High-heat-resistance intrinsic flame-retardant bio-based bismaleimide resin and preparation method thereof

Abstract

The invention discloses a high-heat-resistance intrinsic flame-retardant bio-based bismaleimide resin and a preparation method of the high-heat-resistance intrinsic flame-retardant bio-based bismaleimide resin. Firstly, a 4, 4 '-dithiodiphenylamine bismaleimide monomer (S-BMI) containing a disulfide bond is designed and synthesized, then a renewable biomass compound magnolol (MG) and the S-BMI in different proportions are copolymerized, three kinds of novel resins (BG-1, BG-2 and BG-3) are prepared, and the comprehensive performance of the BG novel resin is systematically investigated. The novel BG resin provided by the invention has a multiphase structure, and the Tg of all BG resins is between 298 DEG C and 374 DEG C. In addition, the novel BG resin not only has good impact and bending properties, but also shows excellent flame retardancy, and is at the top level in the currently reported intrinsic flame-retardant polymer. The BG resin disclosed by the invention has excellent heat resistance and flame retardance, and can be used as a key basic material urgently needed in emerging strategic fields such as new energy, electronic information, aerospace and the like.

Description

Technical Field

[0001] The present invention belongs to the new material technology, relates to a flame retardant material, and specifically relates to a highly heat-resistant intrinsically flame-retardant bio-based bismaleimide resin and a preparation method thereof. Background Art

[0002] High heat-resistant thermosetting resins (HR thermosetting resins) are key basic materials urgently needed in emerging strategic fields such as new energy, electronic information, and aerospace. Flame retardancy and sustainable development have become important performance characteristics of the new generation of HR thermosetting resins. Flammability is almost a performance shortcoming of all polymer materials, resulting in low service reliability of polymer materials. Therefore, excellent flame retardancy has become a necessary performance index for the new generation of HR thermosetting resins. On the other hand, the serious shortage of petrochemical resources, environmental protection, and sustainable development are important issues around the world today, forcing people to actively explore the research and development of recyclable high-performance resins based on biomass. So far, the methods for preparing flame-retardant thermosetting resins are mainly divided into two types: additive type and intrinsic type. The former is to add a flame retardant to the polymer, and the latter is to start from the polymer's own structure and introduce elements with flame-retardant functions into the molecular chain structure. At present, there is less research on the intrinsic flame-retardant method. The main reason is that it is necessary to modify the molecular structure for specific polymers. For commercially available resins, this method requires changing the original production line, which is not as convenient as the additive flame-retardant scheme.

[0003] There are mainly two problems with the currently disclosed intrinsic flame-retardant thermosetting resins. One is that the preparation strategies of intrinsic flame-retardant thermosetting resins focus on introducing phosphorus elements, and phosphorus flame retardants have been proven to be harmful to the environment [see: A review of a class of emerging contaminants: the classification, distribution, intensity of consumption, synthesis routes, environmental effects and expectation of pollution abatement to organophosphate flame retardants (OPFRs)]; the other is that the heat resistance of intrinsic flame-retardant thermosetting resins is not high, and the introduction of flame-retardant elements usually causes a decrease in the heat resistance of the resin [see: Phosphorus / sulfur-containing aliphatic polyamide curing agent endowing epoxy resin with well-balanced flame safety, transparency and refractive index]. Therefore, the green and highly heat-resistant intrinsic flame-retardant thermosetting resin has important and long-term research significance, and the relevant research is still in its infancy so far. Summary of the Invention

[0004] The present invention develops a new generation of HR thermosetting resin. When designing the molecule, the flame-retardant requirements are considered. Starting from the structural source, the disadvantage of the polymer being flammable is completely changed. In response to the research requirements of the new generation of HR thermosetting resin, the present invention develops a highly heat-resistant intrinsic flame-retardant bio-based bismaleimide resin. First, 4,4'-dithiobis(aniline) bismaleimide monomer (S-BMI) containing a disulfide bond was designed and synthesized, and then different proportions of the renewable biomass compound magnolol (MG) were copolymerized with S-BMI to prepare three new resins (BG-1, BG-2, BG-3), and the comprehensive properties of the BG new resins were systematically investigated. The BG new resins have a multiphase structure, and the glass transition temperature (T g g) of all BG resins is between 298 - 374 °C. In addition, the BG new resins not only have good impact and bending properties, but also all show excellent flame retardancy, ranking among the top in the currently reported intrinsic flame-retardant polymers (SCI database).

[0005] The present invention adopts the following technical solutions: A high heat-resistant intrinsically flame-retardant bio-based bismaleimide resin, the raw materials for its preparation including 4,4'-dithiobis(diphenylamine) bismaleimide and magnolol.

[0006] A high heat-resistant intrinsically flame-retardant bio-based bismaleimide resin prepolymer, the raw materials for its preparation including 4,4'-dithiobis(diphenylamine) bismaleimide and magnolol.

