Highly flame-retardant recyclable epoxy thermoset and method of making same

Abstract

A highly flame-retardant recyclable epoxy thermosetting material and its preparation method are disclosed. By weight, the material comprises the following components, which are cured via a curing reaction: 100 parts epoxy resin, 15-30 parts curing agent, 20-40 parts phosphorus-nitrogen synergistic flame retardant containing Diels-Alder dynamic bonds, 0.5-2 parts accelerator, 1-3 parts defoamer, and silane coupling agent-modified nano-silica. The phosphorus-nitrogen synergistic flame retardant containing Diels-Alder dynamic bonds integrates double-terminated reactive hydroxyl groups, DA dynamic covalent bonds, and dual phosphorus-nitrogen flame-retardant groups in its molecular structure. It can directly react with epoxy resin to construct a dynamic cross-linked flame-retardant network. The material has a limiting oxygen index of 39%-45% and a flame retardant rating of V-0. After hot-pressing and remodeling for recycling, the tensile strength recovery rate is 90%-95%. The DA dynamic bonds endow the epoxy thermosetting material with excellent recyclability, while also exhibiting excellent water resistance and salt spray resistance.

Description

Technical Field

[0001] This invention belongs to the field of flame-retardant epoxy thermosetting resins, specifically relating to a highly flame-retardant recyclable epoxy thermosetting material and its preparation method. Background Technology

[0002] Epoxy resins hold an irreplaceable core position in industrial fields such as electronic packaging, aerospace composite materials, rail transportation, and chemical corrosion protection due to their excellent adhesion, chemical corrosion resistance, electrical insulation, and mechanical stability. However, general-purpose epoxy resins face two major industry pain points that restrict their sustainable development: First, their inherent flammability, with a limiting oxygen index of only about 22%, results in continuous melting and dripping during combustion, releasing large amounts of toxic fumes, which cannot meet the fire safety requirements of high-end fields; second, their permanent cross-linked network is non-recyclable. Traditional epoxy resins form an irreversible three-dimensional covalent network after curing, which cannot be melted and reshaped, chemically degraded, or efficiently recycled once formed, resulting in huge resource waste and environmental pollution after disposal.

[0003] For flame retardant modification of epoxy resins, existing technologies are mainly divided into two categories: additive and reactive. Additive flame retardants (such as ammonium polyphosphate and organic phosphinates) have simple processes and wide applicability, but they have inherent defects such as large addition amounts, poor compatibility with the resin matrix, easy migration and precipitation after long-term use, and serious deterioration of the mechanical properties and thermal stability of the material. Reactive flame retardants introduce flame retardant elements into the epoxy crosslinking network through chemical bonding, solving the problems of dispersion and migration. However, most existing systems only focus on improving flame retardant performance and have not solved the core problem of the non-recyclability of thermosetting materials. Moreover, there is a common contradiction of "difficulty in balancing flame retardant efficiency and mechanical properties", which makes it impossible to achieve synergistic effects of multiple properties.

[0004] In recent years, the development of dynamic covalent chemistry has provided a core solution for recyclable thermosetting materials. Diels-Alder (DA) bonds, as typical thermally reversible dynamic covalent bonds, can undergo a [4+2] cycloaddition reaction at 60-120℃ to build a cross-linked network, and undergo a reverse DA reaction at temperatures above 120℃ to achieve decrosslinking. With mild reaction conditions, excellent dynamic reversibility, and no side reactions, they have become a research hotspot in the field of recyclable epoxy resins. However, existing epoxy systems containing DA bonds still have significant technical shortcomings: First, their flame retardant performance is severely insufficient. Most systems do not introduce highly efficient flame-retardant structures, and their limiting oxygen index is generally below 30%, failing to meet the V-0 flame retardant rating of the UL-94 standard, making them difficult to apply to high-end fields with stringent fire safety requirements. Second, their functional structure integration is low. Existing technologies often involve the physical blending of multiple components, including DA-bonded monomers, flame retardants, and curing agents, which easily leads to poor compatibility and uneven distribution of dynamic bonds and flame-retardant elements, resulting in low material recycling efficiency and poor flame retardant stability.

[0005] Therefore, developing an epoxy thermosetting material that combines high flame retardancy, excellent mechanical properties, and high recyclability to solve the industry pain point of the difficulty in achieving both flame retardancy, mechanical properties, and recyclability in existing technologies is a technical challenge that urgently needs to be overcome in this field. Summary of the Invention

[0006] To address the issues of existing epoxy thermosetting materials being flammable and non-recyclable, this invention provides a highly flame-retardant and recyclable epoxy thermosetting material. Through molecular structure design, DA dynamic bonds, phosphorus and nitrogen synergistic flame-retardant groups, and reactive curing groups (hydroxyl groups) are integrated into the same functional molecule to construct a dynamic cross-linked flame-retardant network, achieving a synergistic unity of high flame retardancy, high mechanical properties, and recyclability.

[0007] To achieve the above objectives, the present invention is implemented through the following technical solution: A highly flame-retardant and recyclable epoxy thermosetting material, by weight, comprises the following components obtained through a curing reaction: 100 parts epoxy resin, 15-30 parts curing agent, 20-40 parts phosphorus-nitrogen synergistic flame retardant containing Diels-Alder dynamic bonds, 0.5-2 parts accelerator, 1-3 parts defoamer, and 10-15 parts silane coupling agent modified nano-silica;

[0008] The chemical structure of the phosphorus-nitrogen synergistic flame retardant containing Diels-Alder dynamic bonds is as follows: ; Wherein, R is phenyl, diphenylmethane, diphenyl ether, or tetramethylene.

