Method for enhancing performance of bio-based benzoxazine through copolymerization of modified lignin

Abstract

The invention discloses a method for enhancing the performance of bio-based benzoxazine through modified lignin copolymerization, and belongs to the technical field of biomass-based high polymer materials. The invention aims to solve the problems of high curing temperature, low crosslinking density and poor mechanical properties (especially toughness) of the existing benzoxazine resin reinforced by using lignin as a raw material, and also solve the problems of insufficient mechanical properties and poor thermal stability of the benzoxazine resin prepared by using a small-molecular natural phenolic compound as a raw material. The method comprises the following steps: 1, preparing a lignin-based benzoxazine monomer; 2, preparation of a phlorenic acid group bisphenol type benzoxazine monomer; and 3, preparing the bio-based benzoxazine copolymer resin. The method is used for modifying lignin copolymerization to enhance the performance of bio-based benzoxazine.

Description

Technical Field

[0001] The invention belongs to the technical field of biomass-based polymer materials. Background Art

[0002] Benzoxazine resins, as high-performance thermoset materials, exhibit great potential for application in aerospace, electronic packaging, composite materials, and other fields due to their excellent mechanical properties, high thermal stability, low dielectric constant, low water absorption, and flexible molecular design. However, traditional benzoxazine resins are primarily synthesized from petroleum-based raw materials (such as phenols and amines), which are non-renewable and pose increasingly serious environmental concerns. Therefore, the development of high-performance bio-based benzoxazine resins derived from renewable biomass resources has become a research hotspot.

[0003] Lignin, the second most abundant natural aromatic polymer in nature, is a major byproduct of wood hydrolysis and the pulp and paper industry. It has significant advantages such as wide availability, low cost, and renewability. More importantly, lignin is rich in phenolic hydroxyl groups in its molecular structure, making it considered an ideal raw material for the preparation of bio-based benzoxazine resins. An effective way to achieve high-value utilization of lignin is to replace part or all of petroleum-based phenol with lignin or its depolymerized derivatives (such as ligninphenol), react them with amine compounds to synthesize lignin-based benzoxazines, and then obtain thermosetting resins through thermal ring-opening polymerization.

[0004] However, the practical application of lignin faces significant challenges due to its complex molecular structure, high polydispersity, poor reactivity, and uneven availability. These inherent properties lead to benzoxazine resins made from it generally suffering from high curing temperatures, low crosslink density, and mechanical properties (especially toughness) that are inferior to those of petroleum-based products, limiting its application in high-performance applications.

[0005] Small molecule natural phenolic compounds derived from plants (such as phloric acid) have also attracted attention in the synthesis of bio-based benzoxazines due to their clear structure and high reactivity. Phloric acid is a natural diphenolic acid compound found in the peels of fruits such as apples and pears, and its molecular structure contains two phenolic hydroxyl groups and one carboxyl group. Studies have shown that it is feasible to use phloric acid as a phenolic source to react with amines to prepare benzoxazine monomers. Its structure is regular, and the introduction of flexible fatty chains (from propionic acid side chains) and additional carboxyl functional groups may theoretically help improve the processability and toughness of the resin or provide additional reaction sites to enhance cross-linking. However, benzoxazine resins that rely solely on small molecule phenols such as phloric acid have the limitations of insufficient mechanical properties and low thermal stability.

[0006] Therefore, how to develop new bio-based benzoxazine resin materials with excellent comprehensive performance, good processability and controllable costs remains a key issue that needs to be urgently addressed in this field. Summary of the Invention

[0007] The present invention aims to solve the problems of high curing temperature, low crosslinking density and poor mechanical properties (especially toughness) of existing benzoxazine resins reinforced with lignin as raw material, and at the same time solve the problems of insufficient mechanical properties and poor thermal stability of benzoxazine resins prepared with small molecule natural phenolic compounds as raw materials, and further provide a method for copolymerizing modified lignin to enhance the performance of bio-based benzoxazine.

[0008] A method for copolymerizing modified lignin to enhance the performance of bio-based benzoxazine is carried out according to the following steps:

[0009] 1. Preparation of lignin-based benzoxazine monomers:

[0010] ① Dissolve lignin in an ethanol / water mixed solvent to obtain a lignin solution;

[0011] ② Under heating and stirring conditions, paraformaldehyde and furfurylamine are added to the lignin solution for reaction, cooled to room temperature after the reaction, and then evaporated to remove the solvent, and finally dried to obtain a lignin-based benzoxazine monomer;

[0012] 2. Preparation of phlorate-based bisphenol-type benzoxazine monomer:

[0013] ① Under nitrogen atmosphere, heating and stirring conditions, phloric acid, 1,12-dodecanediol and p-toluenesulfonic acid are mixed and reacted, and after the reaction, the mixture is cooled to room temperature, and then the product is extracted, washed and the solvent is removed by rotary evaporation to obtain phloric acid-based bisphenol;

[0014] ② Under heating and stirring conditions, phloride-based bisphenol, paraformaldehyde and ethanolamine are mixed and reacted, and then cooled to room temperature to obtain a phloride-based bisphenol-type benzoxazine monomer;

[0015] 3. Preparation of bio-based benzoxazine copolymer resin:

[0016] ① Adding lignin-based benzoxazine monomer to an ethanol / water mixed solvent for dispersion, and then heating to obtain a lignin-based benzoxazine monomer solution;

[0017] ② dissolving the phloride-based bisphenol-type benzoxazine monomer in dichloromethane to obtain a phloride-based bisphenol-type benzoxazine monomer solution;

[0018] ③ The lignin-based benzoxazine monomer solution and the phlorate-based bisphenol-type benzoxazine monomer solution are mechanically stirred and uniformly mixed, and then ultrasonically treated to obtain a viscous blend;

[0019] ④ The viscous blend is poured and bubbles are removed, and then asymmetric temperature gradient curing is performed under normal pressure. After cooling to room temperature, demolding is performed to obtain a bio-based polybenzoxazine copolymer resin.

