Melamine resin composite material with high heat resistance and preparation method thereof

Abstract

The invention relates to the technical field of melamine resin high polymer materials, in particular to a high-heat-resistance melamine resin composite material which comprises a first component and a second component. The first component comprises a modified melamine resin prepolymer, a crosslinking accelerator, a curing agent and a first dispersing agent; the modified melamine resin prepolymer is prepared by modifying a melamine resin prepolymer through lignosulfonate and modified nano cellulose whiskers; the modified nano cellulose whiskers are prepared by modifying nano cellulose whiskers through a silane coupling agent; the second component comprises a composite reinforcing material; the composite reinforcing material comprises a carboxylated carbon nanotube, a montmorillonite-graphene hybrid, a second dispersing agent and a nucleating agent; the carboxylated carbon nano tube is prepared by reacting a carbon nano tube with strong acid; the montmorillonite-graphene hybrid is prepared from montmorillonite and graphene oxide through a reaction. Melamine resin is modified by adopting lignosulfonate and modified nanocellulose whiskers, meanwhile, a reinforcing material is compounded, and the heat resistance and the mechanical property of the melamine resin are improved in combination with a gradient curing process.

Description

Technical Field

[0001] This application relates to the field of melamine resin polymer materials technology, specifically to a high heat-resistant melamine resin composite material and its preparation method. Background Technology

[0002] Melamine resin (melamine-formaldehyde resin), as a thermosetting plastic, is widely used in tableware, electrical components, and building materials due to its excellent corrosion resistance, mechanical strength, and surface hardness. However, its insufficient heat resistance has long limited its application in high-temperature environments—the thermal decomposition temperature of traditional melamine resin is usually below 260℃, and above 120℃, it may release formaldehyde and melamine monomers due to molecular chain breakage, posing food safety and environmental risks. For example, when melamine tableware is heated in a microwave oven or used to hold hot oil, the local temperature may exceed 185℃, causing the material to decompose and release harmful substances. Long-term use may also exacerbate formaldehyde migration due to repeated heating.

[0003] Existing modification methods often employ single nanofillers (such as graphene oxide and titanium dioxide nanotubes) or chemical crosslinking agents, but these methods generally suffer from poor dispersibility and weak interfacial bonding. For example, Chinese patent CN119264599A discloses a method for preparing cellulose melamine resin composites, which raises the thermal decomposition temperature to 300℃ by introducing a diphenyl sulfone structure. However, the compatibility issues between the inorganic filler and the melamine resin lead to the formation of agglomerates in the matrix, significantly reducing the dispersion uniformity and ultimately resulting in limited improvement in mechanical properties. Furthermore, while single nanofillers (such as carbon fibers and sericite) can improve heat resistance through physical filling, they lack chemical bonding with the resin matrix, resulting in weak interfacial bonding, low load transfer efficiency, low flexural strength improvement, and a tendency to crack due to stress concentration at high temperatures.

[0004] In summary, existing technologies still have bottlenecks in improving the heat resistance, dispersibility, interfacial bonding, and environmental friendliness of melamine resins, and there is an urgent need for a solution for high heat-resistant melamine resins. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this application provides a high-heat-resistant melamine resin composite material and its preparation method. This application modifies melamine resin with lignin sulfonate and modified cellulose nanocrystal (CNC), while simultaneously incorporating reinforcing materials and employing a gradient curing process to improve the heat resistance and mechanical properties of the melamine resin.

[0006] To achieve the above objectives, this application adopts the following technical solution:

[0007] In a first aspect, this application provides a high heat-resistant melamine resin composite material, comprising: a first component and a second component; the first component comprises a modified melamine resin prepolymer, a crosslinking accelerator, a curing agent, and a first dispersant; the modified melamine resin prepolymer is obtained by modifying melamine resin prepolymer with lignin sulfonate and modified CNC; the modified CNC is obtained by modifying CNC with a silane coupling agent; the second component comprises a composite reinforcing material; the composite reinforcing material comprises carboxylated carbon nanotubes, montmorillonite-graphene hybrid, a second dispersant, and a nucleating agent; the carboxylated carbon nanotubes are obtained by reacting carbon nanotubes with a strong acid; the montmorillonite-graphene hybrid is obtained by reacting montmorillonite with graphene oxide.

