio of the amino group of the curing agent to the epoxy monomer of 1.5:1, stir evenly, pour the solution into a Teflon mold, react at room temperature for 4 h, then put it into an oven at 60 °C to evaporate the solvent for 12 h, and then cure by programmed heating. The specific program is: cure at 80 °C for 2 h, cure at 100 °C for 4 h, and cure at 140 °C for 1 h to obtain an epoxy resin polymer. Since the amide product is a mixture of polymers containing a large number of amide bonds and amino groups, the amino groups in the amide product cure the epoxy, and the molecular chains of the epoxy resin are interconnected with the molecular chains of the amide product to form a dense cross-linked network. After introducing the amide product, the mechanical properties of the cured epoxy resin are improved. This is because the long-chain structure and cross-linking points in the amide product increase the intermolecular force and network density of the epoxy resin molecular chains, making it have higher strength and hardness. At the same time, an appropriate cross-linking density can also maintain the toughness of the epoxy resin to a certain extent and avoid the material becoming brittle due to excessive cross-linking. In addition, the aromatic ring structure and cross-linked network in the amide product can improve the thermal stability of the epoxy resin, enabling it to maintain good performance at higher temperatures.

[0054] Reprocessing and recycling test: Cut the above epoxy resin polymer into small pieces, place them between two steel plates, and hot press at 150 °C and 10 MPa for 20 min. After cooling to room temperature, a recycled film is obtained, as Figure 7 shown. As Figure 8, is the tensile stress-strain curve of the original epoxy resin and the reprocessed epoxy resin. The mechanical properties of the reprocessed epoxy resin only slightly decrease.

[0055] In addition, the purity of the recovered solid product (i.e., precipitate) 4,4'-diaminodiphenyl ether (ODA) is very high. As Figure 9 shown, the structure and purity of the recovered ODA are exactly the same as those of the original ODA. The recovery rate of 4,4'-diaminodiphenyl ether in this example is 99.1%.

[0056] Weigh (2.18 g, 10.00 mmol) of the recovered 4,4'-diaminodiphenyl ether into a 250 mL three-necked flask, add 36 mL of anhydrous N,N-dimethylacetamide, and stir it in an ice bath to completely dissolve it; then add (2.00 g, 10 mmol) of pyromellitic dianhydride (PMDA) monomer in three portions. After the feeding is completed, continue to react in an ice bath for 2 h and at room temperature for 14 h to obtain a pale yellow polyamic acid solution. Using the casting method, pour the prepared polyamic acid solution onto a smooth and flat glass substrate, carefully place it in an oven, and after standing for 0.5 h, perform thermal imidization reaction with programmed temperature increase. The main process is: vacuum dry at 80 °C, 120 °C, 150 °C, 180 °C, 210 °C, and 250 °C for 4 h, 2 h, 2 h, 2 h, 2 h, and 1 h respectively. Finally, obtain the PI (recovered ODA-PMDA) film. The thermogravimetric curve of the obtained PI film is as Figure 10 shown, indicating that it has excellent thermal stability.

[0057] Example 2

[0058] (1) Add 5 g of polyimide (PI-2) to 15 mL of ethylenediamine and stir at 90 °C for 2 h. After the polyimide is fully degraded, distill the degradation solution under reduced pressure to recover ethylenediamine and the degradation product. The recovered ethylenediamine can be reused again;

[0059] (2) Add distilled water to the degradation product and ultrasonically disperse it. 4,4'-diaminodiphenyl ether precipitate is insoluble in water. Filter to collect the precipitate and the filtrate. Wash the obtained precipitate and vacuum dry it, that is, the recovered 4,4'-diaminodiphenyl ether; store the collected filtrate in the refrigerator for use;

[0060] (3) Preparation of bio-based epoxy monomer: Weigh 7.6 g (0.05 mol) of vanillin, 18.5 g (0.2 mol) of epichlorohydrin, and 0.4 g (1 mmol) of tetrabutylammonium bromide and add them to a single-necked flask. React at 80 °C for 2 h and then cool to room temperature. Slowly drop 50 wt% aqueous sodium hydroxide solution into the above reaction solution through a constant-pressure dropping funnel, and continue to stir and react at room temperature for 3 h. After the reaction is completed, add dichloromethane for dilution, extract the organic phase with water three times, dry the organic phase with anhydrous magnesium sulfate, filter, concentrate the organic phase, drop it into excessive anhydrous ethanol, precipitate the product, wash and purify the crude product with anhydrous ethanol, and dry it under vacuum to obtain the epoxy monomer.