[0020] The beneficial effects of the present invention are:

[0021] The present invention successfully prepared a bio-based resin with high comprehensive performance by copolymerizing a specific ratio of lignin-based benzoxazine monomers and flexible phlorothiol-based bisphenol-type benzoxazine monomers. Its significant advantages are reflected in the following aspects:

[0022] 1. Significantly enhanced mechanical properties and improved structural stability:

[0023] At a copolymerization ratio of 5wt% CCL-Bz, the resin exhibits a rigid-flexible synergistic effect: the tensile strength reaches 35.43MPa and the flexural strength reaches 56.18MPa.

[0024] Through lignin modification and copolymerization enhancement process, the agglomeration of lignin in benzoxazine is effectively inhibited, macro / micro uniformity is achieved, and the reprocessing deformation rate is extremely low (the deformation is only 1.9° after 360 hours at room temperature).

[0025] 2. Breakthrough in efficient curing reaction and thermal performance:

[0026] FTIR confirmed the characteristic peak of benzoxazine ring (935cm -1 ) indicated that the cyclization modification reaction of lignin was successful; DSC showed that the peak curing temperature dropped to 213°C (>15°C lower than the pure system), which was attributed to the catalytic effect of free phenolic hydroxyl / carboxyl groups in lignin; the glass transition temperature (Tg) increased to 88.36°C (DMA test), which was higher than that of pure PA-D-mea resin, and the thermal decomposition temperature (Td5% = 261.2°C) was significantly better than that of the flexible matrix, meeting the requirements of high-temperature applications.

[0027] 3. Balance between low dielectric properties and moisture resistance:

[0028] After two weeks of immersion, the water absorption rate remained at 2.21%. The main reason for this phenomenon was that the copolymer formed a dense structure, which effectively blocked the entry of water. It also proved that the introduction of lignin did not damage the water resistance.

[0029] The dielectric constant is reduced to 3.69 (low frequency) and the dielectric loss is as low as 0.035. This is because the rigid structure of lignin limits dipole migration, giving the material excellent electronic packaging potential.

[0030] 4. Compatibility between high-value lignin and green technology:

[0031] Only 5wt% lignin is needed to achieve comprehensive performance improvement, opening up a high-value path for lignin resources; the solvent copolymerization process can be expanded on a large scale, which is highly consistent with the concept of green manufacturing. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is the HSQC spectrum of the corncob lignin described in step 1① of Example 1;

[0033] Figure 2 This is the HSQC spectrum of the masson pine lignin described in step 1① of Example 2;

[0034] Figure 3 HSQC spectra of the lignin-based benzoxazine monomer prepared in step 1② of Examples 1 to 2, (a) Example 1, (b) Example 2;

[0035] Figure 4 FTIR spectra of the lignin-based benzoxazine monomer prepared in step 1② of Examples 1 to 2, (a) Example 1, (b) Example 2;

[0036] Figure 5 Stress-strain curves of the bio-based polybenzoxazine copolymer resins prepared in Examples 1 and 2 and Comparative Experiments 1 and 2, (a) tensile stress-strain curve, (b) toughness;

[0037] Figure 6 The stress-strain curves of the bio-based polybenzoxazine copolymer resin prepared in Example 1 are as follows: (a) tensile stress-strain curve, (b) toughness value, (c) three-point bending stress-strain curve, and (d) flexural modulus.

[0038] Figure 7 Processing stability and remolding test of the bio-based polybenzoxazine copolymer resin CCL-Bz-5% prepared in Example 1, (a) Processing stability, (b) Actual image after remolding and holding for 360 hours;

[0039] Figure 8 Microscopic morphology characterization of the bio-based polybenzoxazine copolymer resin prepared in Example 1, (a) CCL-Bz-5%, (b) CCL-Bz-15%;

[0040] Figure 9 Thermal stability test of the bio-based polybenzoxazine copolymer resin CCL-Bz-5% prepared in Example 1, (a) TG graph, (b) DTG graph;

[0041] Figure 10 Differential scanning calorimetry and thermomechanical properties of the bio-based polybenzoxazine copolymer resin CCL-Bz-5% prepared in Example 1, (a) DSC curve, (b) storage modulus, (c) loss factor;

[0042] Figure 11 Water absorption test of the bio-based polybenzoxazine copolymer resin CCL-Bz-5% prepared in Example One;

[0043] Figure 12 Dielectric property test of the bio-based polybenzoxazine copolymer resin CCL-Bz-5% prepared in Example One, (a) dielectric constant, (b) dielectric loss. DETAILED DESCRIPTION

[0044] Detailed implementation one: a method for modifying the performance of lignin copolymer enhanced bio-based benzoxazine, which is carried out according to the following steps:

[0045] I. Preparation of lignin-based benzoxazine monomer:

[0046] ①Dissolve lignin in an ethanol / water mixed solvent to obtain a lignin solution;

[0047] ②Under the conditions of heating and stirring, add paraformaldehyde and furfurylamine to the lignin solution for reaction, cool to room temperature after reaction, then evaporate the solvent by distillation, and finally dry to obtain lignin-based benzoxazine monomer;

[0048] II. Preparation of phloretic acid-based bisphenol type benzoxazine monomer:

[0049] ①Under the conditions of nitrogen atmosphere, heating and stirring, mix phloretic acid, 1,12-dodecanediol and p-toluenesulfonic acid for reaction, cool to room temperature after reaction, then sequentially carry out extraction of the product, washing and rotary evaporation to remove the solvent, to obtain phloretic acid-based bisphenol;

[0050] ②Under the conditions of heating and stirring, mix phloretic acid-based bisphenol, paraformaldehyde and ethanolamine for reaction, cool to room temperature after reaction, to obtain phloretic acid-based bisphenol type benzoxazine monomer;

[0051] III. Preparation of bio-based benzoxazine copolymer resin:

[0052] ①Disperse the lignin-based benzoxazine monomer in an ethanol / water mixed solvent, then heat to obtain a lignin-based benzoxazine monomer solution;

[0053] ②Dissolve the phloretic acid-based bisphenol type benzoxazine monomer in dichloromethane to obtain a phloretic acid-based bisphenol type benzoxazine monomer solution;

[0054] ③Mechanically stir the lignin-based benzoxazine monomer solution and the phloretic acid-based bisphenol type benzoxazine monomer solution uniformly, then ultrasonically treat to obtain a viscous blended liquid;

[0055] ④ The viscous blend is poured and bubbles are removed, and then asymmetric temperature gradient curing is performed under normal pressure. After cooling to room temperature, demolding is performed to obtain a bio-based polybenzoxazine copolymer resin.