[0007] A preparation method of a high heat-resistant intrinsically flame-retardant bio-based bismaleimide resin prepolymer, comprising melting and mixing the raw materials for preparation to obtain a high heat-resistant intrinsically flame-retardant bio-based bismaleimide resin prepolymer; the raw materials for preparation including 4,4'-dithiobis(diphenylamine) bismaleimide and magnolol.

[0008] A preparation method of a high heat-resistant intrinsically flame-retardant bio-based bismaleimide resin, comprising melting and mixing the raw materials for preparation to obtain a high heat-resistant intrinsically flame-retardant bio-based bismaleimide resin prepolymer, and then curing the high heat-resistant intrinsically flame-retardant bio-based bismaleimide resin prepolymer to obtain a high heat-resistant intrinsically flame-retardant bio-based bismaleimide resin; the raw materials for preparation including 4,4'-dithiobis(diphenylamine) bismaleimide and magnolol.

[0009] In the present invention, the molar ratio of 4,4'-dithiobis(diphenylamine) bismaleimide to magnolol is 1∶(0.5 - 1.5); preferably, the molar ratio of 4,4'-dithiobis(diphenylamine) bismaleimide to magnolol is 1∶(0.8 - 1.2); for example, the molar ratio of 4,4'-dithiobis(diphenylamine) bismaleimide to magnolol is 1∶0.9, 1∶1, 1∶1.1 or any ratio within the range.

[0010] In the present invention, 4,4'-dithiobis(diphenylamine) bismaleimide is prepared from maleic anhydride and 4,4'-diaminodiphenyl disulfide as raw materials; specifically, after maleic anhydride and 4,4'-diaminodiphenyl disulfide react, dehydration cyclization occurs to obtain 4,4'-dithiobis(diphenylamine) bismaleimide.

[0011] In the present invention, the temperature for melting and mixing is 100 - 160°C, and the time is 20 - 60 minutes; preferably, the temperature for melting and mixing is 120 - 150°C, and the time is 30 - 50 minutes.

[0012] In the present invention, the temperature for curing is 140 - 240°C, and the time is 5 - 20 hours; preferably, the temperature for curing is 150 - 230°C, and the time is 6 - 15 hours; as common knowledge, curing is a stepwise temperature increase, with a difference of 15 - 40°C between each step, preferably a difference of 20 - 30°C.

[0013] The present invention also discloses a reshaping method for a highly heat-resistant intrinsically flame-retardant bio-based bismaleimide resin, comprising the following steps: crushing the highly heat-resistant intrinsically flame-retardant bio-based bismaleimide resin and then performing hot pressing treatment to obtain a reshaped bismaleimide resin, thereby realizing the reshaping of the highly heat-resistant intrinsically flame-retardant bio-based bismaleimide resin.

[0014] The present invention also discloses a reshaped bismaleimide resin. The preparation method of the reshaped bismaleimide resin comprises the following steps: performing hot pressing treatment on the crushed highly heat-resistant intrinsically flame-retardant bio-based bismaleimide resin to obtain a reshaped bismaleimide resin; the highly heat-resistant intrinsically flame-retardant bio-based bismaleimide resin is the above-mentioned highly heat-resistant intrinsically flame-retardant bio-based bismaleimide resin.

[0015] In the present invention, the temperature of the hot pressing is 10 - 60 °C above the glass transition temperature of the highly heat-resistant intrinsically flame-retardant bio-based bismaleimide resin, the pressure is 1 - 100 MPa, and the time is 1 - 10 h; preferably, the temperature of the hot pressing is 20 - 50 °C above the glass transition temperature of the highly heat-resistant intrinsically flame-retardant bio-based bismaleimide resin, the pressure is 10 - 50 MPa, and the time is 2 - 8 h.

[0016] The present invention also discloses the application of the above-mentioned highly heat-resistant intrinsically flame-retardant bio-based bismaleimide resin in the preparation of reshaping materials or reshaping thermosetting resins.

[0017] The present invention discloses a shape recovery method for the above-mentioned highly heat-resistant intrinsically flame-retardant bio-based bismaleimide resin, comprising the following steps: (1) At the deformation temperature, changing the shape of the highly heat-resistant intrinsically flame-retardant bio-based bismaleimide resin with the original shape to obtain a highly heat-resistant intrinsically flame-retardant bio-based bismaleimide resin with a new shape, thereby completing the shape change; (2) Placing the highly heat-resistant intrinsically flame-retardant bio-based bismaleimide resin with the new shape back at the deformation temperature, and the highly heat-resistant intrinsically flame-retardant bio-based bismaleimide resin with the new shape returns to the original shape, thereby completing the shape recovery.

[0018] Preferably, the deformation temperature is 20 - 40 °C below to 10 - 60 °C above the glass transition temperature of the highly heat-resistant intrinsically flame-retardant bio-based bismaleimide resin; the shape recovery is single or multiple shape recoveries. The glass transition temperature of the above-mentioned highly heat-resistant intrinsically flame-retardant bio-based bismaleimide resin is the lowest glass transition temperature of the highly heat-resistant intrinsically flame-retardant bio-based bismaleimide resin.