[0009] The phosphorus-nitrogen synergistic flame retardant containing Diels-Alder dynamic bonds is prepared through the following steps: S1: Diphenylphosphonic hydroxylamine is added to a reactor containing tetrahydrofuran and dissolved by ultrasonic vibration and stirring to obtain a diphenylphosphonic hydroxylamine solution. 5-hydroxymethylfurfural is dissolved in a first organic solvent to obtain a 5-hydroxymethylfurfural solution, wherein the molar ratio of diphenylphosphonic hydroxylamine to 5-hydroxymethylfurfural is 1:(1.1~1.3). Under inert gas protection, the 5-hydroxymethylfurfural solution is slowly added dropwise to the reactor. 2~3 drops of glacial acetic acid are added dropwise to the reaction system as a catalyst. The mixture is stirred at room temperature for 20~30 min until homogeneous. The reaction temperature is raised to 40~70℃, refluxed, and the water generated in the reaction is removed using an oil-water separator. The reaction is carried out for 4~6 h to obtain the first reaction solution, which is then cooled to room temperature. S2: After the reaction is complete, cool the first reaction solution to room temperature and pour it into a Buchner funnel. Filter under reduced pressure and wash the filter cake with tetrahydrofuran 1 to 3 times to obtain the first reaction filtrate. S3: The filtrate from the first reaction was concentrated under reduced pressure using a rotary evaporator at a temperature of 35-40°C to obtain a crude first solid product. The crude first solid product was washed 2-3 times with deionized water and then vacuum dried at a temperature of 50-70°C to constant weight to obtain a phosphorus-nitrogen synergistic flame retardant intermediate containing a furan ring and terminal hydroxyl groups. The reaction equation for the flame retardant intermediate is as follows: ; S4: The flame-retardant intermediate obtained in step S3 and bismaleimide are added to N,N-dimethylformamide in a molar ratio of (2.0~2.2):1, stirred until completely dissolved, and a Diels-Alder cycloaddition reaction is carried out at 80~100℃ for 18~36h to obtain a second reaction solution, which is then cooled to room temperature. S5: Pour the second reaction solution into a Buchner funnel, filter under reduced pressure, and wash the filter cake with tetrahydrofuran 1-3 times to obtain the second reaction filtrate. S6: The second reaction filtrate was concentrated under reduced pressure using a rotary evaporator at 70-80°C to obtain a second solid crude product. The second solid crude product was washed with acetone 1-3 times and then dried under vacuum at 80-100°C to constant weight, thus obtaining a phosphorus-nitrogen synergistic flame retardant containing Diels-Alder dynamic bonds. The reaction equation for the phosphorus-nitrogen synergistic flame retardant containing Diels-Alder dynamic bonds is as follows: ; Wherein, R is phenyl, diphenylmethane, diphenyl ether, or tetramethylene.

[0010] Preferably, in step S1, the first organic solvent is a solvent with a boiling point below 80°C, and the first organic solvent is one or more of anhydrous methanol, anhydrous ethanol, or acetone.

[0011] Preferably, in step S1, the mass of the tetrahydrofuran is 4 to 8 times the mass of diphenylphosphonohydroxylamine, and in step S4, the mass of the N,N-dimethylformamide is 8 to 12 times the mass of the flame-retardant intermediate.

[0012] Preferably, when the first organic solvent is anhydrous ethanol, the mass fraction ratio of tetrahydrofuran to anhydrous ethanol is 75wt%:25wt%, and an azeotropic point of 66°C is formed between tetrahydrofuran and anhydrous ethanol. When the first organic solvent is anhydrous methanol, the mass fraction ratio of tetrahydrofuran to anhydrous methanol is 88wt%:12wt%, and an azeotropic point of 64°C is formed between tetrahydrofuran and anhydrous methanol.

[0013] Preferably, the inert gas is either nitrogen or argon.

[0014] Preferably, the bismaleimide is one or more of N,N'-(4,4'-methylenediphenyl)bismaleimide, N,N'-m-phenylenebismaleimide, N,N'-(1,4-phenylene)bismaleimide, 4,4'-diphenyl ether dicis-butene diimide, and 1,4-bis(maleimide)butane.

[0015] Preferably, the vacuum degree of vacuum drying in step S3 or step S6 is -0.08 to -0.09 MPa, and the drying time is 12 to 24 hours.

[0016] Preferably, the epoxy resin is one or more of bisphenol A type epoxy resin E-51, bisphenol A type epoxy resin E-44, and phenolic epoxy resin F-51.

[0017] Preferably, the curing agent is one or more of 4,4'-diaminodiphenylmethane and 4,4'-diaminodiphenyl sulfone.

[0018] Preferably, the accelerator is one or more of 1,2-dimethylimidazole, 2-ethyl-4-methylimidazole, 2-methylimidazole, and N,N'-dimethylbenzylamine.

[0019] Preferably, the defoamer is one or more of defoamer BYK-054, defoamer BYK-024, or defoamer BYK-022.

[0020] This invention also provides a method for preparing the above-mentioned highly flame-retardant recyclable epoxy thermosetting material, comprising the following steps: (1) Premixing and degassing: Epoxy resin, curing agent, phosphorus-nitrogen synergistic flame retardant containing Diels-Alder dynamic bonds, accelerator, defoamer and silane coupling agent modified nano silica are mixed in proportion, stirred at 80~100℃ until completely uniform, placed in a vacuum container, and degassed at a vacuum degree of -0.08~-0.1MPa for 5~10min to obtain the mixture; (2) Segmented curing: Pour the mixture into a mold preheated to 90°C, pre-cur at 95~105°C for 2 hours, then heat to 125~135°C and cure for 2 hours. Cool down to 80~95°C and maintain for 2~4 hours to improve the reconstruction of Diels-Alder dynamic bonds and the reversible cross-linking network. After curing, allow it to cool naturally to room temperature and demold to obtain a highly flame-retardant and recyclable epoxy thermosetting material.