[0056] In this embodiment, a lignin raw material rich in reactive phenolic hydroxyl structural units (G-type and H-type) is selected, preferably corn cob lignin (CC-Lignin) and / or masson pine lignin (MP-Lignin).

[0057] In this specific embodiment, in step 1②, paraformaldehyde and furfurylamine are slowly added. The amounts of paraformaldehyde and furfurylamine added are strictly measured based on the total molar number of phenolic hydroxyl groups in the lignin to ensure sufficient reaction of the active sites adjacent to the phenolic hydroxyl groups and maximize the formation of oxazine rings. In step 1②, a reflux condensation reaction is performed to fully induce a Mannich condensation reaction between the phenolic hydroxyl groups of the lignin, the furfurylamine, and the paraformaldehyde to form a lignin-based benzoxazine monomer.

[0058] In step 2 of this specific embodiment, ① solvent-free melt polycondensation is used to synthesize phloroglucinol (PA-D); in step 2 of this specific embodiment, ② the phenolic hydroxyl group of PA-D undergoes in situ Mannich condensation with ethanolamine and paraformaldehyde to form a benzoxazine ring, that is, solvent-free polycondensation is used to in situ construct a phloroglucinol-type benzoxazine monomer (PA-D-mea).

[0059] In step 3 (4) of this specific embodiment, the bubble removal effectively eliminates bubble defects and improves the density of the cured resin; the asymmetric temperature gradient curing ensures that the oxazine ring is fully ring-opened and polymerized to form a cross-linked network.

[0060] The beneficial effects of this embodiment are:

[0061] This embodiment successfully prepared a bio-based resin with high comprehensive performance by copolymerizing a specific ratio of lignin-based benzoxazine monomers and flexible phlorothiolane-based bisphenol-type benzoxazine monomers. Its significant advantages are reflected in the following aspects:

[0062] 1. Significantly enhanced mechanical properties and improved structural stability:

[0063] At a copolymerization ratio of 5wt% CCL-Bz, the resin exhibits a rigid-flexible synergistic effect: the tensile strength reaches 35.43MPa and the flexural strength reaches 56.18MPa.

[0064] Through lignin modification and copolymerization enhancement process, the agglomeration of lignin in benzoxazine is effectively inhibited, macro / micro uniformity is achieved, and the reprocessing deformation rate is extremely low (the deformation is only 1.9° after 360 hours at room temperature).

[0065] 2. Breakthrough in efficient curing reaction and thermal performance:

[0066] FTIR confirmed the characteristic peak of benzoxazine ring (935cm-1 ) indicated that the cyclization modification reaction of lignin was successful; DSC showed that the peak curing temperature dropped to 213°C (>15°C lower than the pure system), which was attributed to the catalytic effect of free phenolic hydroxyl / carboxyl groups in lignin; the glass transition temperature (Tg) increased to 88.36°C (DMA test), which was higher than that of pure PA-D-mea resin, and the thermal decomposition temperature (Td5% = 261.2°C) was significantly better than that of the flexible matrix, meeting the requirements of high-temperature applications.

[0067] 3. Balance between low dielectric properties and moisture resistance:

[0068] After two weeks of immersion, the water absorption rate remained at 2.21%. The main reason for this phenomenon was that the copolymer formed a dense structure, which effectively blocked the entry of water. It also proved that the introduction of lignin did not damage the water resistance.

[0069] The dielectric constant is reduced to 3.69 (low frequency) and the dielectric loss is as low as 0.035. This is because the rigid structure of lignin limits dipole migration, giving the material excellent electronic packaging potential.

[0070] 4. Compatibility between high-value lignin and green technology:

[0071] Only 5wt% lignin is needed to achieve comprehensive performance improvement, opening up a high-value path for lignin resources; the solvent copolymerization process can be expanded on a large scale, which is highly consistent with the concept of green manufacturing.

[0072] Specific embodiment 2: This embodiment differs from specific embodiment 1 in that: the lignin described in step 1 (1) is one or a combination of corncob lignin and Masson pine lignin; the volume ratio of ethanol to water in the ethanol / water mixed solvent described in step 1 (1) is (2-4):1; and the concentration of lignin in the lignin solution described in step 1 (1) is 0.005 g / mL to 0.04 g / mL. Other aspects are the same as specific embodiment 1.

[0073] Specific embodiment 3: This embodiment differs from either specific embodiment 1 or 2 in that the molar ratio of the phenolic hydroxyl groups in the lignin described in step 1 (1) to the aldehyde groups in the paraformaldehyde described in step 1 (2) is 1:(2-4); and the molar ratio of the phenolic hydroxyl groups in the lignin described in step 1 (1) to the amine groups in the furfurylamine described in step 1 (2) is 1:(1-1.5). Other aspects are the same as specific embodiments 1 or 2.

[0074] Specific embodiment 4: This embodiment differs from Specific embodiments 1 to 3 in that, in step 1 (2), paraformaldehyde and furfurylamine are added to the lignin solution at a temperature of 60°C to 80°C and a stirring rate of 400 rpm to 800 rpm for a reaction of 8 to 16 hours. Other steps are the same as Specific embodiments 1 to 3.