[0019] The present invention discloses the application of the above-mentioned highly heat-resistant intrinsically flame-retardant bio-based bismaleimide resin in the preparation or as a bismaleimide resin material.

[0020] The present invention firstly designed and synthesized a class of intrinsic flame-retardant resins (BG) that combine high heat resistance, halogen-free and phosphorus-free, and biomass. It is copolymerized from a novel bismaleimide containing disulfide bonds (S-BMI) and the renewable biomass compound magnolol (MG), and has the following technological advancements: (1) The three BG resins have high T g (298 - 374 °C) and high T di (391 - 405 °C), and have the best heat resistance among the halogen-free and phosphorus-free intrinsic flame-retardant bio-based thermosetting resins reported so far.

[0021] (2) Compared with the classic BD resin, BG-1 has improved impact strength (14.23 kJ / m 2 ).

[0022] (3) The three BG resins have excellent flame retardancy. The limiting oxygen indices (LOIs) of BG-1, BG-2, and BG-3 are 41.8%, 40.7%, and 37.3% respectively.

[0023] (4) The BG-1 resin can be reshaped from powder form by hot pressing at 340 °C under a pressure of 30 MPa for 2 h. The relatively high T g makes the reshaping conditions of BG-1 relatively harsh, and the further increase in the curing degree of BG-1 resin under hot pressing makes BG-1 more brittle. In addition, the BG-1 resin has a rapid shape recovery ability at 340 °C and can quickly recover from the "U" shape to the straight shape within 15 s. Description of the Drawings

[0024] Figure 1 is the 1 H-NMR spectrum of S-BMA and the 1 H-NMR spectrum of S-BMI.

[0025] Figure 2 is the high-resolution mass spectrum of S-BMI.

[0026] Figure 3 is the DSC curve of S-BMI.

[0027] Figure 4 is the Tan δ - temperature curve of the BG cured resin.

[0028] Figure 5 is the curing reaction of the BG resin.

[0029] Figure 6 is the TGA and DTG curves of the cured resin.

[0030] Figure 7 is the HRR - temperature curve (MCC test) of the BG resin.

[0031] Figure 8 THR-Temperature curve of BG resin (MCC test).

[0032] Figure 9 Raman spectrum of the char layer after combustion of BG resin.

[0033] Figure 10 SEM image of the char layer after combustion of BG resin.

[0034] Figure 11 Impact strength of BG cured resin.

[0035] Figure 12 Flexural modulus and flexural strength of BG cured resin.

[0036] Figure 13 Storage modulus - temperature curve of cured resin.

[0037] Figure 14 Shape memory effect of BG.

[0038] Figure 15 Shape reconstruction of BG-1.

[0039] Figure 16 Remolding experiment of BG-1 resin. Detailed implementation manners

[0040] In existing flame retardant technologies, halogen-containing (CI, Br) flame retardants have been prohibited from use worldwide due to the high biological toxicity and corrosiveness of their decomposition products at high temperatures; phosphorus flame retardants can significantly improve the flame retardancy of polymers, but they may reduce the thermal stability of heat-resistant polymers and increase soot generation during combustion; inorganic silicon is often used in combination with other flame retardants to achieve good flame retardant effects, and organosilicon (such as polysiloxanes) has a condensed-phase flame retardant effect, but it is often effective only for specific polymers; other additive flame retardants such as carbon-based materials and inorganic nanofillers all have problems such as incompatibility with polymers, difficulty in dispersion, and easy aggregation. Although the intrinsic flame retardant method can overcome the above-mentioned compatibility and aggregation problems, most of the existing technologies introduce phosphorus elements into resins. Existing studies have shown that phosphorus-containing flame retardants have biological toxicity and bioaccumulation, and they have been identified as a new type of pollutant. The inventor previously synthesized a new type of halogen-free and phosphorus-free aryl allyl ether compound (DS), and an intrinsic flame retardant bismaleimide resin (BDS) was obtained by copolymerizing DS with BDM. BDS has excellent flame retardancy (LOI: 32.6%) and high heat resistance (T g : 336 °C, T di : 356 °C), but the T g and T diToo close, which limits its practical application and is not conducive to industrialization.

[0041] Due to its high crosslinking density and brittleness, BMI needs to be modified in practical applications. Currently, DBA is one of the most successful compounds for modifying bismaleimide. However, DBA is derived from non-renewable petrochemical resources, and bisphenol A in it can cause endocrine disorders and is harmful to the human body. Therefore, it is necessary to develop a healthy, environmentally friendly and renewable compound to modify BMI.

[0042] So far, biomass intrinsic flame-retardant polymers with both high heat resistance and halogen-free and phosphorus-free have not been reported. In this invention, 4,4'-dithiobis(phenylamine) bismaleimide (S-BMI) containing disulfide bonds was synthesized and copolymerized with the biomass compound magnolol to obtain a biomass intrinsic flame-retardant BMI resin with both high heat resistance and halogen-free and phosphorus-free.

[0043] The following illustrates the technical progress of this invention through specific experiments. The raw materials used are existing products, and the specific preparation operations and performance tests are all conventional technologies.