[0008] In this application, after ultrasonic dispersion, the surface hydroxyl groups of CNC are exposed, undergoing a condensation reaction with a silane coupling agent. Under acidic conditions of pH 4-4.5, the silane coupling agent is first hydrolyzed to silanol (Si-OH). The silanol and the siloxane oligomers generated by its condensation are adsorbed onto the CNC surface through hydrogen bonding, resulting in modified CNC. The introduction of low surface energy organic groups reduces the surface energy of the modified CNC, enabling it to be uniformly dispersed in the system and construct a three-dimensional nanonetwork. This not only improves the heat resistance of the material by restricting the thermal motion of the melamine resin molecular chains but also prevents crack propagation under stress. The polar groups of lignin sulfonate are bonded to the residual siloxane groups or silane organic functional groups (such as amino groups) of the modified CNC through hydrogen bonding or electrostatic interactions. The aromatic rings of lignin and the triazine rings of melamine undergo π-π stacking, which enhances interfacial compatibility through the charge transfer effect of the conjugated system and reduces phase separation between lignin sulfonate and melamine prepolymer, thereby reducing heat resistance defects caused by phase separation. At the same time, the long-chain alkyl segments of lignin sulfonate are embedded in the melamine prepolymer through molecular chain entanglement, further alleviating resin brittleness.

[0009] Simultaneously, the carboxyl groups of the carboxylated carbon nanotubes are bonded to the amino groups of the melamine resin prepolymer through electrostatic interactions, while the sp of the carboxylated carbon nanotubes... 2 The carbon domains and melamine triazine rings undergo π-π stacking, forming an effective stress transfer network and heat conduction pathway through a tubular structure. In the montmorillonite-graphene hybrid, the interlayer hydroxyl groups of montmorillonite form hydrogen bonds with the oxygen-containing functional groups of graphene, and the layered structure is entangled with the melamine molecular chains through van der Waals forces, constructing a planar high-temperature barrier network. The second dispersant forms steric hindrance by adsorbing onto the surface of the reinforcing material, preventing agglomeration and promoting interfacial bonding with the melamine resin matrix. During the gradient heating process, the crosslinking accelerator and curing agent induce the second component and resin molecules to form a network structure with high local crosslinking density. The highly dense crosslinking network effectively restricts the thermal motion of molecular chain segments, thereby improving the heat resistance of the composite material.

[0010] Preferably, the lignin sulfonate comprises any one of sodium lignin sulfonate, calcium lignin sulfonate, and ammonium lignin sulfonate; the crosslinking accelerator comprises any one of triethanolamine, diethanolamine, and N,N-dimethylethanolamine; the curing agent comprises any one of ammonium sulfate, ammonium dihydrogen phosphate, and citric acid; the first dispersant comprises any one of glyceryl stearate, polyethylene glycol stearate, and sorbitan monostearate; the silane coupling agent comprises KH550; the strong acid comprises at least one of concentrated nitric acid and concentrated sulfuric acid; the second dispersant comprises any one of polyethylene glycol, sodium dodecylbenzene sulfonate, and polyvinylpyrrolidone; and the nucleating agent comprises any one of sodium benzoate, nano-calcium carbonate, and zinc stearate.

[0011] Preferably, the lignin sulfonate has a molecular weight of 2000-5000 Da, a degree of sulfonation of 1.7-2.0 mmol / g, and the CNC has a diameter of 20-50 nm and a length of 100-300 nm.

[0012] Preferably, the mass ratio of the first component to the second component is (6-8):1; the mass ratio of the melamine resin prepolymer, lignin sulfonate, and modified CNC is (16-19):(2-4):1; the amount of the crosslinking accelerator added is 0.5%-1.5% of the mass of the melamine resin prepolymer; the amount of the curing agent added is 2%-6% of the total mass of the first component; the amount of the first dispersant added is 1%-3% of the mass of the lignin sulfonate; The amount of silane coupling agent added is 3%-8% of the mass of CNC; the mass ratio of the carboxylated carbon nanotubes to the montmorillonite-graphene hybrid is 1:(2-3); the amount of the second dispersant added is 1.5%-3.5% of the total mass of the composite reinforcing material; the amount of the nucleating agent added is 2%-5% of the total mass of the second component; the mass ratio of the strong acid to the carbon nanotubes is (5-10):1; the mass ratio of the montmorillonite to the graphene oxide is (3-5):1.