[0075] Specific embodiment 5: This embodiment differs from Specific embodiments 1 to 4 in that: the molar ratio of phloric acid to 1,12-dodecanediol in step 2 (1) is 1:(0.6-0.8); the mass ratio of the total mass of phloric acid and 1,12-dodecanediol in step 2 (1) to p-toluenesulfonic acid is 1:(0.005-0.02); the molar ratio of phloric acid-based bisphenol to paraformaldehyde in step 2 (2) is 1:(4-6); and the molar ratio of phloric acid-based bisphenol to ethanolamine in step 2 (2) is 1:(2-2.2). Other aspects are the same as Specific embodiments 1 to 4.

[0076] Specific embodiment 6: This embodiment differs from Specific embodiments 1 to 5 in that: Step 2 (1) phloric acid, 1,12-dodecanediol, and p-toluenesulfonic acid are mixed and reacted for 18 to 30 hours under a nitrogen atmosphere at a temperature of 120°C to 140°C and a stirring rate of 200 rpm to 500 rpm. The other steps are the same as Specific embodiments 1 to 5.

[0077] Specific embodiment 7: This embodiment differs from Specific embodiments 1 to 6 in that in step 2②, phloroglucinol, paraformaldehyde, and ethanolamine are mixed and reacted at a temperature of 80°C to 100°C and a stirring rate of 200 rpm to 500 rpm for 2 to 4 hours. Other steps are the same as Specific embodiments 1 to 6.

[0078] Specific embodiment 8: This embodiment differs from any one of specific embodiments 1 to 7 in that the volume ratio of ethanol to water in the ethanol / water mixed solvent described in step 3 (1) is (2-4):1; the concentration of the lignin-based benzoxazine monomer solution described in step 3 (1) is 0.01 g / mL to 0.05 g / mL; the concentration of the phlorate-based bisphenol-type benzoxazine monomer solution described in step 3 (2) is 0.15 g / mL to 0.19 g / mL; and the mass percentage of the lignin-based benzoxazine monomer in the viscous mixed solution described in step 3 (3) is 5% to 25% of the total solute. Other aspects are the same as specific embodiments 1 to 7.

[0079] Specific embodiment 9: This embodiment differs from any one of specific embodiments 1 to 8 in that: in step 3 (1), the lignin-based benzoxazine monomer is added to an ethanol / water mixed solvent for dispersion, and then the temperature is raised to 40°C to 80°C; in step 3 (3), the lignin-based benzoxazine monomer solution and the phlorate-based bisphenol-type benzoxazine monomer solution are mechanically stirred and mixed at a speed of 200 rpm to 500 rpm, and then ultrasonically treated at a power of 20 W to 50 W for 1 hour to 3 hours. Other steps are the same as specific embodiments 1 to 8.

[0080] Specific embodiment 10: This embodiment differs from specific embodiments 1 to 9 in that: the bubble removal described in step 3 (4) is specifically carried out at a temperature of 110°C to 140°C, with a vacuum degassing treatment for 10 to 60 minutes; the asymmetric temperature gradient curing described in step 3 (4) is specifically carried out at a temperature of 125°C to 135°C, with a holding time of 1 to 3 hours, then at a temperature of 145°C to 155°C, with a holding time of 2 to 4 hours, then at a temperature of 160°C to 170°C, with a holding time of 1 to 3 hours, then at a temperature of 175°C to 180°C, with a holding time of 1 to 3 hours, and finally at a temperature of 185°C to 195°C, with a holding time of 2 to 4 hours. Other aspects are the same as specific embodiments 1 to 9.

[0081] The following examples are used to verify the beneficial effects of the present invention:

[0082] Example 1:

[0083] A method for copolymerizing modified lignin to enhance the performance of bio-based benzoxazine is carried out according to the following steps:

[0084] 1. Preparation of lignin-based benzoxazine monomers:

[0085] ① Dissolve lignin in an ethanol / water mixed solvent to obtain a lignin solution;

[0086] The lignin is corn cob lignin (CC-Lignin); the volume ratio of ethanol to water in the ethanol / water mixed solvent is 7:3; the concentration of lignin in the lignin solution is 0.01 g / mL;

[0087] ② Paraformaldehyde and furfurylamine were added to the lignin solution at a temperature of 80°C and a stirring rate of 500 rpm for 12 hours. After the reaction, the solution was cooled to room temperature and then evaporated to remove the solvent. Finally, the solution was dried at a temperature of 60°C for 24 hours to obtain a black powdery solid product, namely, lignin-based benzoxazine monomer (CCL-Bz).

[0088] The molar ratio of the phenolic hydroxyl group in the lignin described in step 1① to the aldehyde group in the paraformaldehyde described in step 1② is 1:2.25; the molar ratio of the phenolic hydroxyl group in the lignin described in step 1① to the amine group in the furfurylamine described in step 1② is 1:1.25;

[0089] 2. Preparation of phlorate-based bisphenol-type benzoxazine monomer:

[0090] ① Under a nitrogen atmosphere, a temperature of 130°C, and a stirring rate of 300 rpm, phloric acid, 1,12-dodecanediol, and p-toluenesulfonic acid were mixed and reacted for 24 hours. After the reaction, the mixture was cooled to room temperature and then extracted with dichloromethane. The product was then washed with deionized water to separate the phases, and finally the solvent was removed by rotary evaporation to obtain a dark yellow transparent liquid product, phloric acid-based bisphenol (PA-D), with a yield of 92.5%;

[0091] The molar ratio of the phloric acid to 1,12-dodecanediol is 1:0.6; the mass ratio of the total mass of the phloric acid and 1,12-dodecanediol described in step 2① to p-toluenesulfonic acid is 1:0.01;

[0092] ② Phloride-based bisphenol, paraformaldehyde, and ethanolamine were reacted at 90°C and a stirring rate of 200 rpm for 2.5 hours. After the reaction, the mixture was cooled to room temperature to obtain a light yellow viscous liquid product, namely, phloride-based bisphenol-based benzoxazine monomer (PA-D-mea), with a yield of 92.5%;