[0021] The present invention has the following beneficial effects: This invention is the first to integrate di-terminated reactive hydroxyl groups, DA dynamic bonds, and diphosphorus-nitrogen flame-retardant groups into a single functional molecule. Using this molecule as a curing agent, an epoxy network combining "dynamic crosslinking" and "flame retardancy" is constructed in one step, solving the industry pain point of traditional epoxy materials' difficulty in simultaneously achieving "flame retardancy, mechanical properties, and recyclability." The active hydroxyl groups can react with epoxy groups, completely entering the epoxy crosslinking network through chemical bonding, eliminating the migration, precipitation, and blooming problems that occur with traditional additive flame retardants during long-term use. Simultaneously, the molecular structure has excellent compatibility with the epoxy resin matrix due to its polarity match; the DA dynamic bonds endow epoxy thermosetting materials with excellent recyclability; and the diphosphorus-nitrogen groups achieve intramolecular synergistic flame retardancy, with a flame retardant efficiency far exceeding that of multi-component physical blend systems.

[0022] This invention uses bio-based 5-hydroxymethylfurfural as the core raw material, which is in line with the trend of green chemistry development. The core flame retardant only requires two reaction steps to synthesize, the reaction conditions are mild (normal pressure, medium and low temperature reaction), no highly toxic by-products are generated, and the preparation does not require a complicated purification process. The epoxy material is prepared using the industry-standard segmented curing process, which does not require additional special equipment and is easy to realize industrial continuous production.

[0023] The modified nano-silica of this invention and the phosphorus-nitrogen synergistic flame retardant containing Diels-Alder dynamic bonds form a good synergistic effect. The introduction of silane coupling agent modified nano-silica allows it to effectively fill the cavity formed by the phosphorus-nitrogen structure. Furthermore, the active groups of the modified nano-silica undergo grafting reactions with epoxy resin and flame retardant during the curing process, thereby enabling the silane coupling agent modified nano-silica to be well anchored, avoiding the migration, precipitation, and blooming of the filler, and giving the epoxy thermosetting material excellent permanent water resistance.

[0024] The epoxy thermosetting material prepared by this invention has a limiting oxygen index of 39%~45% and a flame retardant rating of V-0. After being recycled by hot pressing at 150℃, the epoxy thermosetting material has a tensile strength recovery rate of 90%~95%. The DA dynamic bond gives the epoxy thermosetting material excellent recycling performance, while also having excellent water resistance and salt spray resistance. Attached Figure Description

[0025] Figure 1 The ³¹P-NMR spectrum of the flame-retardant intermediate DPHM-1.

[0026] Figure 2 The infrared spectrum of the flame-retardant intermediate DPHM-1.

[0027] Figure 3 The infrared spectrum of phosphorus-nitrogen synergistic flame retardant 1 containing Diels-Alder dynamic bonds. Detailed Implementation

[0028] The present invention will be further described in detail below with reference to embodiments. It should also be understood that the following embodiments are only for further illustration of the present invention and should not be construed as limiting the scope of protection of the present invention. The specific mass, reaction time, temperature, process parameters, etc., in the examples are merely examples within a suitable range. Any non-essential improvements and adjustments made by those skilled in the art based on the above description of the present invention are within the scope of protection of the present invention.

[0029] In this embodiment of the invention, the modification method of silane coupling agent modified nano-silica is as follows: 150 mL of anhydrous ethanol is poured into a beaker and placed in a 40°C constant temperature water bath. The pH is adjusted to 4-5 with glacial acetic acid. 0.6 g of silane coupling agent KH560 is added dropwise to the above system. After the addition is complete, the temperature is maintained at 40°C, the stirring speed at 800 rpm, and the stirring time at 30 min until the system becomes a transparent and uniform liquid, completing the hydrolysis. 40 g of nano-silica is slowly added to the above hydrolysate while stirring at 800 rpm. After the addition is complete, stirring is continued for 10 min to complete the initial dispersion. The beaker is then removed and placed in an ultrasonic disperser for ultrasonic dispersion for 30 min, stirring for 1 min every 10 min to break up the agglomerates and obtain a milky white uniform suspension. The ultrasonically dispersed suspension is then placed back into the constant temperature water bath, the temperature is adjusted to 75°C, and the stirring speed is maintained at 1200 rpm. The mixture was stirred at a constant temperature for 4 hours to carry out the grafting reaction. After the grafting reaction was completed, the system was poured into a centrifuge tube, placed in a centrifuge, and centrifuged at 5000 rpm for 10 minutes to precipitate the silane coupling agent modified nano-silica. The supernatant was poured off, anhydrous ethanol was added to the centrifuge tube, and the mixture was shaken evenly. The mixture was centrifuged again at 5000 rpm for 10 minutes. The washing was repeated 2-3 times until the supernatant had no obvious silane odor. The washed precipitate was transferred to a clean petri dish, spread evenly, and placed in a vacuum drying oven preheated to 110℃. The vacuum degree was adjusted to -0.09 MPa, and the product was vacuum dried for 2 hours until it was completely dry. The dried product was placed in a planetary ball mill, and agate balls were added at a ball-to-material ratio of 3:1 by mass. The milling speed was adjusted to 300 rpm, and the milling time was 30 minutes to grind the product into a uniform powder. The powder was then passed through a 100-mesh sieve to remove lumps, and the silane coupling agent modified nano-silica product was obtained.