[0061] (4) Dissolve 2 g of the epoxy monomer obtained in step (3) in 5 mL of acetonitrile, add the curing agent obtained in step (2) according to the molar ratio of the amino group of the curing agent to the epoxy monomer of 1.5:1, stir evenly, pour the solution into a Teflon mold, react at room temperature for 4 h, then place it in an oven at 60 °C to evaporate the solvent for 12 h, and then cure by programmed heating. The specific procedure is: cure at 80 °C for 2 h, cure at 100 °C for 4 h, and cure at 140 °C for 1 h to obtain the epoxy resin polymer.

[0062] In this Example 4, the recovery rate of 4,4'-diaminodiphenyl ether is 98.6%.

[0063] Example 3

[0064] (1) Add 5 g of polyimide (PI-1) to 15 mL of 1,3-propanediamine, stir at 110 °C for 3 h. After the polyimide is fully degraded, distill the degradation solution under reduced pressure to recover 1,3-propanediamine and the degradation product. The recovered 1,3-propanediamine can be reused.

[0065] (2) Add distilled water to the degradation product and ultrasonically disperse it. 4,4'-Diaminodiphenyl ether precipitates and is insoluble in water. Filter to collect the precipitate and the filtrate. Wash the obtained precipitate and dry it under vacuum to obtain the recovered 4,4'-diaminodiphenyl ether. Store the collected filtrate in the refrigerator and use it as an epoxy resin curing agent.

[0066] (3) Preparation of bio-based epoxy monomer with aldehyde group: Weigh 7.6 g (0.05 mol) of vanillin, 18.5 g (0.2 mol) of epichlorohydrin, and 0.4 g (1 mmol) of tetrabutylammonium bromide and add them to a single-necked flask. React at 80 °C for 2 h and then cool to room temperature. Slowly drop 50 wt% aqueous sodium hydroxide solution into the above reaction solution through a constant-pressure dropping funnel, and continue to stir and react at room temperature for 3 h. After the reaction is completed, add dichloromethane for dilution, extract the organic phase with water three times, dry the organic phase with anhydrous magnesium sulfate, filter, concentrate the organic phase, drop it into excessive anhydrous ethanol, precipitate the product, wash and purify the crude product with anhydrous ethanol, and dry it under vacuum to obtain the epoxy monomer with aldehyde group.

[0067] (4) Dissolve 2 g of the epoxy monomer obtained in step (3) in 5 mL of acetonitrile. Add the curing agent obtained in step (2) in a ratio of 1.5:1 of the amino groups of the curing agent to the epoxy monomer. After stirring evenly, pour the solution into a Teflon mold, react at room temperature for 4 h, then place it in an oven at 60 °C to evaporate the solvent for 12 h, and then cure by programmed temperature rise. The specific procedure is: cure at 80 °C for 2 h, cure at 100 °C for 4 h, and cure at 140 °C for 1 h to obtain an epoxy resin polymer.

[0068] In this Example 4, the recovery rate of 4,4'-diaminodiphenyl ether is 98.7%.

[0069] Example 4

[0070] (1) Add 5 g of polyimide (PI-2) to 15 mL of 1,3-propanediamine, stir at 110 °C for 3 h. After the polyimide is fully degraded, distill the degradation solution under reduced pressure to recover 1,3-propanediamine and the degradation product. The recovered 1,3-propanediamine can be reused again;

[0071] (2) Add distilled water to the degradation product and ultrasonically disperse it. 4,4'-Diaminodiphenyl ether precipitates and is insoluble in water. Filter to collect the precipitate and the filtrate. The obtained precipitate is washed and vacuum dried to obtain the recovered 4,4'-diaminodiphenyl ether; the collected filtrate is stored in the refrigerator and used as an epoxy resin curing agent;

[0072] (3) Preparation of bio-based epoxy monomer: Weigh 7.6 g (0.05 mol) of vanillin, 18.5 g (0.2 mol) of epichlorohydrin, and 0.4 g (1 mmol) of tetrabutylammonium bromide and add them to a single-necked flask. React at 80 °C for 2 h and then cool to room temperature; slowly drip 50 wt% aqueous sodium hydroxide solution into the above reaction solution through a constant pressure dropping funnel, and continue to stir and react at room temperature for 3 h; after the reaction is completed, add dichloromethane for dilution, extract the organic phase with water three times, dry the organic phase with anhydrous magnesium sulfate, filter, concentrate the organic phase, drop it into an excess of absolute ethanol, precipitate the product, and wash and purify the crude product with absolute ethanol and then vacuum dry to obtain an epoxy monomer with an aldehyde group;

[0073] (4) Dissolve 2 g of the epoxy monomer obtained in step (3) in 5 mL of acetonitrile. Add the curing agent obtained in step (2) in a ratio of 1.5:1 of the amino groups of the curing agent to the epoxy monomer. After stirring evenly, pour the solution into a Teflon mold, react at room temperature for 4 h, then place it in an oven at 60 °C to evaporate the solvent for 12 h, and then cure by programmed temperature rise. The specific procedure is: cure at 80 °C for 2 h, cure at 100 °C for 4 h, and cure at 140 °C for 1 h to obtain an epoxy resin polymer.