[0093] The molar ratio of the phloride bisphenol to paraformaldehyde is 1:4; the molar ratio of the phloride bisphenol to ethanolamine is 1:2;

[0094] 3. Preparation of bio-based benzoxazine copolymer resin:

[0095] ① Add the lignin-based benzoxazine monomer to an ethanol / water mixed solvent for dispersion, and then heat it to 60° C. to obtain a lignin-based benzoxazine monomer solution;

[0096] The volume ratio of ethanol to water in the ethanol / water mixed solvent is 7:3; the concentration of the lignin-based benzoxazine monomer solution is 0.01 g / mL;

[0097] ② dissolving the phloride-based bisphenol-type benzoxazine monomer in dichloromethane to obtain a phloride-based bisphenol-type benzoxazine monomer solution;

[0098] The concentration of the phloride-based bisphenol-type benzoxazine monomer solution is 0.19 g / mL;

[0099] ③ The lignin-based benzoxazine monomer solution and the phlorothiolane-based bisphenol-type benzoxazine monomer solution were mechanically stirred and mixed at a rotation speed of 300 rpm, and then ultrasonically treated at a power of 30 W for 1 h to obtain a viscous blend;

[0100] The mass percentages of the lignin-based benzoxazine monomer in the viscous blend liquid to the total solute are 5%, 10%, 15%, 20% and 25%, respectively; that is, the mass percentages of the phloride-based bisphenol-type benzoxazine monomer in the viscous blend liquid to the total solute are 95%, 90%, 85%, 80% and 75%, respectively;

[0101] ④ The viscous blend was injected into a polytetrafluoroethylene mold and vacuum degassed at 120°C for 30 minutes. Then, under normal pressure, the mixture was kept at 130°C for 2 hours, then at 150°C for 3 hours, then at 170°C for 2 hours, then at 180°C for 2 hours, and finally at 190°C for 2 hours. After cooling to room temperature, the mixture was demoulded to obtain a bio-based polybenzoxazine copolymer resin.

[0102] Given that the mass percentage of the lignin-based benzoxazine monomer in the total solute in the viscous blend described in step 3③ is 5%, 10%, 15%, 20% and 25%, the bio-based polybenzoxazine copolymer resins prepared in step 3④ are named CCL-Bz-5%, CCL-Bz-10%, CCL-Bz-15%, CCL-Bz-20% and CCL-Bz-25%, respectively.

[0103] Example 2: This example differs from Example 1 in that the lignin described in step 1 (1) is masson pine lignin (MP-Lignin); the lignin-based benzoxazine monomer prepared in step 1 (2) is named MPL-Bz; and the bio-based polybenzoxazine copolymer resins prepared in step 3 (4) are named MPL-Bz-5%, MPL-Bz-10%, MPL-Bz-15%, MPL-Bz-20%, and MPL-Bz-25%, respectively. All other aspects are the same as in Example 1.

[0104] Comparative Experiment 1: This comparative experiment differs from Example 1 in that the lignin-based benzoxazine monomer in step 3 (1) is replaced with corncob lignin (CC-Lignin). The bio-based polybenzoxazine copolymer resins prepared in step 3 (4) are named CC-Lig-5%, CC-Lig-10%, CC-Lig-15%, CC-Lig-20%, and CC-Lig-25%, respectively. All other conditions are the same as in Example 1.

[0105] Comparative Experiment 2: This comparative experiment differs from Example 1 in that the lignin-based benzoxazine monomer in step 3 (1) is replaced with Masson pine lignin (MP-Lignin); and the bio-based polybenzoxazine copolymer resins prepared in step 3 (4) are named MP-Lig-5%, MP-Lig-10%, MP-Lig-15%, MP-Lig-20%, and MP-Lig-25%. All other aspects are the same as in Example 1.

[0106] Figure 1 This is the HSQC spectrum of the corncob lignin described in step 1① of Example 1. As can be seen from the figure, the signals in the side chain region of the corncob lignin are relatively well preserved (δC / δH=63-91 / 2.85-5.53). In the aromatic region (δC / δH=105-134 / 6.33-7.71), the corncob lignin has obvious structural unit signals such as syringyl (S type), guaiacyl (G type) and paraben (PB).

[0107] Figure 2 This is the HSQC spectrum of the masson pine lignin described in step 1① of Example 2. As can be seen from the figure, the guaiacyl (G-type) unit signal is mainly observed in the masson pine lignin, which is very beneficial for the subsequent synthesis of benzoxazine.

[0108] Figure 3 HSQC spectra of the lignin-based benzoxazine monomer prepared in Step 1② of Examples 1 to 2, (a) Example 1, (b) Example 2; As can be seen from the figure, its structure was characterized by 2D HSQC nuclear magnetic resonance. The spectra of the two lignin benzoxazines were compared. During the reaction, the introduction of formaldehyde will cause lignin to undergo polycondensation, so only the characteristic peaks of benzoxazine were observed. The characteristic peaks of benzoxazine from corncob lignin are 3.8 / 48.9ppm and 4.8 / 82.1ppm, and the characteristic peaks of benzoxazine from Masson pine lignin are 3.9 / 49.2ppm and 5.2 / 95.7ppm. These peaks refer to the methylene groups in the benzoxazine ring (Ar-CH2-N and O-CH2-N, respectively). According to these nuclear magnetic resonance analyses, the benzoxazine structure was clearly formed in both samples.