[0030] In this embodiment of the invention, the preparation steps of the flame-retardant intermediate DPHM-1 are as follows: S1: 14.00 g (60 mmol) of diphenylphosphonic hydroxylamine was added to a reactor containing 84.5 mL (75 g) of tetrahydrofuran, the mass of which was 5.4 times the mass of diphenylphosphonic hydroxylamine. The solution was dissolved by ultrasonic agitation and stirring to obtain a diphenylphosphonic hydroxylamine solution. 8.32 g (66 mmol) of 5-hydroxymethylfurfural was dissolved in 31.8 mL (25 g) of anhydrous ethanol to obtain a 5-hydroxymethylfurfural solution, wherein the molar ratio of diphenylphosphonic hydroxylamine to 5-hydroxymethylfurfural was 1:1.1. Nitrogen gas was used as an inert gas. Under the protection of a volatile gas, a 5-hydroxymethylfurfural solution was slowly added dropwise to the reactor. 2-3 drops of glacial acetic acid were added dropwise to the reaction system as a catalyst. The mixture was stirred at room temperature for 30 minutes. After thorough mixing, the reactor was heated to 66°C, refluxed, and the water generated in the reaction was removed using an oil-water separator. Anhydrous ethanol was used as the first organic solvent. The mass fraction ratio of tetrahydrofuran to anhydrous ethanol was 75wt%:25wt%. The azeotropic point of tetrahydrofuran and anhydrous ethanol was 66°C. The reaction was carried out at a constant temperature at the azeotropic point for 4 hours to obtain the first reaction solution, which was then cooled to room temperature. S2: Pour the first reaction solution into a Buchner funnel, filter under reduced pressure, and wash the filter cake three times with tetrahydrofuran to obtain the first reaction filtrate. S3: The filtrate from the first reaction was concentrated under reduced pressure at 40°C using a rotary evaporator to obtain the first crude solid product. The first crude solid product was washed three times with deionized water and then vacuum-dried at 70°C to constant weight. The vacuum degree of vacuum drying was -0.08 MPa, and the drying time was 12 h, yielding 21.42 g of a phosphorus-nitrogen synergistic flame-retardant intermediate containing a furan ring and terminal hydroxyl groups, denoted as DPHM-1. DPHM-1 was determined by ³¹P-NMR, referring to… Figure 1 This confirmed that diphenylphosphonohydroxylamine and 5-hydroxymethylfurfural underwent a Schiff base addition reaction. DPHM-1 was analyzed by FT-IR spectroscopy, with a spectral range of 400-4000 cm⁻¹. -1 The measurement results are as follows Figure 2 The IR spectrum of the sample, the yield of DPHM-1 product: 95.96%, and the reaction equation for the flame retardant intermediate are as follows:

[0031] Figure 2 The flame-retardant intermediate DPHM-1 at 3356 cm⁻¹ -1 The peak at 3100-3000 cm⁻¹ is an absorption peak of OH stretching vibration, a typical absorption of hydrogen-bonded associated hydroxyl groups. The peak is broad and of moderate intensity, perfectly consistent with the presence of terminal hydroxymethyl groups. -1 The weak peaks at 2950-2850 cm⁻¹ correspond to the stretching vibrations of unsaturated CH bonds on the benzene and furan rings, respectively. This indicates the presence of sp² hybridized CH bonds on the aromatic rings (benzene or furan rings). -1The peak intensity at 1622 cm⁻¹ corresponds to the saturated CH stretching vibration of the furan ring side chain -CH₂, and is relatively weak, consistent with the characteristics of a small amount of methylene in the structure. -1 The value at 1600-1450 cm⁻¹ corresponds to the stretching vibration of the C=N double bond in the oxime ether structure, which perfectly matches the characteristic peaks of oxime compounds, proving that the C=N bond was successfully formed; -1 The peak at 1236 cm⁻¹ corresponds to the multiple absorption peaks of the skeletal vibrations of the benzene and furan rings. The superposition of the characteristic skeletal vibration peaks of the benzene ring and the C=C skeletal vibrations of the furan ring is consistent with the multiple peaks in this region of the spectrum; -1 The position corresponds to the stretching vibration of the P=O double bond; 1100-1000 cm. -1 The stretching vibrations at 961 cm⁻¹ correspond to the stretching vibrations of the COC bond in the furan ring and the CO bond in the alcohol hydroxyl group. -1 The stretching vibration of the PO single bond in the PON bond is observed at 900-650 cm⁻¹, and the dense, strong peaks in this region perfectly match the characteristics of these three types of oxygen-containing single bonds; -1 The multiple peaks corresponding to the out-of-plane bending vibrations of the CH bonds on the benzene and furan rings can further verify the substitution reaction of the aromatic rings, which is consistent with the substitution characteristics of the phenyl and furan rings in the flame retardant intermediate DPHM-1.

[0032] In this embodiment of the invention, the preparation of flame retardant intermediate DPHM-2 is the same as that of flame retardant intermediate DPHM-1, except that in step S1, the first organic solvent is 13 mL (10 g) of anhydrous methanol, the mass fraction ratio of tetrahydrofuran to anhydrous methanol is 88 wt%: 12 wt%, the azeotropic point of tetrahydrofuran and anhydrous methanol is 64 °C, the reaction is carried out at a constant temperature at the azeotropic point for 5 h, and 21.25 g of a phosphorus-nitrogen synergistic flame retardant intermediate containing a furan ring and a terminal hydroxyl group is obtained, denoted as DPHM-2, with a product yield of 95.20%.

[0033] In this embodiment of the invention, the preparation of flame retardant intermediate DPHM-3 is the same as that of flame retardant intermediate DPHM-1. 14.00 g (60 mmol) of diphenylphosphonic hydroxylamine was dissolved in 120 mL (106.8 g) of tetrahydrofuran, the mass of which was 7.6 times the mass of diphenylphosphonic hydroxylamine. The solution was dissolved by ultrasonic agitation and stirring. 9.90 g (78.6 mmol) of 5-hydroxymethylfurfural was dissolved in 20 mL (15.8 g) of acetone to obtain a 5-hydroxymethylfurfural solution. The molar ratio of diphenylphosphonic hydroxylamine to 5-hydroxymethylfurfural was 1:1.3. The reaction temperature was 50 °C and the reaction time was 6 h. 20.05 g of a phosphorus-nitrogen synergistic flame retardant intermediate containing a furan ring and terminal hydroxyl groups was obtained, denoted as DPHM-3. The yield of DPHM-3 product was 89.83%.