[0044] Structure characterization and performance testing are conventional technologies.

[0045] Thermogravimetric analysis (TGA): Under a nitrogen atmosphere, each sample was heated from 25°C to 800°C at a heating rate of 10°C / min, and the nitrogen flow rate was 10 mL / min.

[0046] Dynamic mechanical analysis (DMA): In an air atmosphere, the frequency was 1 Hz, and it was heated from 25°C to 380°C at a heating rate of 3°C / min.

[0047] Thermogravimetry-infrared spectroscopy (TG-IR) test: Under a nitrogen atmosphere, it was heated from 25°C to 800°C at a heating rate of 10°C / min, and the nitrogen flow rate was 10 mL / min.

[0048] The impact strength was measured according to the standard ASTM D2990-2017; the flexural strength was measured according to the standard ASTM D2990-2017; according to the standard ASTM D7309, microcalorimetry (MCC) test was carried out using a microcalorimeter, the heating rate was 1°C / s, the temperature range was from room temperature to 750°C, and the test atmosphere was a mixed gas of nitrogen and oxygen with a volume ratio of 80:20; according to the standard ASTM D2863, the LOI test was carried out using an oxygen index meter.

[0049] Example 1 Synthesis of S-BMI Maleic anhydride (0.22 mol) and 4,4'-diaminodiphenyl disulfide (0.1 mol) were reacted at room temperature for 2 h to form the product 4,4'-dithiobismaleamic acid (S-BMA), and the yield was 98%.1 1H NMR (400 MHz, DMSO-d6) δ 13.02(s, 2H), 10.52 (s, 2H), 7.67 (d, J = 8.8 Hz, 4H), 7.51 (d, J = 8.8 Hz, 4H), 6.49 (d, J = 12.0 Hz, 2H), 6.33 (d, J = 12.0 Hz, 2H). 13 13C NMR (101 MHz, DMSO) δ 167.34, 163.83, 139.21, 132.07, 130.76, 130.74, 130.35, 120.70. Calculated [C 20 H 16 O 6 N 2 S 2 -2H] / 2 + The molecular weight of [] is 221.0000, and the high-resolution mass spectrometry test result is 221.0141. Then, the acetic anhydride dehydration and ring closure method was used to dehydrate and cyclize S-BMA to obtain the product S-BMI, with a yield of 90%. 1 1H NMR (400 MHz, DMSO-d6) δ 7.66 (dt, J = 8.8, 1.0, 0.0 Hz, 4H), 7.38 (d, J = 8.8 Hz, 4H), 7.19 (s, 4H). 13 13C NMR (101 MHz, DMSO) δ 169.77, 134.94, 134.84, 131.09, 127.74, 127.65. Calculated [C 20 H 12 O 4 N 2 S 2 Na] + The molecular weight of [] is 431.0000, and the high-resolution mass spectrometry test result is 431.0126. The specific preparation process is a conventional method, such as referring to CN2025100676794, a reshaping and degradable bismaleimide resin and its preparation method.

[0050] Using 1 1H-NMR, 13 13C-NMR and HRMS spectra to characterize the structures of S-BMA and S-BMI. Figure 1 Shown is the 1 1H NMR (400 MHz, DMSO-d6) of S-BMA, the 1 1H NMR (400 MHz, DMSO-d6) of S-BMI,1 The results of \(^1H\) NMR confirmed the synthesis of S-BMA and S-BMI. The HRMS of S-BMA showed that the measured \([M - 2H] / 2\) was 221.0141, which was consistent with the theoretical value of 221.0000. The HRMS spectrum of S-BMI ( Figure 2 showed that the measured \([M + Na] + was 431.0126, which was consistent with the theoretical value of 431.0000.

[0051] The above test results proved that the synthesized intermediate product S-BMA compound was 4,4'-dithiobis maleamic acid, and the final product S-BMI was 4,4'-dithiobis aniline bismaleimide. S-BMI was a creamy yellow solid at room temperature, and its DSC curve was as Figure 3 shown. It can be seen from the figure that S-BMI had a sharp melting endothermic peak at 186 °C, which was the melting point of S-BMI. In addition, there was an exothermic peak of curing reaction at 261 °C, corresponding to the self-polymerization reaction of bismaleimide.

[0052] Example 2 Preparation of cured resin Weigh magnolol (MG) and S-BMI according to Table 1, and carry out melt pre-polymerization at 140 °C for 40 min to obtain a clear and transparent reddish-brown prepolymer; then pour the prepolymer into a preheated mold while it is still hot, evacuate at 140 °C for 30 min, and then carry out curing and post-curing according to the procedures of 150 °C / 2 h + 180 °C / 2 h + 200 °C / 2 h and 230 °C / 8 h. After natural cooling to room temperature, demold to obtain the cured resin BG.

[0053] Table 1 Formulation of cured resin

[0054] DBA-modified BDM was one of the most successful modified resins. However, the raw materials for the synthesis of DBA came from petrochemical resources, and bisphenol A in it could cause endocrine disorders. In this invention, magnolol was first used to copolymerize with BMI to develop a halogen-free, phosphorus-free, intrinsically flame-retardant biomass resin (BG) with high heat resistance, solving the problem that the existing BMI modifiers were not environmentally friendly, especially the problem of toxicity.