[0109] Figure 4 FTIR spectra of the lignin-based benzoxazine monomer prepared in step 1② of Examples 1 to 2, (a) Example 1, (b) Example 2; As can be seen from the figure, the various functional groups in lignin and lignin-based benzoxazine were detected by infrared spectroscopy, 3400 cm -1 ~3200cm -1 The broad peak at 1291 cm is the stretching vibration of the OH bond of the phenolic hydroxyl group and alcohol hydroxyl group, which are the most abundant. -1 and 1125cm -1 The characteristic vibrations of the phenolic OH and alcoholic OH bonds are 2925cm -1 、2874cm -1 The absorption peaks appearing at 3380 cm-1 are the stretching vibration peaks of the methoxy group on the benzene ring and the methyl and methylene CH bonds on the side chains. -1A broad and strong absorption peak is shown at 935cm, which is attributed to the stretching vibration characteristics of phenolic hydroxyl groups. After the synthesis of lignin benzoxazine by Mannich reaction, the peak intensity is significantly reduced. This phenomenon indicates that most of the phenolic hydroxyl groups in the lignin molecule participate in the construction of the benzoxazine ring. It is worth noting that the new spectrum at 935cm -1 A distinct characteristic peak appears at , corresponding to the out-of-plane bending vibration of the HCH bond in the benzoxazine ring, and its intensity is positively correlated with the degree of cyclization. The synchronization of the appearance of this characteristic peak and the consumption of phenolic hydroxyl groups provides evidence for the successful synthesis of lignin benzoxazine.

[0110] Figure 5 Figures 1 and 2 show stress-strain curves of the bio-based polybenzoxazine copolymers prepared in Examples 1 and 2 and Comparative Experiments 1 and 2, (a) tensile stress-strain curve, and (b) toughness. As shown in the figure, when the lignin benzoxazine addition level is 15%, the corncob lignin benzoxazine (CCL-Bz) copolymer exhibits the best overall mechanical properties, with a tensile strength of 26.01 MPa and a toughness of 1.62 MJ / m 3 , which is higher than the mechanical properties of Masson pine lignin benzoxazine (MPL-Bz) copolymer (tensile strength 24.93MPa, toughness value 1.22MJ / m 3 ), in sharp contrast, the mechanical properties of the two non-oxazinized lignin copolymers with flexible chains are much lower than those of the oxazinized ones. The tensile strength of the corncob lignin (CC-Lig) copolymer is 22.7MPa and the toughness value is 1.15MJ / m 3 , Masson pine lignin (MP-Lig) copolymer, tensile strength of 20.4MPa, toughness value of 1.05MJ / m 3 .

[0111] Figure 6 The stress-strain curves of the bio-based polybenzoxazine copolymer resin prepared in Example 1 are as follows: (a) tensile stress-strain curve, (b) toughness value, (c) three-point bending stress-strain curve, and (d) flexural modulus. As can be seen from the figure, the mechanical properties of the material reach their optimal state when the addition of corncob lignin benzoxazine is 5%. At this point, the material's tensile strength is 35.43 MPa and its toughness is 5.08 MJ / m 3, and the flexural strength was 56.18MPa, which were better than samples with other proportions. The study found that with the continuous increase in the amount of corncob lignin benzoxazine added, the overall mechanical properties of the material showed a downward trend. The main reason for this phenomenon may be the microphase separation at the interface between the two phases. When the content of lignin benzoxazine increases, aggregation is likely to occur, resulting in defects inside the material, thereby weakening its mechanical properties. Specifically, an appropriate amount of corncob lignin benzoxazine can effectively enhance the mechanical properties of the material, but excessive addition will lead to unevenness in the internal structure of the material, triggering microphase separation and aggregation. These negative effects will destroy the uniformity and continuity of the material, thereby reducing its tensile strength, toughness and flexural strength. Finally, 5% corncob lignin benzoxazine was selected as the preferred raw material, and it was used as the optimal copolymerization ratio for adjustable mechanical properties for subsequent characterization of related properties.

[0112] In order to evaluate the processing stability of the copolymer, a 50 × 5 × 1 mm 3 The bio-based polybenzoxazine copolymer resin CCL-Bz-5% long strip material prepared in Example 1 was heat-treated at 100°C to form a right-angled long strip, and then stored at room temperature. The angle change was recorded every 60 hours. Plasticity test: The bio-based polybenzoxazine copolymer resin CCL-Bz-5% long strip material prepared in Example 1 was heat-treated at 100°C to form various three-dimensional curved structures and maintained for 360 hours. Figure 7 Figure 1 shows the processing stability and remolding performance of the bio-based polybenzoxazine copolymer resin CCL-Bz-5% prepared in Example 1. (a) Processing stability, (b) Remolding and 360-hour stability. Figure (a) shows that due to the presence of dynamic ester bonds in the flexible benzoxazine segments, after heat treatment to maintain a specific 90° angle, the copolymer exhibits only a 1.9° change in stability over 360 hours at room temperature, demonstrating enhanced room-temperature stability after the introduction of lignin-based benzoxazine. As shown in Figure (b), the inclusion of lignin gives the overall material a black color. The long sample of the copolymer resin was cut into strips and molded into various three-dimensional curved structures using thermoplastics. These strips exhibit excellent stability, demonstrating the processability of the bio-based polybenzoxazine resin.

[0113] Figure 8Figure 1 shows the micromorphology of the bio-based polybenzoxazine copolymer resin prepared in Example 1, (a) CCL-Bz-5%, (b) CCL-Bz-15%. As shown in the figure, when the addition level of lignin benzoxazine increases from 5% to 15%, the SEM image shows a significant change in the resin fracture morphology. This is manifested by an increase in the number of fracture lines, indicating that the introduction of lignin benzoxazine leads to a change in the brittleness of the material. By comparison, when the addition level of lignin benzoxazine is 15%, the copolymer exhibits obvious microscopic aggregation. This suggests that the compatibility between the excess lignin benzoxazine and the flexible segment benzoxazine (PA-D-mea) may be unsatisfactory, leading to a decrease in mechanical properties.