[0034] The reaction yields of phosphorus nitrogen flame retardant intermediates containing furan rings and terminal hydroxyl groups, as shown by DPHM-1, DPHM-2, and DPHM-3, indicate that the azeotropic reaction using the first organic solvent and tetrahydrofuran can effectively remove the water produced in the reaction, which is conducive to the Schiff base addition direction of diphenylphosphonohydroxylamine and 5-hydroxymethylfurfural, resulting in a more complete reaction and thus a higher yield of the product.

[0035] In this embodiment of the invention, the phosphorus-nitrogen synergistic flame retardant 1 containing Diels-Alder dynamic bonds has the following structural formula:

[0036] The preparation steps are as follows: S4: 14.33 g (42 mmol) of flame retardant intermediate DPHM-1 and 7.2 g (20 mmol) of N,N-(4,4'-methylenediphenyl)bismaleimide were added to 150 mL (141.7 g) of N,N-dimethylformamide. The molar ratio of DPHM-1 to N,N-(4,4'-methylenediphenyl)bismaleimide was 2.1:1. The mixture was stirred until completely dissolved and then subjected to a Diels-Alder cycloaddition reaction at 90 °C for 24 h to obtain a second reaction solution. The solution was then cooled to room temperature. S5: Pour the second reaction solution into a Buchner funnel, filter under reduced pressure, and wash the filter cake three times with tetrahydrofuran to obtain the second reaction filtrate. S6: The filtrate from the second reaction was concentrated under reduced pressure at 80°C using a rotary evaporator to obtain a second solid crude product. The second solid crude product was washed three times with acetone and then vacuum-dried at 90°C to constant weight. The vacuum degree of vacuum drying was -0.09 MPa, and the drying time was 12 h, yielding 20.36 g of phosphorus-nitrogen synergistic flame retardant 1 containing Diels-Alder dynamic bonds, denoted as DPHM-1-BMI. DPHM-1-BMI was analyzed by FT-IR spectroscopy, and the spectral range was 400-4000 cm⁻¹. -1 The measurement results are as follows Figure 3 The IR spectrum of the sample shows that the DPHM-1-BMI product yield is 94.6%. The reaction equation for the phosphorus-nitrogen synergistic flame retardant containing Diels-Alder dynamic bonds is as follows:

[0037] Figure 3 The flame retardant DPHM-1-BMI is at 3326 cm⁻¹ -1 The peak at 2920 cm⁻¹ is an absorption peak due to the OH stretching vibration, a typical absorption of hydroxyl groups associated with hydrogen bonding, corresponding to the -OH groups at both ends of the molecule; -1 2852cm -1The stretching vibration of saturated CH on the methylene (-CH2) norbornene skeleton is 3050-3000 cm⁻¹. -1 The peak at 1718 cm⁻¹ corresponds to the weak absorption peak of the stretching vibration of the CH group in the benzene ring and the CH group in the norbornene double bond. -1 The strong absorption peak at 1512 cm⁻¹ is due to the stretching vibration of the carbonyl group (-CO-N-CO-) of the imide ring, a characteristic peak of the imide group, and matches the diimide structure in the molecule; -1 The absorption peak at 1200-1100 cm⁻¹ is due to the skeletal vibration (C=C) of the benzene ring, proving the presence of a benzene ring structure in the molecule; -1 The peak at 1050-1000 cm⁻¹ corresponds to the stretching vibration of the P=O bond in the diphenylphosphono oxime group, which is a characteristic peak of the phosphorus-containing flame-retardant group, verifying the introduction of the diphenylphosphono oxime structure; -1 The absorption peak at 850-800 cm⁻¹ corresponds to the stretching vibration of the PON bond in the diphenylphosphonooxime group, and the absorption peak in this region corresponds to the vibration of the phosphorus-oxygen bond, further confirming the presence of the phosphorus-containing group; -1 The presence of out-of-plane bending vibrations of the CH group at the para-disubstituted benzene ring indicates that the benzene ring has a para-substituted structure, consistent with the methylene diphenylamine skeleton.

[0038] In this embodiment of the invention, the preparation steps of the phosphorus-nitrogen synergistic flame retardant 2 containing Diels-Alder dynamic bonds are the same as those of DPHM-1-BMI, except that the flame retardant intermediate is 14.33 g (42 mmol) of DPHM-2, the bismaleimide is 5.36 g (20 mmol) of N,N'-(1,4-phenylene)bismaleimide, the molar ratio of DPHM-2 to N,N'-(1,4-phenylene)bismaleimide is 2.1:1, the reaction temperature is 90 °C, the reaction time is 30 h, and 18.79 g of phosphorus-nitrogen synergistic flame retardant 2 containing Diels-Alder dynamic bonds is obtained, denoted as DPHM-2-PDM, with a product yield of 95.4%.

[0039] In this embodiment of the invention, the preparation steps of the phosphorus-nitrogen synergistic flame retardant 3 containing Diels-Alder dynamic bonds are the same as those for the preparation of DPHM-1-BMI. The flame retardant intermediate is 14.33 g (42 mmol) of DPHM-1. The difference is that the bismaleimide is 7.21 g (20 mmol) of 4,4'-diphenyl ether dicis-butenediamide. The molar ratio of DPHM-1 to 4,4'-diphenyl ether dicis-butenediamide is 2.1:1. The reaction temperature is 80 °C and the reaction time is 36 h, yielding 20.33 g of phosphorus-nitrogen synergistic flame retardant 3 containing Diels-Alder dynamic bonds, denoted as DPHM-1-ODA, with a product yield of 94.4%.