[0013] Secondly, this application provides a high heat-resistant melamine resin composite material and its preparation method, comprising the following steps:

[0014] S1. Preparation of composite modified solution: Dissolve lignin sulfonate in deionized water, adjust pH to 8-9, add the first dispersant, and stir until completely dissolved to obtain lignin sulfonate solution; mix CNC with deionized water, ultrasonically disperse, add silane coupling agent, and stir evenly to obtain modified CNC; mix the above lignin sulfonate solution and modified CNC to obtain the first mixture;

[0015] S2. Preparation of composite reinforced material dispersion: Strong acid and carbon nanotubes are mixed at a set temperature, cooled to room temperature, washed with deionized water until neutral, and vacuum dried to obtain carboxylated carbon nanotubes; Montmorillonite and graphene oxide are dispersed in deionized water to obtain a montmorillonite and graphene oxide solution, ascorbic acid is added, and freeze-dried to obtain a montmorillonite-graphene hybrid; Carboxylated carbon nanotubes and montmorillonite-graphene hybrid are mixed, deionized water is added to prepare a suspension, a second dispersant is added, a nucleating agent is added, and high-speed shearing is performed to obtain a second mixture;

[0016] S3, Blending reaction: Add melamine prepolymer to the first mixture and stir; then add the second mixture and crosslinking accelerator and stir to obtain a blend system;

[0017] S4. Gradient curing molding: Add curing agent to the blend system, stir to make the system uniform, and then transfer it into the mold for gradient temperature curing molding.

[0018] In this application, a gradient curing molding process is used in step S4. In the low-temperature stage, hydrogen bond networks are preferentially formed due to the thermodynamic properties of hydrogen bonds and covalent bonds, which fixes the spatial distribution of modified CNC and lignin. In the high-temperature stage, rapid polycondensation of melamine prepolymer increases the degree of crosslinking, reduces unreacted groups and internal stress, and allows the modified CNC to fully contact the melamine resin matrix, hindering thermal deformation. The crystalline phase and crosslinking network improve the heat resistance and mechanical properties of the material.

[0019] Preferably, in step S1, the mass concentration of the lignin sulfonate solution is 20-30%, the pH range is 8-10, the mass ratio of CNC to deionized water is 1:(30-50), and the ultrasonic dispersion power is 400-600W and the time is 40-60min.

[0020] Preferably, the set temperature in step S2 is 50-70℃, the vacuum degree of the vacuum drying process is 0.05-0.1MPa, the temperature is 60-100℃, and the time is 8-12h; the total mass concentration of the montmorillonite and graphene oxide solution is 0.5-2mg / mL, the ascorbic acid is 5%-10% of the mass of montmorillonite, and the freezing temperature of the freeze-drying process is -40 to -20℃, and the time is 20-24h.

[0021] Preferably, in step S2, the total mass concentration of carboxylated carbon nanotubes and montmorillonite-graphene hybrid in the suspension is 2%-4%; the high-speed shearing speed is 8000-12000 rpm and the time is 20-30 min.

[0022] Preferably, the melamine resin prepolymer in step S3 is prepared by reacting melamine and formaldehyde in a mass ratio of 1:(0.7-0.9) at pH 8-10 and temperature 60-80°C.

[0023] Preferably, the gradient curing molding includes a first gradient curing molding and a second gradient curing molding; the first gradient curing molding heats the material to 120-150°C at a heating rate of 5-20°C / min and holds it at a pressure of 10-20 MPa for 15-30 minutes; the second gradient curing molding heats the material to 160-190°C at a heating rate of 3-10°C / min and holds it at a pressure of 20-30 MPa for 20-40 minutes.

[0024] Beneficial technical effects:

[0025] In this application, after ultrasonic dispersion, the surface hydroxyl groups of CNC are exposed, undergoing a condensation reaction with a silane coupling agent. Under acidic conditions of pH 4-4.5, the silane coupling agent is first hydrolyzed to silanol (Si-OH). The silanol and the siloxane oligomers generated by its condensation are adsorbed onto the CNC surface through hydrogen bonding, resulting in modified CNC. The introduction of low surface energy organic groups reduces the surface energy of the modified CNC, enabling it to be uniformly dispersed in the system and construct a three-dimensional nanonetwork. This not only improves the heat resistance of the material by restricting the thermal motion of the melamine resin molecular chains but also prevents crack propagation under stress. The polar groups of lignin sulfonate are bonded to the residual siloxane groups or silane organic functional groups (such as amino groups) of the modified CNC through hydrogen bonding or electrostatic interactions. The aromatic rings of lignin and the triazine rings of melamine undergo π-π stacking, which enhances interfacial compatibility through the charge transfer effect of the conjugated system and reduces phase separation between lignin sulfonate and melamine prepolymer, thereby reducing heat resistance defects caused by phase separation. At the same time, the long-chain alkyl segments of lignin sulfonate are embedded in the melamine prepolymer through molecular chain entanglement, further alleviating resin brittleness.