[0055] Example 3 Thermal properties of BG resin T g is the temperature at which the resin transforms between the high elastic state and the glassy state, and it is the upper limit temperature for its use. Figure 4It is the Tanδ-temperature curves of three BG cured resins. It can be seen that the Tan δ peaks of the three resins are all asymmetric wide peaks, and as the molar ratio of S-BMI:MG decreases, the peak height gradually decreases and the full width at half maximum becomes wider. After peak fitting, three peaks are obtained (Table 2), indicating that the BG cured resin is a multiphase structure, which can be explained by the curing mechanism ( Figure 5 ). In the BG system, first, the diene addition reaction between maleimide and allyl group occurs to generate an intermediate product; when the temperature further rises, the intermediate product reacts with the maleimide group through the Diels-Alder reaction to form a crosslinked network; at the same time, the maleimide ring can undergo homopolymerization. Therefore, BDM molecules can self-polymerize, and homopolymerization of the maleimide ring can occur between the copolymer intermediates formed by BDM and MG or between the intermediate and BDM molecules.

[0056] The TGA and DTG curves of BG resin in a nitrogen atmosphere are as Figure 6 shown, and the initial thermal decomposition temperature (T di ), the temperature of maximum degradation rate (T max ), and the char yield at 800 °C ( Y c ) obtained therefrom are summarized in Table 3. It can be seen from this that as the molar ratio of S-BMI:MG decreases, the T di of BG resin gradually increases, Y c slightly decreases, and T max remains basically unchanged. For the allyl compound copolymer-modified BDM system without sulfur element reported in the prior art, its Y c gradually decreases as the ratio of allyl compound in the system increases. In particular, when the ratio of maleimide group to allyl group is the same, their corresponding Y c (≤40 wt%) are all lower than those of BG resin.

[0057] Table 2 Thermal properties of cured resins

[0058] Table 3 TGA and DTG data of BG resin

[0059] The heat resistance of the reported intrinsic flame-retardant polymers in the prior art is summarized in Table 4. It can be found that among the reported intrinsic flame-retardant thermosetting resins so far, the highest T g of the phosphorus-free intrinsic flame-retardant thermosetting resin is only 168.4 °C, while the T g of the BG resin of the present invention is between 298 - 307 °C, compared with T diThe difference is about 100 °C. Therefore, BG resin has the best heat resistance among the halogen-free and phosphorus-free intrinsic flame-retardant thermosetting resins and has a good industrial application foundation.

[0060] Table 4 Thermal properties and flame retardancy of the intrinsic flame-retardant thermosetting resins in the prior art and the present invention

[0061]

[62] Research on High-Performance Flame-Retardant Cyanate Ester Resin [D];

[70] Vanillin-derived High-Performance Flame-Retardant Epoxy Resins: Facile Synthesis and Properties [J];

[72] Synthesis of an Acrylate Constructed by Phosphaphenanthrene and Triazine-Trione and Its Application in Intrinsic Flame-Retardant Vinyl Ester Resin [J];

[74] A Novel Biobased Epoxy Resin with High Mechanical Stiffness and Low Flammability: Synthesis, Characterization and Properties [J];

[80] High Residue Bio-based Structural-Functional Integration Epoxy and Intrinsic Flame Retardant Mechanism Study [J];

[83] Magnolol-based Bio-epoxy Resin with Acceptable Glass Transition Temperature, Processability and Flame Retardancy [J];

[84] Biobased heat resistant epoxy resin with extremely high biomasscontent from 2,5-furandicarboxylic acid and eugenol[J];

[85] Biobased epoxy resin derived from eugenol with excellentintegrated performance and high renewable carbon content[J];

[86] Hexa(eugenol)cyclotripho- sphazene modified bismaleimide resinswith unique thermal stability and flame retardancy[J];

[121] Synthesis of a novel reactive flame retardant containingphosphaphenanthrene and piperidine groups and its application in epoxy resin[J];

[122] Intrinsic flame-retardant and thermally stable epoxy endowed bya highly efficient, multifunctional curing agent[J];

[123] Flame-retardant performance and mechanism of epoxy thermosetsmodified with a novel reactive flame retardant containing phosphorus,nitrogen, and sulfur[J];

[124] Synthesis of an intrinsically flame retardant bio-basedbenzoxazine resin[J]。

[0062] Flame Retardancy of BG Resin in Example 4 Microcalorimetry test (MCC) and LOI are effective methods for characterizing the flame retardancy of materials. The heat release rate (HRR) of the resin is the most important performance parameter for characterizing the fire intensity. The peak value of HRR represents the PHRR during the combustion process of the resin. The magnitude of PHRR characterizes the maximum heat dissipation during material combustion. The larger the PHRR, the more heat is released by the material, and the greater the fire risk. In addition, the MCC test can also obtain the heat release capacity (HRC) of the material.