[0040] In this embodiment of the invention, the preparation steps of the phosphorus-nitrogen synergistic flame retardant 4 containing Diels-Alder dynamic bonds are the same as those of DPHM-2-PDM. The flame retardant intermediate is 14.33 g (42 mmol) of DPHM-2. The difference is that the bismaleimide is 4.96 g (20 mmol) of 1,4-bis(maleimide)butane, the molar ratio of DPHM-2 to 1,4-bis(maleimide)butane is 2.1:1, the reaction temperature is 100 °C, and the reaction time is 18 h. 18.13 g of phosphorus-nitrogen synergistic flame retardant 4 containing Diels-Alder dynamic bonds is obtained, denoted as DPHM-2-TME, with a product yield of 94.0%.

[0041] In this embodiment of the invention, the preparation steps of the phosphorus-nitrogen synergistic flame retardant 5 containing Diels-Alder dynamic bonds are the same as those of DPHM-1-BMI, except that the flame retardant intermediate is 14.96 g (44 mmol) of DPHM-3, the bismaleimide is 5.36 g (20 mmol) of N,N'-m-phenylenebismaleimide, the molar ratio of DPHM-3 to N,N'-m-phenylenebismaleimide is 2.2:1, the reaction temperature is 90 °C, the reaction time is 24 h, and 20.74 g of phosphorus-nitrogen synergistic flame retardant 5 containing Diels-Alder dynamic bonds is obtained, denoted as DPHM-3-PDM, with a product yield of 93.6%. Example 1

[0042] A highly flame-retardant and recyclable epoxy thermosetting material, by weight, comprises the following components obtained through a curing reaction: 100 parts of bisphenol A type epoxy resin E-51, 21 parts of curing agent 4,4'-diaminodiphenylmethane, 40 parts of DPHM-1-BMI, 1 part of accelerator 1,2-dimethylimidazole, 1 part of defoamer BYK-054, and 15 parts of silane coupling agent modified nano-silica. The preparation steps are as follows: Premixing and degassing: Bisphenol A type epoxy resin E-51, 4,4'-diaminodiphenylmethane, DPHM-1-BMI, and 1,2-dimethylimidazole are mixed... Imidazole and defoamer BYK-054 were mixed in proportion and stirred at 90°C until completely homogeneous. The mixture was placed in a vacuum container and degassed for 8 minutes under a vacuum of -0.08 to -0.1 MPa to obtain the mixture. Segmented curing: The mixture was poured into a mold preheated to 90°C and pre-cured at 100°C for 2 hours. The temperature was then raised to 130°C and cured for 2 hours. The temperature was then lowered to 80°C and maintained for 4 hours to improve the reconstruction of the Diels-Alder dynamic bonds and the reversible crosslinking network. After curing, the mixture was naturally cooled to room temperature and demolded to obtain a highly flame-retardant and recyclable epoxy thermosetting material, denoted as EP-1. Example 2

[0043] A highly flame-retardant and recyclable epoxy thermosetting material, by weight, comprises the following components obtained through a curing reaction: 100 parts of bisphenol A type epoxy resin E-44, 23 parts of curing agent 4,4'-diaminodiphenyl sulfone, 30 parts of DPHM-2-PDM, 0.5 parts of accelerator 2-ethyl-4-methylimidazolium, 1.5 parts of defoamer BYK-024, and 12 parts of silane coupling agent modified nano-silica. The preparation steps are the same as in Example 1. The resulting highly flame-retardant and recyclable epoxy thermosetting material is designated as EP-2. Example 3

[0044] A highly flame-retardant and recyclable epoxy thermosetting material, by weight, comprises the following components obtained through a curing reaction: 100 parts of phenolic epoxy resin F-51, 29 parts of curing agent 4,4'-diaminodiphenyl sulfone, 20 parts of DPHM-1-ODA, 2 parts of accelerator 2-methylimidazole, 2 parts of defoamer BYK-022, and 10 parts of silane coupling agent modified nano-silica. The preparation steps are the same as in Example 1. The resulting highly flame-retardant and recyclable epoxy thermosetting material is designated as EP-3. Example 4

[0045] A highly flame-retardant and recyclable epoxy thermosetting material, by weight, comprises the following components prepared by a curing reaction: 100 parts of bisphenol A type epoxy resin E-51, 17 parts of curing agent 4,4'-diaminodiphenylmethane, 30 parts of DPHM-2-TME, 1.5 parts of accelerator 2-methylimidazole, 3 parts of defoamer BYK-024, and 12 parts of silane coupling agent modified nano-silica. The preparation steps are the same as in Example 1. The resulting highly flame-retardant and recyclable epoxy thermosetting material is designated as EP-4. Example 5

[0046] A highly flame-retardant and recyclable epoxy thermosetting material, by weight, comprises the following components prepared by a curing reaction: 100 parts of bisphenol A type epoxy resin E-51, 24 parts of curing agent 4,4'-diaminodiphenylmethane, 30 parts of DPHM-3-PDM, 1.5 parts of accelerator 2-methylimidazole, 3 parts of defoamer BYK-024, and 10 parts of silane coupling agent modified nano-silica. The preparation steps are the same as in Example 1. The resulting highly flame-retardant and recyclable epoxy thermosetting material is designated as EP-5. Comparative Example 1

[0047] The epoxy thermosetting material, by weight, comprises the following components obtained through a curing reaction: 100 parts of bisphenol A type epoxy resin E-51, 30 parts of curing agent 4,4'-diaminodiphenyl sulfone, 1 part of accelerator 1,2-dimethylimidazole, 1 part of defoamer BYK-054, and 12 parts of silane coupling agent modified nano-silica. The preparation steps are the same as in Example 1, except that no phosphorus-nitrogen synergistic flame retardant is added. The resulting epoxy thermosetting material is denoted as EP-A. Comparative Example 2

[0048] The epoxy thermosetting material, by weight, comprises the following components obtained through a curing reaction: 100 parts of bisphenol A type epoxy resin E-51, 30 parts of curing agent 4,4'-diaminodiphenylmethane, 30 parts of DPHM-1-BMI, 2 parts of accelerator 2-methylimidazole, and 2 parts of defoamer BYK-024. The preparation steps are the same as in Example 1, except that no silane coupling agent is added to modify the nano-silica. The resulting epoxy thermosetting material is denoted as EP-B. Comparative Example 3

[0049] The epoxy thermosetting material is the same as in Example 1, except that the added nano-silica is commercially available unmodified nano-silica. The resulting epoxy thermosetting material is denoted as EP-C.