[0074] In this Example 4, the recovery rate of 4,4'-diaminodiphenyl ether is 98.6%.

[0075] Example 5

[0076] (1) Add 5 g of polyimide (PI-1) to 15 mL of 1,4-butanediamine, stir at 120 °C for 3 h. After the polyimide is fully degraded, distill the degradation solution under reduced pressure to recover 1,4-butanediamine and the degradation product. The recovered 1,4-butanediamine can be reused.

[0077] (2) Add distilled water to the degradation product and ultrasonically disperse it. 4,4'-Diaminodiphenyl ether precipitates and is insoluble in water. Filter to collect the precipitate and the filtrate. Wash the obtained precipitate and dry it under vacuum to obtain the recovered 4,4'-diaminodiphenyl ether; put the collected filtrate in the refrigerator for storage and use it as an epoxy resin curing agent.

[0078] (3) Preparation of bio-based epoxy monomer: Weigh 7.6 g (0.05 mol) of vanillin, 18.5 g (0.2 mol) of epichlorohydrin, and 0.4 g (1 mmol) of tetrabutylammonium bromide and add them to a single-neck flask. React at 80 °C for 2 h and then cool to room temperature; slowly drop 50 wt% aqueous sodium hydroxide solution into the above reaction solution through a constant pressure dropping funnel, and continue to stir and react at room temperature for 3 h; after the reaction is completed, add dichloromethane for dilution, extract the organic phase with water three times, dry the organic phase with anhydrous magnesium sulfate, filter, concentrate the organic phase, drop it into excess absolute ethanol, precipitate the product, and wash and purify the crude product with absolute ethanol and dry it under vacuum to obtain the epoxy monomer with aldehyde groups.

[0079] (4) Dissolve 2 g of the epoxy monomer obtained in step (3) in 5 mL of acetonitrile, add the curing agent obtained in step (2) according to the molar ratio of the amino group of the curing agent to the epoxy monomer of 1.5:1, stir evenly, pour the solution into a Teflon mold, react at room temperature for 4 h, then place it in an oven at 60 °C to evaporate the solvent for 12 h, and then cure it by programmed heating. The specific program is: cure at 80 °C for 2 h, cure at 100 °C for 4 h, and cure at 140 °C for 1 h to obtain an epoxy resin polymer.

[0080] In this Example 4, the recovery rate of 4,4'-diaminodiphenyl ether is 98.8%.

[0081] Example 6

[0082] (1) Add 5 g of polyimide (PI-2) to 15 mL of 1,4-butanediamine, stir at 120 °C for 3 h. After the polyimide is fully degraded, distill the degradation solution under reduced pressure to recover 1,4-butanediamine and the degradation product. The recovered 1,4-butanediamine can be reused.

[0083] (2) Add distilled water to the degradation product, and disperse it thoroughly by ultrasonication. The 4,4'-diaminodiphenyl ether precipitate is insoluble in water, and the precipitate and filtrate are collected by filtration. The obtained precipitate is washed and vacuum dried to recover the obtained 4,4'-diaminodiphenyl ether; the collected filtrate is stored in a refrigerator and used as an epoxy resin curing agent;

[0084] (3) Preparation of bio-based epoxy monomer: 7.6 g (0.05 mol) of vanillin, 18.5 g (0.2 mol) of epichlorohydrin, and 0.4 g (1 mmol) of tetrabutylammonium bromide were weighed and added into a single-necked flask, reacted at 80° C. for 2 h, and then cooled to room temperature; 50 wt % sodium hydroxide aqueous solution was slowly dripped into the above reaction solution through a constant pressure dropping funnel, and the reaction was continued at room temperature for 3 h with stirring; after the reaction was completed, dichloromethane was added to dilute, and the organic phase was extracted with water three times, the organic phase was dried with anhydrous magnesium sulfate, filtered, and the organic phase was concentrated, and dripped into excess anhydrous ethanol to precipitate the product. The crude product was washed and purified with anhydrous ethanol, and vacuum dried to obtain an epoxy monomer with an aldehyde group;

[0085] (4) Dissolve 2 g of the epoxy monomer obtained in step (3) in 5 mL of acetonitrile, add the curing agent obtained in step (2) at a molar ratio of 1.5:1 between the amino group of the curing agent and the epoxy monomer, stir evenly, pour the solution into a Teflon mold, react at room temperature for 4 h, then place in a 60°C oven to evaporate the solvent for 12 h, and then perform programmed temperature curing. The specific procedure is: curing at 80°C for 2 h, curing at 100°C for 4 h, and curing at 140°C for 1 h to obtain an epoxy resin polymer.