[0063] Figure 7 is the heat release rate (HRR)-temperature curve of BG resin. All BG resins show a broad peak, indicating that the resin burns slowly after ignition and releases less heat. Figure 8 is the total heat release (THR)-temperature curve of BG resin; the physicochemical data obtained from the figure are listed in Table 5. The flame retardancy of BG-3 is inferior to that of BG-1 and BG-2. After LOI testing, the LOIs of BG-1, BG-2, and BG-3 are 41.8%, 40.7%, and 37.3% respectively. The LOI of BG resin is higher than that of the reported halogen-free and phosphorus-free intrinsically flame-retardant bismaleimide (32.6%). Comprehensive analysis of these data shows that BG resin has excellent flame retardancy. According to the reported LOI and T of intrinsically flame-retardant thermosetting resins in the prior art g , it can be intuitively seen that the previously reported intrinsically flame-retardant resins cannot have both high LOI and high heat resistance, while BG resin has excellent heat resistance (T g > 300 °C) and a high LOI (37.3% - 41.8%). Therefore, BG is a high-heat-resistant intrinsically flame-retardant biomass thermosetting resin.

[0064] Table 5 MCC and LOI test data of cured resins

[0065] To further study the microstructure of the residual carbon layer after resin combustion, Raman spectroscopy was performed on it to characterize the graphitization degree of the carbon layer. Generally, the graphitization degree is determined by the intensity ratio of the G peak to the D peak (I G / I D ) in the Raman spectrum. As Figure 9 shown, the I G / I D value of BG resin is between 1.18 - 1.25. As the molar ratio of S-BMI:MG decreases, the I G / I D gradually decreases. The graphitization degree of all three BG resins is greater than that of the previously reported intrinsically flame-retardant thermosetting resins (I G / I D<1). The residual carbon layer acts as a protective barrier on the material surface. The higher the graphitization degree of the carbon layer, the stronger the inhibition of heat and mass transfer, and the better the flame retardant effect in the condensed phase.

[0066] Figure 10 Figure is the SEM photograph of the residual carbon after the LOI test of BG resins. It can be seen that the residual carbons of the three BG resins are all continuous and dense. The dense carbon layer can effectively isolate the heat and mass transfer between the inside of the resin and the outside, thereby improving the flame retardant effect in the condensed phase. In summary, the carbon layer after the combustion of BG resins has excellent flame retardant effect in the condensed phase.

[0067] The gases generated during the thermal degradation of the resins were detected by thermogravimetry-infrared spectroscopy (TG-IR). It can be known that the peak signals of the three resins are the strongest at 2075 cm -1 and 2045 cm -1 , which correspond to the non-combustible gases (such as S=C=O, N=C=S) generated by the interaction of sulfur free radicals generated by the cleavage of S-S bonds and / or C-S bonds with other thermal degradation products, and the intensity of the absorption peak decreases with the decrease of the S-BMI:MG molar ratio because the content of disulfide bonds in the system decreases accordingly. At the same time, non-combustible carbon dioxide (2370 and 2310 cm -1 ), sulfur dioxide (1340 and 1360 cm -1 ), nitrogen oxides (1500 cm -1 ) and water (3580 cm -1 ) are also generated during the pyrolysis of the resins. These non-combustible gases can dilute the combustible gases. Since BG resins contain a large number of disulfide bonds, the sulfur free radicals released by their thermal decomposition can quench the active free radicals generated during combustion and block the chain reaction to extinguish the flame. In addition, a small amount of organic gaseous combustibles are also generated during the thermal decomposition of BG resins: hydrocarbons (2800 - 3100 cm -1 ), carbonyl compounds (1740 cm -1 ), carbonitrogen compounds (1175 cm -1 ).

[0068] The above research results show that disulfide bonds have both flame retardant effects in the condensed phase and the gas phase, endowing BG resins with excellent flame retardant properties.

[0069] Example 5 Static and Dynamic Mechanical Properties of BG Resins The impact strength is an index used to evaluate the impact resistance of materials and measure the toughness of materials, such as Figure 11As shown, the impact strength of BG-1 and BG-2 is similar, while the impact strength of BG-3 is about 62.6 - 65.5% of the former two. In particular, the three BG resins have improved impact strength compared with the existing BD-0.86 (8.19 kJ / m 2 ).

[0070] Flexural properties can comprehensively reflect the rigidity and toughness of materials, Figure 12 which are the flexural strength and flexural modulus of BG resins. As the molar ratio of S-BMI:MG decreases, the flexural strength of BG resins increases, and BG-3 has the highest flexural strength (105.3 MPa). In addition, the flexural modulus of the three BG resins decreases slightly as the molar ratio of S-BMI:MG decreases.

[0071] The storage modulus represents the ability of materials to store elastic deformation energy, Figure 13 which is the storage modulus - temperature curve of the novel cured resin. It can be seen that at 50 °C, BG-1, BG-2 and BG-3 are 2719 MPa, 3188 MPa and 3330 MPa respectively.