[0050] The performance test results of the products obtained from Examples 1 to 5 and Comparative Examples 1 to 3 above, and the reprocessed samples obtained from the hot-pressing and recycling of epoxy thermosetting materials at 150°C, are shown in Table 1. The performance test results are explained below: The limiting oxygen index of epoxy thermosetting materials was determined by a digital oxygen index tester (model K-R2406S) in accordance with GB / T2406.2-2009. The limiting oxygen index is the minimum volume percentage of oxygen in the oxygen-nitrogen mixture required for the material to maintain combustion for 50 mm or 3 minutes after ignition.

[0051] The flame retardancy rating of epoxy thermosetting materials is determined according to UL94 standards using a horizontal and vertical combustion tester (model HD-R807-S).

[0052] The tensile strength of epoxy thermosetting materials shall be determined in accordance with the national standard GB / T528-2009.

[0053] Water vapor transmission rate test of epoxy thermosetting materials: According to the national standard GB / T1037-2021, water vapor transmission performance test of plastic films and sheets, a plastic film permeation cup is used. The epoxy thermosetting material is cured into a uniform rectangular sheet using a mold. The length, width, and thickness of the sheet are accurately measured (accurate to 0.01 mm), and the external surface area is calculated (unit: m²). 2The sheet was placed in a constant temperature and humidity chamber, and the temperature was set at 38 ± 0.6℃ and the relative humidity at 90 ± 2%. The weight gain method was used to measure the weight gain of the sheet at exposure times of 1 day, 2 days, 3 days, 4 days and 5 days (accurate to 0.0001g). The average value was taken and divided by the surface area of ​​the sheet to obtain the water vapor transmission rate.

[0054] Salt spray resistance test of epoxy thermosetting materials: According to the national standard GB / T2423.17-2024 Environmental Testing Part 2: Test Methods Test Ka: Salt spray, the neutral salt spray test method is adopted, using 5% sodium chloride aqueous solution as spray solution, the solution value is adjusted to the neutral range pH (6.5~7.2), the test temperature is 35℃, the exposure time is 72h, the epoxy thermosetting material is taken out, and its surface corrosion is observed.

[0055] Table 1

[0056] In Examples 1-5 of this invention, the epoxy thermosetting materials obtained by adding the phosphorus-nitrogen synergistic flame retardant containing Diels-Alder dynamic bonds prepared in this invention have a limiting oxygen index of 39%-45%, which is significantly improved compared to Comparative Example 1. The flame retardant rating reaches V-0. After hot pressing and recycling at 150°C, the tensile strength recovery rate of the epoxy thermosetting materials is 90%-95%. The phosphorus-nitrogen synergistic flame retardant containing Diels-Alder dynamic bonds endows the epoxy thermosetting materials with excellent recycling performance due to its DA dynamic bonds. Compared to Comparative Example 1, the material cannot be hot-pressed and recycled after curing because it does not contain the aforementioned DA dynamic bonds.

[0057] In Examples 1-5 of this invention, the water vapor permeability was tested to be 0.5-0.8 g / m³. 2 • After 24 hours, the results showed that the epoxy thermosetting material prepared by this invention has excellent water resistance. Compared with Comparative Example 2, the epoxy thermosetting material obtained by grafting the phosphorus-nitrogen synergistic flame retardant containing Diels-Alder dynamic bonds into the epoxy resin did not have good internal cavities formed between the phosphorus and nitrogen structures through the filling and anchoring of the silane coupling agent modified nano-silica. During the water vapor transmission test, water vapor could easily penetrate into the internal cavities, resulting in poor water resistance of the material. Compared with Comparative Example 3, which used unmodified nano-silica as filling, the water resistance was improved during the water vapor transmission test, but not for a long time. Moreover, the unmodified nano-silica could not be well anchored and fixed inside. After the test, frost and bumps appeared on the surface of the material, indicating that the unmodified nano-silica was prone to migration and precipitation.

[0058] The above embodiments merely illustrate implementation methods of the present invention, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention.

Claims

1. A highly flame-retardant, recyclable epoxy thermosetting material, characterized by, in parts by weight, The product is prepared by curing reaction of the following components: 100 parts epoxy resin, 15-30 parts curing agent, 20-40 parts phosphorus-nitrogen synergistic flame retardant containing Diels-Alder dynamic bonds, 0.5-2 parts accelerator, 1-3 parts defoamer and 10-15 parts silane coupling agent modified nano silica. The chemical structure of the phosphorus-nitrogen synergistic flame retardant containing Diels-Alder dynamic bonds is as follows: ; Wherein, R is phenyl, diphenylmethane, diphenyl ether, or tetramethylene.