[0114] Figure 9 Thermal stability test of the bio-based polybenzoxazine copolymer resin CCL-Bz-5% prepared in Example 1, (a) TG graph, (b) DTG graph; As can be seen from the figures, the thermal decomposition of the bio-based polybenzoxazine copolymer resin presents three main stages. In the first stage, the temperature ranges from room temperature to 230°C, and the polymer begins to lose weight. The main reason may be the breakage of ester bonds in the copolymer chain. The second stage of thermal degradation occurs in the range of 230°C to 400°C, accompanied by significant mass loss. The main reasons for this are the structural breakage of the cross-linked Mannich bridges of the copolymer and the continuous degradation of flexible long alkane chains, as well as the decomposition of low-molecular fragments in the lignin structural units. The third stage is in the range of 400°C to 500°C. In this stage, the main manifestation is the comprehensive degradation of the polymer, the continuous loss of connecting bonds and its own mass. However, the residual carbon content at 800°C is still higher than that of pure flexible chain benzoxazine resin poly(PA-D-mea), with a residual carbon content of 26.5%. The reason for this is that the introduced lignin contains more aromatic rings and furan rings in the benzoxazine structure. Overall, the introduction of lignin benzoxazine improves the thermal stability of the copolymer.

[0115] The curing behavior of the bio-based polybenzoxazine copolymer resin CCL-Bz-5% prepared in Example 1 was measured using a TA Netzsch DSC differential scanning calorimeter: Under a nitrogen atmosphere, 5 mg to 10 mg of the viscous blend prepared in step 3 of Example 1 was sealed in an aluminum pot with a sealing lid. The temperature was raised from room temperature to 100°C at a rate of 10°C / min, held for 1 minute, then cooled to room temperature at a rate of 10°C / min, and then heated to 250°C at a rate of 10°C / min. Figure 10Differential scanning calorimetry and thermomechanical properties of the bio-based polybenzoxazine copolymer resin CCL-Bz-5% prepared in Example 1. (a) DSC curve, (b) storage modulus, and (c) loss factor. The curing behavior of the copolymer system exhibits some changes, with both the initial and peak curing temperatures decreasing with the addition of lignin benzoxazine. Compared to pure flexible segment benzoxazine, the initial curing temperature decreases from 170°C to 125°C, and the peak curing temperature decreases from 222°C to 213°C. This phenomenon is attributed to the fact that the unreacted acidic functional groups (phenolic hydroxyl and carboxyl groups) in lignin benzoxazine catalyze the ring-opening polymerization of benzoxazine, lowering the initial curing temperature. The phenolic hydroxyl groups generated by the ring-opening of the flexible segment benzoxazine further catalyze the polymerization reaction, accelerating the reaction rate and reducing the peak curing temperature. Dynamic mechanical properties of a pure flexible-chain benzoxazine resin (poly(PA-D-mea)) and a bio-based polybenzoxazine copolymer (CCL-Bz-5%) demonstrate that, at room temperature, the bio-based polybenzoxazine copolymer (CCL-Bz-5%) exhibits significantly superior storage modulus compared to the pure flexible system. This improvement in mechanical properties is primarily attributed to the introduction of rigid benzene rings into the lignin molecular chain, which enhances the overall rigidity of the material through a physical stiffening effect. Notably, the cured system exhibits a unique dual glass transition behavior. This is attributed to the microphase separation phenomenon induced by the addition of lignin-benzoxazine. When the rigid lignin phase and the flexible benzoxazine matrix form a heterogeneous system, the physical entanglement and potential chemical crosslinking at the interface between the two phases effectively restrict the molecular mobility of the flexible segments. Consequently, the previously single glass transition temperature shifts to 88.36°C, while the inherent heat resistance of lignin contributes to the characteristic rigid phase peak at 228.63°C. Overall, the introduction of lignin benzoxazine significantly increased the glass transition temperature of the copolymer, indicating that the thermal stability and mechanical properties of the material were significantly improved.

[0116] Table 1 Thermal degradation rate parameters of CCL-Bz-5% and pure flexible chain benzoxazine resin prepared in Example 1

[0117]

[0118] The poly(PA-D-mea) described in the table and above is the phlorate-based bisphenol-type benzoxazine monomer prepared in step 2 ② of Example 1, and is cured by asymmetric temperature gradient according to step 3 ④ of Example 1 to obtain a pure flexible chain benzoxazine resin.

[0119] Figure 11This figure shows the water absorption of the bio-based polybenzoxazine copolymer resin CCL-Bz-5% prepared in Example 1. The figure shows that the bio-based polybenzoxazine copolymer resin CCL-Bz-5% was immersed in water for two weeks to test its water absorption. After three days of immersion in water, the water absorption of the bio-based polybenzoxazine copolymer resin was 2.02%. After one week of immersion, the water absorption was 2.18%, and after two weeks, the water absorption was 2.21%. The change during this period was relatively stable. The main reason for this phenomenon is that the copolymer forms a dense structure, which effectively blocks the ingress of water.

[0120] Figure 12 The dielectric properties of the bio-based polybenzoxazine copolymer resin CCL-Bz-5% prepared in Example 1 are shown in Figures 1 (a) and 2 (b) respectively. As shown in the figure, the bio-based polybenzoxazine copolymer exhibits excellent frequency stability across the entire test range, with no observed resonance. The dielectric constant of the bio-based polybenzoxazine copolymer resin at the test frequency is 3.69, and the dielectric loss is 0.035. Compared to the purely flexible segments, its dielectric constant is lower, indicating that the incorporation of lignin benzoxazine has a significant effect. This is due to the large number of benzene and furan rings in lignin benzoxazine. The introduction of these rigid structures not only reduces dipole mobility but also disrupts the disordered molecular arrangement in the amorphous region, reducing the ordered arrangement of dipoles. Furthermore, the steric hindrance of lignin limits the mobility of the molecular chains, further hindering the oriented polarization of the dipoles, thereby reducing the dielectric constant. Based on the above analysis, the incorporation of lignin benzoxazine monomers not only imparts specific functionality to the material but also optimizes its dielectric properties. Taking all the performance into consideration, bio-based polybenzoxazine copolymer resins show great application potential in the field of electronic packaging and are expected to achieve high-value application of biomass resources.