[0026] Simultaneously, the carboxyl groups of the carboxylated carbon nanotubes are bonded to the amino groups of the melamine resin prepolymer through electrostatic interactions, while the sp of the carboxylated carbon nanotubes... 2 The carbon domains and melamine triazine rings undergo π-π stacking, forming an effective stress transfer network and heat conduction pathway through a tubular structure. In the montmorillonite-graphene hybrid, the interlayer hydroxyl groups of montmorillonite form hydrogen bonds with the oxygen-containing functional groups of graphene, and the layered structure is entangled with the melamine molecular chains through van der Waals forces, constructing a planar high-temperature barrier network. The second dispersant forms steric hindrance by adsorbing onto the surface of the reinforcing material, preventing agglomeration and promoting interfacial bonding with the melamine resin matrix. During the gradient heating process, the crosslinking accelerator and curing agent induce the second component and resin molecules to form a network structure with high local crosslinking density. The highly dense crosslinking network effectively restricts the thermal motion of molecular chain segments, thereby improving the heat resistance of the composite material. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of a process for preparing a high heat-resistant melamine resin composite material. Detailed Implementation

[0028] To make the technical problems, technical solutions, and beneficial effects to be solved by this application clearer, the application will be further described in detail below with reference to embodiments. However, this should not be construed as limiting the scope of this application to the following examples. All other embodiments obtained by those skilled in the art without creative effort without departing from the above-described methodological spirit of this application are within the scope of protection of this application.

[0029] In this application, the terminology used is for the purpose of describing particular embodiments only and is not intended to be limiting of this application.

[0030] The singular forms “for,” “or,” “a,” “any,” and “the” used in this application are intended to include the plural forms unless the context clearly indicates otherwise.

[0031] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0032] The following will describe in detail, with reference to different embodiments, a high heat-resistant melamine resin composite material and its preparation method provided in this application.

[0033] Example 1

[0034] This embodiment provides a method for preparing a high heat-resistant melamine resin composite material, including the following steps:

[0035] S1. Preparation of composite modified solution: Sodium lignosulfonate with a molecular weight of 2000 Da and a sulfonation degree of 1.7 mmol / g was dissolved in deionized water to prepare a 20% mass concentration solution. The pH was adjusted to 8, and 1% by mass of glyceryl stearate of sodium lignosulfonate was added and stirred to dissolve. CNCs with a diameter of 20 nm and a length of 100 nm were mixed with deionized water at a mass ratio of 1:30 and ultrasonically dispersed at a power of 400 W for 40 min. KH550 at 3% by mass of CNCs was added and stirred evenly to obtain modified CNCs. The two solutions above were mixed, with the mass ratio of sodium lignosulfonate to modified CNCs being 2:1, to obtain the first mixture.

[0036] S2. Composite reinforced material dispersion: Concentrated sulfuric acid and carbon nanotubes are mixed at a mass ratio of 5:1 at 50°C. After cooling to room temperature, the mixture is washed with deionized water until neutral and dried at 0.05 MPa vacuum and 60°C for 8 hours to obtain carboxylated carbon nanotubes.

[0037] Montmorillonite and graphene oxide were dispersed in deionized water at a mass ratio of 3:1 to obtain a montmorillonite and graphene oxide solution. 5% ascorbic acid by mass of montmorillonite was added, and the solution was freeze-dried at -40℃ for 20h to obtain a montmorillonite-graphene hybrid.

[0038] Carboxylated carbon nanotubes and montmorillonite-graphene hybrid were mixed at a mass ratio of 1:2, and deionized water was added to prepare a 2% mass concentration suspension. Polyethylene glycol of 1.5% of the total mass of the composite reinforcing material was added, and sodium benzoate of 2% of the total mass of the second component was added. The mixture was sheared at 8000 rpm for 20 min to obtain the second mixture.

[0039] S3. Blending reaction: First, melamine resin prepolymer is prepared by reacting melamine and formaldehyde at a mass ratio of 1:0.7 under pH 8 and temperature of 60℃. The mass ratio of melamine resin prepolymer to modified CNC is 16:1. Melamine prepolymer and 0.5% of triethanolamine by mass of melamine prepolymer are added to the first mixture and stirred. Then, the second mixture is added to obtain a blend system. The mass ratio of the first mixture to the second mixture is 6:1.

[0040] S4. Gradient curing molding: Add 2% ammonium sulfate by mass of the first component to the blend system, stir to make the system uniform, and then transfer it to the mold. First, heat it to 120°C at a heating rate of 5°C / min and hold it at 10MPa pressure for 15 minutes. Then, perform the second gradient heating curing molding, heat it to 160°C at a heating rate of 3°C / min and hold it at 20MPa pressure for 20 minutes.