[0086] The recovery rate of 4,4'-diaminodiphenyl ether in this example is 98.7%.

[0087] refer to Figure 6 The DSC curve (a) of the epoxy resin polymer prepared in Example 1-5 shows that the curing agent is a polymer, which reduces the crosslinking density of the epoxy resin and introduces a dynamic Schiff base bond, so that the glass transition temperature of the epoxy resin prepared in Example 1-5 is between 90°C and 120°C; the thermogravimetric curve (b) shows that the epoxy resin prepared above has excellent thermal properties and the residual carbon rate at 800°C is as high as 49%. According to the equation LOI×100=17.5+0.4CR (where CR is the residual carbon content (wt%) at 800°C under N2 atmosphere), the limiting oxygen index (LOI) is calculated to be 37% [Li, Y. et al. Polymer 297, 126836 (2024)].

[0088] The above embodiments are only used to further illustrate a method for recycling waste polyimide for curing epoxy resin according to the present invention. However, the present invention is not limited to the embodiments. Any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention shall fall within the protection scope of the technical solution of the present invention.

Claims

1. A method for recycling waste polyimide for curing epoxy resin, characterized in that, It includes the following steps: (1) Mix the waste polyimide with an organic diamine, stir at 80 - 120 °C for 2 - 6 hours until complete degradation, and separate the excess organic diamine and degradation products by vacuum distillation; (2) Mix the degradation product with pure water and disperse it evenly. After filtration, the filtrate is used as an epoxy resin curing agent; (3) Mix the filtrate evenly with an epoxy monomer with an aldehyde group, pour it into a mold, evaporate the solvent, and cure by heating to obtain an epoxy resin polymer.

2. The method for recycling waste polyimide for curing epoxy resin according to claim 1, characterized in that: The organic diamine is at least one of ethylenediamine, 1,3 - propanediamine or 1,4 - butanediamine.

3. The method for recycling waste polyimide for curing epoxy resin according to claim 1, characterized in that: The mass ratio of the waste polyimide to the organic diamine is 1:2 - 20.

4. The method for recycling waste polyimide for curing epoxy resin according to claim 1, characterized in that: The structural formula of the waste polyimide is:

5. The method for recycling waste polyimide for curing epoxy resin according to claim 1, characterized in that: In step (2), after filtration, a solid product and the filtrate are obtained. The solid product is 4,4'-diaminodiphenyl ether, and the filtrate contains an amide product, and the amide product contains an amide bond and an amino group.

6. The method for recycling waste polyimide for curing epoxy resin according to claim 1, wherein: The preparation steps of the epoxy monomer with an aldehyde group include: A. Mix vanillin, epichlorohydrin and a phase transfer catalyst, and stir - react at 60 °C - 110 °C for 2 - 10 h; B. Cool the reaction solution in step A to room temperature, dropwise add an aqueous sodium hydroxide solution, and stir - react at room temperature for 2 - 5 h; C. Add dichloromethane to dilute the reaction solution in step B, extract the organic phase with water, dry, filter and concentrate the organic phase, then drop it into absolute ethanol to precipitate the product, and wash and purify the crude product with absolute ethanol; vacuum - dry to obtain.

7. The method for recycling waste polyimide for curing epoxy resin according to claim 6, characterized in that: The phase transfer catalyst is tetrabutylammonium bromide or benzyltriethylammonium chloride, and the molar ratio of vanillin, epichlorohydrin, and the phase transfer catalyst is 1∶1 - 20∶0.1 - 1.

8. The method for recycling waste polyimide for curing epoxy resin according to claim 1, wherein: Step (3) specifically includes: 3.1) Dissolve the epoxy monomer with an aldehyde group in an organic solvent, drop it into the filtrate, stir evenly and pour it into a mold, react at room temperature for 2 - 8 h, and slowly evaporate the solvent at 40 - 60 °C; 3.2) Carry out programmed - temperature curing, and the curing program is: cure at 70 - 90 °C for 1 - 3 h, cure at 100 - 120 °C for 4 - 8 h, and cure at 140 - 150 °C for 1 - 3 h.

9. The method for recycling waste polyimide for curing epoxy resin according to claim 8, characterized in that: The organic solvent is absolute ethanol, methanol, acetonitrile, N,N - dimethylacetamide or N,N - dimethylformamide.

10. The method for recycling waste polyimide for curing epoxy resin according to claim 8, characterized in that: The molar ratio of the curing group in the filtrate to the epoxy monomer with an aldehyde group is 1.5 - 2.5∶1; where the curing group is an amino group.

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