[0072] Example 6 Shape memory and reconstruction properties of BG resins Figure 14 The schematic diagram of the shape memory process of BG-1 is given. The rectangular linear BG-1 sample is first heated at 340 °C for about 5 s, the sample is bent into a "U" shape, and then cooled at room temperature to release the stress, so that the sample is fixed from the linear shape to the temporary shape "U". Finally, the "U" shaped sample is reheated at 340 °C, and it is found that the "U" shaped sample quickly returns to the linear shape within 15 s. According to the following formula, after 15 s, the shape recovery rate R of BG-1 is calculated to be 100%, and BG-1 has excellent shape memory performance.

[0073] R = ( θ D - θ t ) / θ D × 100% Where, θ D represents the total deformation angle of the specimen, θ t is the residual deformation angle at time t.

[0074] The test method for the shape memory process of BG-2 and BG-3 is similar to that of BG-1. First, heat the linear sample at 340 °C for about 5 s, then bend it into a right angle, place it at room temperature to cool, release the stress, and the shape of the sample is fixed. When BG-2 is heated at 340 °C again, BG-2 and BG-3 recover their shapes within 10 s, showing excellent shape recovery ability.

[0075] Due to its unique three-dimensional network structure, traditional thermosetting resins are insoluble and infusible once formed, and their shapes cannot be changed. Dynamic covalent bonds bring the possibility of reprogramming the shape of thermosetting resins. Figure 15 It is a schematic diagram of the permanent shape reconstruction of BG-1. First, heat the linear sample at 340 °C and fix its shape into an "S" shape with an external force. The crosslinked network rearranges and combines under the dynamic exchange of disulfide bonds, the molecular chains are oriented and internal stress is stored. Then, keep it at 340 °C for another 0.5 h to completely relax the internal stress, the orientation of the molecular chains disappears, the network structure is fixed, and a new permanent shape is formed.

[0076] Example 7 Remolding Performance of BG Resin Existing thermosetting resins are three-dimensional networks with high crosslinking density. Once formed, they are insoluble and infusible, resulting in the inability to recycle waste thermosetting resins. Generally, waste thermosetting resins are disposed of by burial or incineration. The research on remoldable thermosetting resins has important research value for resource conservation and the sustainable development of society.

[0077] The remolding process of BG-1 resin is as Figure 16 shown. Grind the resin sample into powder and place it in a mold, then hot press it at 340 °C under a pressure of 30 MPa for 2 h in a hot press. After the hot pressing is completed, cool it naturally and demold. Experiments found that BG-1 can be remolded from powder into regular sheet resins with a smooth surface and no cracks, indicating that the resin powder reconnects to form a complete resin.

[0078] High heat-resistant thermosetting resins are key basic materials in emerging strategic fields such as new energy, electronic information, and aerospace. Among them, service reliability and sustainable development have become important indicators. Flammability is almost a performance shortcoming of all polymer materials, and thermosetting resins are no exception. This deficiency leads to low service reliability of polymer materials. Therefore, excellent flame retardancy has become a necessary performance indicator for thermosetting resins in recent years. On the other hand, the serious shortage of petrochemical resources, environmental protection, and sustainable development are important issues around the world today, forcing people to actively explore the research and development of high-performance resins based on biomass.

[0079] At present, the methods for preparing flame-retardant polymer materials are mainly divided into two types: additive type and intrinsic type. The former refers to adding flame retardants to polymers, and the latter is to modify the molecular structure of the polymers themselves. The key point of the additive flame-retardant strategy is to develop various flame retardants. In order to obtain an efficient flame-retardant effect, the compatibility between the flame retardant and the polymer needs to be considered. The currently known green and environmentally friendly flame retardants are mainly silicone and inorganic fillers (such as graphite, boron nitride, carbon nanotubes, etc.). The flame-retardant effect of silicone compounds is related to the type of polymer and often reduces the heat resistance of the resin; the flame-retardant effect of inorganic flame retardants is generally not high, and usually a high content needs to be added, which often deteriorates the processability and mechanical properties of the polymer; in addition, there are also compatibility problems with organic polymers, and organic functionalization is required.

[0080] Relatively speaking, there is less research work on the intrinsic flame-retardant method. The main reason is that it is necessary to modify the molecular structure for specific polymers. For commercially available thermosetting resins, this method requires changing the original production line, which is not as convenient as the additive flame-retardant scheme. BMI is a typical representative of thermosetting resins. As the matrix of advanced composite materials, heat-resistant adhesives, class C insulating paints, etc., it is applied in cutting-edge fields such as new energy, electronic information, and aerospace. There are only three literature reports (SCI database) on the research of intrinsic BMI so far. Although two of them prepared bio-based BMI, phosphorus is contained in the system; the research group previously prepared a non-bio-based halogen-free and phosphorus-free flame-retardant BMI resin, but its T g and T di are very close, which is not conducive to practical applications.