[0041] Example 2

[0042] This embodiment provides a method for preparing a high heat-resistant melamine resin composite material, including the following steps:

[0043] S1. Preparation of composite modified solution: Calcium lignosulfonate with a molecular weight of 3000 Da and a sulfonation degree of 1.8 mmol / g was dissolved in deionized water to prepare a 25% mass concentration solution. The pH was adjusted to 8.5, and 2% (by mass) of polyethylene glycol stearate of calcium lignosulfonate was added and stirred to dissolve. CNCs with a diameter of 30 nm and a length of 200 nm were mixed with deionized water at a mass ratio of 1:40 and ultrasonically dispersed at 500 W for 50 min. KH550 at 6% (by mass) of CNCs was added and stirred evenly to obtain modified CNCs. The two solutions were mixed, with the mass ratio of sodium lignosulfonate to modified CNCs being 3:1, to obtain the first mixture.

[0044] S2. Composite reinforced material dispersion: Concentrated nitric acid and carbon nanotubes are mixed at a mass ratio of 7:1 at 60°C. After cooling to room temperature, the mixture is washed with deionized water until neutral and dried at 0.07 MPa vacuum and 80°C for 10 h to obtain carboxylated carbon nanotubes.

[0045] Montmorillonite and graphene oxide were dispersed in deionized water at a mass ratio of 4:1 to obtain a montmorillonite and graphene oxide solution. Ascorbic acid of 7% by mass of montmorillonite was added, and the solution was freeze-dried at -30℃ for 22 hours to obtain a montmorillonite-graphene hybrid.

[0046] Carboxylated carbon nanotubes and montmorillonite-graphene hybrids were mixed at a mass ratio of 1:2.5, and deionized water was added to prepare a 3% mass concentration suspension. Composite reinforcing material, sodium dodecylbenzenesulfonate, total mass 2% was added, and then nano-calcium carbonate, total mass 3% of the second component, was added. The mixture was sheared at 10000 rpm for 25 min to obtain the second mixture.

[0047] S3. Blending Reaction: First, melamine resin prepolymer is prepared by reacting melamine and formaldehyde at a mass ratio of 1:0.8 under conditions of pH 9 and temperature 70℃. Then, melamine prepolymer and 1% (by mass) diethanolamine of the melamine prepolymer are added to the first mixture, stirred, and then a second mixture is added to obtain a blend system. The mass ratio of the first mixture to the second mixture is 7:1.

[0048] S4. Gradient curing molding: Add 4% of ammonium dihydrogen phosphate by mass of the first component to the blend system, stir to make the system uniform, and then transfer it to the mold. First, heat to 130°C at a heating rate of 10°C / min, and hold at 15MPa pressure for 20 minutes. Then, perform second gradient heating curing molding, heat to 170°C at a heating rate of 5°C / min, and hold at 25MPa pressure for 30 minutes.

[0049] Example 3

[0050] This embodiment provides a method for preparing a high heat-resistant melamine resin composite material, including the following steps:

[0051] S1. Preparation of composite modified solution: Ammonium lignosulfonate with a molecular weight of 5000 Da and a sulfonation degree of 2.0 mmol / g was dissolved in deionized water to prepare a 30% mass concentration solution. The pH was adjusted to 9, and 3% (by mass) of sorbitan monostearate of ammonium lignosulfonate was added and stirred to dissolve. CNCs with a diameter of 50 nm and a length of 300 nm were mixed with deionized water at a mass ratio of 1:50 and ultrasonically dispersed at a power of 600 W for 60 min. KH550 at 8% (by mass) of CNCs was added and stirred evenly to obtain modified CNCs. The two solutions above were mixed, with the mass ratio of sodium lignosulfonate to modified CNCs being 4:1, to obtain the first mixture.

[0052] S2. Composite reinforced material dispersion: Concentrated sulfuric acid and carbon nanotubes were mixed at a mass ratio of 10:1 at 70°C. After cooling to room temperature, the mixture was washed with deionized water until neutral and dried at 0.1 MPa vacuum and 100°C for 12 h to obtain carboxylated carbon nanotubes.

[0053] Montmorillonite and graphene oxide were dispersed in deionized water at a mass ratio of 5:1 to obtain a montmorillonite and graphene oxide solution. Ascorbic acid of 10% by mass of montmorillonite was added, and the solution was freeze-dried at -20℃ for 24 hours to obtain a montmorillonite-graphene hybrid.

[0054] Carboxylated carbon nanotubes and montmorillonite-graphene hybrids were mixed at a mass ratio of 1:3, and deionized water was added to prepare a 4% mass concentration suspension. Polyvinylpyrrolidone (3.5% of the total mass of the composite reinforcing material) was added, and zinc stearate (5% of the total mass of the second component) was added. The mixture was then subjected to high-speed shearing at 12000 rpm for 30 min to obtain the second mixture.