Claims

1. A method for copolymerizing modified lignin to enhance the performance of bio-based benzoxazine, characterized in that It is carried out in the following steps:

1. Preparation of lignin-based benzoxazine monomers: ① Dissolve lignin in an ethanol / water mixed solvent to obtain a lignin solution; ② Under heating and stirring conditions, paraformaldehyde and furfurylamine are added to the lignin solution for reaction, cooled to room temperature after the reaction, and then evaporated to remove the solvent, and finally dried to obtain a lignin-based benzoxazine monomer; 2. Preparation of phlorate-based bisphenol-type benzoxazine monomer: ① Under nitrogen atmosphere, heating and stirring conditions, phloric acid, 1,12-dodecanediol and p-toluenesulfonic acid are mixed and reacted, and after the reaction, the mixture is cooled to room temperature, and then the product is extracted, washed and the solvent is removed by rotary evaporation to obtain phloric acid-based bisphenol; ② Under heating and stirring conditions, phloride-based bisphenol, paraformaldehyde and ethanolamine are mixed and reacted, and then cooled to room temperature to obtain a phloride-based bisphenol-type benzoxazine monomer; 3. Preparation of bio-based benzoxazine copolymer resin: ① Adding lignin-based benzoxazine monomer to an ethanol / water mixed solvent for dispersion, and then heating to obtain a lignin-based benzoxazine monomer solution; ② dissolving the phloride-based bisphenol-type benzoxazine monomer in dichloromethane to obtain a phloride-based bisphenol-type benzoxazine monomer solution; ③ The lignin-based benzoxazine monomer solution and the phlorate-based bisphenol-type benzoxazine monomer solution are mechanically stirred and uniformly mixed, and then ultrasonically treated to obtain a viscous blend; ④ The viscous blend is poured and bubbles are removed, and then asymmetric temperature gradient curing is performed under normal pressure. After cooling to room temperature, demolding is performed to obtain a bio-based polybenzoxazine copolymer resin.

2. The method for enhancing the performance of bio-based benzoxazine by copolymerization of modified lignin according to claim 1, characterized in that The lignin described in step 1① is one or a combination of corn cob lignin and masson pine lignin; the volume ratio of ethanol to water in the ethanol / water mixed solvent described in step 1① is (2-4):1; the concentration of lignin in the lignin solution described in step 1① is 0.005g / mL to 0.04g / mL.

3. The method for copolymerizing modified lignin to enhance the performance of bio-based benzoxazine according to claim 1, characterized in that The molar ratio of the phenolic hydroxyl group in the lignin described in step 1① to the aldehyde group in the paraformaldehyde described in step 1② is 1:(2-4); the molar ratio of the phenolic hydroxyl group in the lignin described in step 1① to the amine group in the furfurylamine described in step 1② is 1:(1-1.5).

4. The method for copolymerizing modified lignin to enhance the performance of bio-based benzoxazine according to claim 1, characterized in that In step 1②, paraformaldehyde and furfurylamine are added to the lignin solution at a temperature of 60° C. to 80° C. and a stirring rate of 400 rpm to 800 rpm, and the reaction is carried out for 8 h to 16 h.

5. The method for copolymerizing modified lignin to enhance the performance of bio-based benzoxazine according to claim 1, characterized in that The molar ratio of phloride and 1,12-dodecanediol described in step 2① is 1:(0.6~0.8); the mass ratio of the total mass of phloride and 1,12-dodecanediol described in step 2① to p-toluenesulfonic acid is 1:(0.005~0.02); the molar ratio of phloride-based bisphenol described in step 2② to paraformaldehyde is 1:(4~6); the molar ratio of phloride-based bisphenol described in step 2② to ethanolamine is 1:(2~2.2).

6. The method for copolymerizing modified lignin to enhance the performance of bio-based benzoxazine according to claim 1, characterized in that Step 2: ① Under nitrogen atmosphere, temperature of 120° C. to 140° C. and stirring speed of 200 rpm to 500 rpm, phloric acid, 1,12-dodecanediol and p-toluenesulfonic acid are mixed and reacted for 18 h to 30 h.

7. The method of claim 1, wherein the modified lignin copolymerization method enhances the performance of bio-based benzoxazine. In step 2②, phlorate-based bisphenol, paraformaldehyde and ethanolamine are mixed and reacted at a temperature of 80° C. to 100° C. and a stirring rate of 200 rpm to 500 rpm for 2 h to 4 h.

8. The method for copolymerizing modified lignin to enhance the performance of bio-based benzoxazine according to claim 1, characterized in that The volume ratio of ethanol to water in the ethanol / water mixed solvent described in step 3① is (2-4):1; the concentration of the lignin-based benzoxazine monomer solution described in step 3① is 0.01 g / mL to 0.05 g / mL; the concentration of the phlorate-based bisphenol-type benzoxazine monomer solution described in step 3② is 0.15 g / mL to 0.19 g / mL; the mass percentage of the lignin-based benzoxazine monomer in the viscous blend described in step 3③ to the total solute is 5% to 25%.

9. The method for copolymerizing modified lignin to enhance the performance of bio-based benzoxazine according to claim 1, characterized in that In step 3①, the lignin-based benzoxazine monomer is added to an ethanol / water mixed solvent for dispersion, and then the temperature is raised to 40°C to 80°C; in step 3③, the lignin-based benzoxazine monomer solution and the phlorotactic acid-based bisphenol-type benzoxazine monomer solution are mechanically stirred and mixed at a speed of 200rpm to 500rpm, and then ultrasonically treated for 1h to 3h at a power of 20W to 50W.

10. The method for copolymerizing modified lignin to enhance the performance of bio-based benzoxazine according to claim 1, characterized in that The bubble removal described in step 3 ④ is specifically a vacuum degassing treatment for 10min to 60min at a temperature of 110℃ to 140℃; the asymmetric temperature gradient curing described in step 3 ④ is specifically a heat preservation at a temperature of 125℃ to 135℃ for 1h to 3h, then at a temperature of 145℃ to 155℃ for 2h to 4h, then at a temperature of 160℃ to 170℃ for 1h to 3h, then at a temperature of 175℃ to 180℃ for 1h to 3h, and finally at a temperature of 185℃ to 195℃ for 2h to 4h.

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