2. The highly flame-retardant recyclable epoxy thermosetting material according to claim 1, characterized in that, The phosphorus-nitrogen synergistic flame retardant containing Diels-Alder dynamic bonds is prepared through the following steps: S1: Diphenylphosphonic hydroxylamine is added to a reactor containing tetrahydrofuran and dissolved by ultrasonic vibration and stirring to obtain a diphenylphosphonic hydroxylamine solution. 5-hydroxymethylfurfural is dissolved in a first organic solvent to obtain a 5-hydroxymethylfurfural solution, wherein the molar ratio of diphenylphosphonic hydroxylamine to 5-hydroxymethylfurfural is 1:(1.1~1.3). Under inert gas protection, the 5-hydroxymethylfurfural solution is slowly added dropwise to the reactor. 2~3 drops of glacial acetic acid are added dropwise to the reaction system as a catalyst. The mixture is stirred at room temperature for 20~30 min until homogeneous. The reaction temperature is raised to 40~70℃, refluxed, and the water generated in the reaction is removed using an oil-water separator. The reaction is carried out for 4~6 h to obtain the first reaction solution, which is then cooled to room temperature. S2: Pour the first reaction solution into a Buchner funnel, filter under reduced pressure, and wash the filter cake with tetrahydrofuran 1-3 times to obtain the first reaction filtrate. S3: The filtrate from the first reaction was concentrated under reduced pressure using a rotary evaporator at a temperature of 35-40°C to obtain a crude first solid product. The crude first solid product was washed 2-3 times with deionized water and then vacuum dried at a temperature of 50-70°C to constant weight to obtain a phosphorus-nitrogen synergistic flame retardant intermediate containing a furan ring and terminal hydroxyl groups. The reaction equation for the flame retardant intermediate is as follows: ; S4: The flame-retardant intermediate obtained in step S3 and bismaleimide are added to N,N-dimethylformamide in a molar ratio of (2.0~2.2):1, stirred until completely dissolved, and a Diels-Alder cycloaddition reaction is carried out at 80~100℃ for 18~36h to obtain a second reaction solution, which is then cooled to room temperature. S5: Pour the second reaction solution into a Buchner funnel, filter under reduced pressure, and wash the filter cake with tetrahydrofuran 1 to 3 times to obtain the second reaction filtrate; S6: The filtrate from the second reaction was concentrated under reduced pressure using a rotary evaporator at 70-80°C to obtain a second solid crude product. The second solid crude product was washed with acetone 1-3 times and then vacuum dried at 80-100°C to constant weight to obtain a phosphorus-nitrogen synergistic flame retardant containing Diels-Alder dynamic bonds. The reaction equation for the phosphorus-nitrogen synergistic flame retardant containing Diels-Alder dynamic bonds is as follows: ; Wherein, R is phenyl, diphenylmethane, diphenyl ether, or tetramethylene.

3. The highly flame-retardant recyclable epoxy thermosetting material according to claim 2, characterized in that, In step S1, the first organic solvent is a solvent with a boiling point below 80°C, and the first organic solvent is one or more of anhydrous methanol, anhydrous ethanol, or acetone.

4. The highly flame-retardant recyclable epoxy thermosetting material according to claim 2, characterized in that, In step S1, the mass of the tetrahydrofuran is 4 to 8 times the mass of diphenylphosphonohydroxylamine, and in step S4, the mass of the N,N-dimethylformamide is 8 to 12 times the mass of the flame-retardant intermediate.

5. The highly flame-retardant recyclable epoxy thermosetting material according to claim 3, characterized in that, When the first organic solvent is anhydrous ethanol, the mass fraction ratio of tetrahydrofuran to anhydrous ethanol is 75wt%:25wt%, and an azeotropic point of 66℃ is formed between tetrahydrofuran and anhydrous ethanol. When the first organic solvent is anhydrous methanol, the mass fraction ratio of tetrahydrofuran to anhydrous methanol is 88wt%:12wt%, and an azeotropic point of 64℃ is formed between tetrahydrofuran and anhydrous methanol.

6. The highly flame-retardant recyclable epoxy thermosetting material according to claim 2, characterized in that, The bismaleimide mentioned in step S4 is one or more of N,N'-(4,4'-methylenediphenyl)bismaleimide, N,N'-m-phenylenebismaleimide, N,N'-(1,4-phenylene)bismaleimide, 4,4'-diphenyl ether dicis-butene diimide, and 1,4-bis(maleimide)butane.

7. The highly flame-retardant recyclable epoxy thermosetting material according to claim 1, characterized in that, The epoxy resin is one or more of bisphenol A type epoxy resin E-51, bisphenol A type epoxy resin E-44, and phenolic epoxy resin F-51.

8. The highly flame-retardant recyclable epoxy thermosetting material according to claim 1, characterized in that, The curing agent is one or more of 4,4'-diaminodiphenylmethane and 4,4'-diaminodiphenyl sulfone; the accelerator is one or more of 1,2-dimethylimidazole, 2-ethyl-4-methylimidazole, 2-methylimidazole, and N,N'-dimethylbenzylamine; and the defoamer is one or more of defoamer BYK-054, defoamer BYK-024, or defoamer BYK-022.

9. The method for preparing the highly flame-retardant recyclable epoxy thermosetting material according to any one of claims 1 to 8, characterized in that, Includes the following steps: (1) Premixing and degassing: Epoxy resin, curing agent, phosphorus-nitrogen synergistic flame retardant containing Diels-Alder dynamic bonds, accelerator, defoamer and silane coupling agent modified nano silica are mixed in proportion, stirred at 80~100℃ until completely uniform, placed in a vacuum container, and degassed at a vacuum degree of -0.08~-0.1MPa for 5~10min to obtain the mixture; (2) Segmented curing: Pour the mixture into a mold preheated to 90°C, pre-cur at 95~105°C for 2 hours, then heat to 125~135°C and cure for 2 hours. Cool down to 80~95°C and maintain for 2~4 hours to improve the reconstruction of Diels-Alder dynamic bonds and the reversible cross-linking network. After curing, allow it to cool naturally to room temperature and demold to obtain a highly flame-retardant and recyclable epoxy thermosetting material.

Images