[0055] S3. Blending Reaction: First, melamine resin prepolymer is prepared by reacting melamine and formaldehyde at a mass ratio of 1:0.9 under conditions of pH 10 and temperature 80℃. The mass ratio of melamine resin prepolymer to modified CNC is 19:1. Melamine prepolymer and 1.5% N,N-dimethylethanolamine (by mass of melamine prepolymer) are added to the first mixture, and after stirring, a second mixture is added to obtain a blend system. The mass ratio of the first mixture to the second mixture is 8:1, which is the mass ratio of the first component to the second component.

[0056] S4. Gradient curing molding: Add 6% citric acid by mass of the first component to the blend system, stir to make the system uniform, and then transfer it to the mold. First, heat it to 150°C at a heating rate of 20°C / min and hold it at 20MPa pressure for 30 minutes. Then, perform second gradient heating curing molding, heat it to 190°C at a heating rate of 10°C / min and hold it at 30MPa pressure for 40 minutes.

[0057] Comparative Example 1

[0058] This comparative example provides a method for preparing a high heat-resistant melamine resin composite material. The difference between this method and Example 1 is that in step S1, only modified CNC-modified melamine resin is added, without the addition of lignin sulfonate.

[0059] Comparative Example 2

[0060] This comparative example provides a method for preparing a high heat-resistant melamine resin composite material, which differs from Example 1 in that only lignin dispersion is added in S1, and no modified CNC is used.

[0061] Comparative Example 3

[0062] This comparative example provides a method for preparing a high heat-resistant melamine resin composite material, which differs from Example 1 in that S4 is a constant temperature curing process under a pressure of 20 MPa and a temperature of 160°C for 30 minutes.

[0063] Performance testing:

[0064] 1. Thermal Decomposition Temperature Test: Samples of a high-heat-resistant melamine resin composite material from Examples 1-3 and Comparative Examples 1-3 were tested using a thermogravimetric analyzer (TGA) under a nitrogen atmosphere at a flow rate of 50 mL / min, with the temperature increased from room temperature to 600°C at a rate of 10°C / min. The temperature (T) at which 5% mass loss was achieved was recorded as the thermal decomposition temperature. This was used to evaluate the material's heat resistance limit.

[0065] 2. High-temperature sustained load test: The high heat-resistant melamine resin composite material samples of Examples 1-3 and Comparative Examples 1-3 were subjected to a static load of 50 MPa at a constant temperature of 200℃ for 100 hours. The residual flexural strength retention rate was measured to evaluate the mechanical stability of the material under long-term high temperature.

[0066] 3. Interfacial integrity test after thermal cycling: The high heat-resistant melamine resin composite material samples of Examples 1-3 and Comparative Examples 1-3 were subjected to 100 cycles of thermal cycling from -40% to 200℃ (30 minutes each time), and the shear strength reduction rate was measured to verify the interface's resistance to fatigue cracking under drastic temperature changes.

[0067] Table 1 Performance test data of Examples 1-3 and Comparative Examples 1-3

[0068] Group Thermal decomposition temperature (°C) High-temperature endurance load (%) Shear strength reduction rate (%) Example 1 320 90 5 Example 2 315 88 6 Example 3 310 85 8 Comparative Example 1 275 65 25 Comparative Example 2 280 70 20 Comparative Example 3 290 75 15

[0069] As shown in Table 1, Examples 1 and 3 are superior to Comparative Examples 1-3 in terms of thermal decomposition temperature, retention rate of residual strength under high-temperature sustained load, and decrease rate of shear strength after thermal cycling. This is because the aromatic rings of lignin sulfonate form a carbon layer through π-π stacking to block heat transfer, the silane groups of modified CNC form ether bonds with the amino groups (-NH2) of melamine prepolymer, the rigid crystalline region of modified CNC inhibits the disordered movement of molecular chains at high temperatures, and the nano-size effect of CNC hinders the heat conduction path. The synergistic effect of modified CNC and lignin sulfonate enhances the activation energy of melamine resin thermal decomposition. However, Comparative Example 1, lacking lignin and relying solely on the rigid support of CNC, suffers from a lack of lignin's hydrogen bond network, leading to easy breakage of the molecular chains at high temperatures and a thermal decomposition temperature dropping to 275℃. This is because the CNC surface is rich in hydroxyl (-OH) groups, and polar groups readily aggregate through hydrogen bonding. In contrast, the molecular structure of lignin sulfonate contains both polar groups, such as phenolic hydroxyl and sulfonic acid groups, and non-polar aromatic ring structures, which inhibit CNC aggregation. Aggregated CNC cannot form a continuous nanoskeleton, existing only as isolated large particles that cannot uniformly distribute thermal stress. Furthermore, the difference in thermal expansion coefficients with the melamine resin matrix results in microcracks at the interface. Comparative Example 3, due to uneven cross-linking caused by isothermal curing, and Comparative Example 2, lacking rigid support due to the absence of CNC, both exhibit decreased thermal stability.