[0081] To address the above problems, the present invention discloses a highly heat-resistant intrinsic flame-retardant bio-based BMI resin. First, the synthesis and characterization of 4,4'-dithiobis(diphenylamine) bismaleimide monomer (S-BMI) are carried out; then different proportions of the biomass compound magnolol (MG) are copolymerized with S-BMI to prepare three BG resins. The effects of the ratio of MG to S-BMI on the thermal and mechanical properties of the BG resins are investigated, and the flame-retardant properties of the BG resins are studied. The three BG resins have high T g (298 - 374 °C) and high T di (391 - 405 °C), and have the best heat resistance among the halogen-free and phosphorus-free intrinsic flame-retardant bio-based thermosetting resins reported so far; compared with the classic BD resin, BG-1 has improved impact strength (14.23 kJ / m 2 ); the three BG resins have excellent flame retardancy, and the LOIs of BG-1, BG-2, and BG-3 are 41.8%, 40.7%, and 37.3% respectively; the BG-1 resin can be reshaped from powder form by hot pressing at 340 °C under a pressure of 30 MPa for 2 h. The higher T gThe reshaping conditions of BG-1 are relatively harsh, and the further increase in the degree of curing of BG-1 resin under hot pressing makes BG-1 more brittle. In addition, BG-1 resin has a rapid shape recovery ability at 340 °C and can quickly recover from the "U" shape to the straight shape within 15 s.

Claims

1. A highly heat-resistant intrinsic flame-retardant bio-based bismaleimide resin, the raw materials for its preparation include 4,4'-dithiodiphenylamine bismaleimide and magnolol; or a highly heat-resistant intrinsic flame-retardant bio-based bismaleimide resin prepolymer, the raw materials for its preparation include 4,4'-dithiodiphenylamine bismaleimide and magnolol.

2. The highly heat-resistant intrinsic flame-retardant bio-based bismaleimide resin or highly heat-resistant intrinsic flame-retardant bio-based bismaleimide resin prepolymer according to claim 1, characterized in that: The molar ratio of 4,4'-dithiodiphenylamine bismaleimide to magnolol is 1:(0.5-1.5).

3. A method for preparing a highly heat-resistant intrinsic flame-retardant bio-based bismaleimide resin prepolymer, The invention comprises the following steps: melting and mixing the raw materials to obtain a high heat-resistant intrinsic flame-retardant bio-based bismaleimide resin prepolymer; or a method for preparing a high heat-resistant intrinsic flame-retardant bio-based bismaleimide resin, The method comprises the following steps: melting and mixing the raw materials to obtain a high heat-resistant intrinsic flame-retardant bio-based bismaleimide resin prepolymer; and then curing the high heat-resistant intrinsic flame-retardant bio-based bismaleimide resin prepolymer to obtain a high heat-resistant intrinsic flame-retardant bio-based bismaleimide resin; the raw materials comprise 4,4'-dithiodiphenylamine bismaleimide and magnolol.

4. The preparation method according to claim 3, characterized in that: 4,4'-Dithiodiphenylamine bismaleimide was prepared from maleic anhydride and 4,4'-diaminodiphenyl disulfide.

5. The preparation method according to claim 3, characterized in that: The temperature of melt mixing is 100-160° C., and the time is 20-60 minutes; the temperature of curing is 140-240° C., and the time is 5-20 hours.

6. The method for reshaping the highly heat-resistant intrinsic flame-retardant bio-based bismaleimide resin according to claim 1 comprises the following steps: The highly heat-resistant intrinsic flame-retardant bio-based bismaleimide resin is crushed and then subjected to hot pressing to obtain a reshaped bismaleimide resin, thereby achieving the reshaping of the highly heat-resistant intrinsic flame-retardant bio-based bismaleimide resin.

7. A reshaped bismaleimide resin, wherein the preparation method of the reshaped bismaleimide resin comprises the following steps: The crushed high heat resistant intrinsic flame retardant bio-based bismaleimide resin is heat-pressed to obtain a reshaped bismaleimide resin; the high heat resistant intrinsic flame retardant bio-based bismaleimide resin is the high heat resistant intrinsic flame retardant bio-based bismaleimide resin according to claim 1.

8. The deformation recovery method of the highly heat-resistant intrinsic flame-retardant bio-based bismaleimide resin according to claim 1 comprises the following steps: (1) at a deformation temperature, changing the shape of a highly heat-resistant intrinsic flame-retardant bio-based bismaleimide resin having an original shape to obtain a highly heat-resistant intrinsic flame-retardant bio-based bismaleimide resin having a new shape, thereby completing the shape change; (2) The high heat-resistant intrinsic flame-retardant bio-based bismaleimide resin with a new shape is placed back at the deformation temperature, and the high heat-resistant intrinsic flame-retardant bio-based bismaleimide resin with a new shape is restored to the original shape, thereby completing the shape recovery.

9. The method according to claim 8, characterized in that The deformation temperature is 20-40°C below and 10-60°C above the glass transition temperature of the highly heat-resistant intrinsic flame-retardant bio-based bismaleimide resin; and the deformation recovery is single or multiple deformation recovery.

10. Use of the highly heat-resistant intrinsic flame-retardant bio-based bismaleimide resin according to claim 1 in the preparation of or as a bismaleimide resin material.

Images