[0070] In the high-temperature endurance load test, the residual flexural strength retention rate of Examples 1 and 3 (85%-90%) was better than that of Comparative Example 3 (75%). The key lies in the dense cross-linked network formed by the gradient curing in stages: hydrogen bonds pre-fix the distribution of lignin and CNC in the low-temperature stage, and covalent cross-linking in the high-temperature stage forms a three-dimensional network, which improves the cross-linking density compared with isothermal curing. Moreover, the phenolic hydroxyl groups of lignin sulfonate and the hydroxyl groups of CNC and melamine prepolymer form a hydrogen bond-covalent bond composite interface, which resists stress relaxation and interface debonding at high temperature. Comparative Example 1 has interface defects caused by CNC agglomeration, and stress concentration at high temperature leads to rapid strength decay. Comparative Example 2 has no CNC and lacks a rigid skeleton, so the melamine resin matrix is ​​prone to creep and has low residual strength.

[0071] In the thermal cycling test, the shear strength reduction rate of Examples 1-3 (5%-8%) was much lower than that of Comparative Examples 1-3 (15%-25%). This was due to the buffering effect of hydrogen bonds and covalent bonds. The hydrogen bonds between lignin sulfonate and modified CNC absorbed thermal stress through fracture reconstruction, while the ether bonds between melamine prepolymer and lignin sulfonate provided interfacial bonding force and inhibited crack initiation. At the same time, the carboxyl groups of carboxylated carbon nanotubes formed acid-base coordination bonds with the amino groups of the melamine matrix, and the tubular structure interspersed in the matrix could resist load deformation under high temperature. The sheets of montmorillonite-graphene hybrid were entangled with the matrix through hydrogen bonds to form a "barrier barrier" and inhibit the slippage of molecular chains under load. Meanwhile, the ordered crystalline phase induced by the nucleating agent further strengthened the interfacial bonding. In Comparative Example 1, the interface was maintained solely by physical adsorption without lignin sulfonate, making it prone to debonding under alternating hot and cold temperatures. In Comparative Example 2, the unmodified CNC showed a 20% decrease in shear strength during the hot and cold cycling test. This was because the lack of a rigid framework and interfacial regulation function of the CNC prevented the composite material from effectively buffering the internal stress caused by drastic temperature changes. In Comparative Example 3, uneven cross-linking during the isothermal curing process led to localized stress concentration, resulting in microcrack propagation after cycling and a decrease in shear strength. In summary, Examples 1 and 3 improved heat resistance and interfacial stability through the interaction of lignin sulfonate, modified CNC, modified melamine resin, and gradient curing processes.

[0072] The above results demonstrate and describe the basic principles and main features of this application, as well as its advantages.

[0073] Those skilled in the art should understand that this application is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of this application. Various changes and modifications can be made to this application without departing from the spirit and scope thereof, and all such changes and modifications fall within the scope of this application as claimed. The scope of protection of this application is defined by the equivalents of the appended claims.

Claims

1. A high heat-resistant melamine resin composite material, characterized in that, It includes a first component and a second component; the first component includes a modified melamine resin prepolymer, a crosslinking accelerator, a curing agent, and a first dispersant; the modified melamine resin prepolymer is prepared by modifying melamine resin prepolymer with lignin sulfonate and modified nanocellulose whiskers; The modified cellulose nanofiber whiskers are prepared by modifying cellulose nanofiber whiskers with a silane coupling agent; the second component includes a composite reinforcing material; the composite reinforcing material includes carboxylated carbon nanotubes, montmorillonite-graphene hybrid, a second dispersant, and a nucleating agent; the carboxylated carbon nanotubes are prepared by reacting carbon nanotubes with a strong acid; the montmorillonite-graphene hybrid is prepared by reacting montmorillonite with graphene oxide.

2. The high heat-resistant melamine resin composite material according to claim 1, characterized in that, The lignin sulfonate salt includes any one of sodium lignin sulfonate, calcium lignin sulfonate, and ammonium lignin sulfonate; the crosslinking accelerator includes any one of triethanolamine, diethanolamine, and N,N-dimethylethanolamine; the curing agent includes any one of ammonium sulfate, ammonium dihydrogen phosphate, and limonene; the first dispersant includes any one of stearyl glycerol, polyethylene glycol stearate, and sorbitan monostearyl ester; the silane coupling agent includes KH550; the strong dispersant includes at least one of concentrated nitric acid and concentrated sulfuric acid; the second dispersant includes any one of polyethylene glycol, sodium dodecylbenzene sulfonate, and polyvinylpyrrolidone; the nucleating agent includes any one of sodium benzoate, nano-carbon calcium, and zinc stearate.

3. The high heat-resistant melamine resin composite material according to claim 1, characterized in that, The lignin sulfonate has a molecular weight of 2000-5000 Da and a degree of sulfonation of 1.7-2.0 mmol / g. The nanocellulose whiskers have a diameter of 20-50 nm and a length of 100-300 nm.

4. The high heat-resistant melamine resin composite material according to claim 1, characterized in that, The mass ratio of the first component to the second component is (6-8):1; the mass ratio of the melamine resin prepolymer, lignin sulfonate, and modified nanocellulose whiskers is (16-19):(2-4):1; the amount of the crosslinking accelerator added is 0.5%-1.5% of the mass of the melamine resin prepolymer; the amount of the curing agent added is 2%-6% of the total mass of the first component; the amount of the first dispersant added is 1%-3% of the mass of the lignin sulfonate; the silane... The amount of coupling agent is 3%-8% of the mass of nanocellulose whiskers; the mass ratio of the carboxylated carbon nanotubes to the montmorillonite-graphene hybrid is 1:(2-3); the amount of the second dispersant added is 1.5%-3.5% of the total mass of the composite reinforcing material; the amount of the nucleating agent added is 2%-5% of the total mass of the second component; the mass ratio of the strong acid to the carbon nanotubes is (5-10):1; the mass ratio of the montmorillonite to the graphene oxide is (3-5):

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5. A method for preparing a high heat-resistant melamine resin composite material as described in any one of claims 1-4, characterized in that, Includes the following steps: S1. Preparation of composite modified solution: Dissolve lignin sulfonate in deionized water, adjust the pH to 8-9, add the first dispersant, and stir until completely dissolved to obtain lignin sulfonate solution; mix nanocellulose whiskers with deionized water, ultrasonically disperse, add silane coupling agent, and stir evenly to obtain modified nanocellulose whiskers; mix the lignin sulfonate solution and modified nanocellulose whiskers to obtain the first mixture; S2. Preparation of composite reinforced material dispersion: Strong styrene and carbon nanotubes are mixed at a set temperature, cooled to room temperature, washed with deionized water until neutral, and vacuum dried to obtain carboxylated carbon nanotubes; montmorillonite and graphene oxide are dispersed in deionized water to obtain a montmorillonite and graphene oxide solution, ascorbic acid is added, and the mixture is freeze-dried to obtain a montmorillonite-graphene hybrid; carboxylated carbon nanotubes are mixed with the montmorillonite-graphene hybrid, deionized water is added to prepare a suspension, a second dispersant is added, a nucleating agent is added, and high-speed shearing is performed to obtain a second mixture; S3, Blending reaction: Add melamine prepolymer to the first mixture and stir; Then add the second mixture and crosslinking accelerator and stir to obtain the blend system; S4. Gradient curing molding: Add curing agent to the blend system, stir to make the system uniform, and then transfer it into the mold for gradient temperature curing molding.

6. The method for preparing a high heat-resistant melamine resin composite material according to claim 5, characterized in that, In step S1, the mass concentration of the lignin sulfonate solution is 20-30%, the pH range is 8-9, the mass ratio of the nanocellulose whiskers to deionized water is 1:(30-50), and the ultrasonic dispersion power is 400-600W for 40-60min.

7. The method for preparing a high heat-resistant melamine resin composite material according to claim 5, characterized in that, In step S2, the set temperature is 50-70℃, the vacuum degree of the vacuum drying process is 0.05-0.1MPa, the temperature is 60-100℃, and the time is 8-12h; the total mass concentration of the montmorillonite and graphene oxide solution is 0.5-2mg / mL, the ascorbic acid is 5%-10% of the mass of montmorillonite, and the freezing temperature of the freeze-drying process is -40 to -20℃, and the time is 20-24h.

8. The method for preparing a high heat-resistant melamine resin composite material according to claim 5, characterized in that, The total mass concentration of carboxylated carbon nanotubes and montmorillonite-graphene hybrids in the suspension in step S2 is 2%-4%; the high-speed shearing speed is 8000-12000 rpm and the time is 20-30 min.

9. The method for preparing a high heat-resistant melamine resin composite material according to claim 5, characterized in that, The melamine resin prepolymer described in step S3 is prepared by reacting melamine and formaldehyde in a mass ratio of 1:(0.7-0.9) at pH 8-10 and temperature 60-80℃.

10. The method for preparing a high heat-resistant melamine resin composite material according to claim 5, characterized in that, The gradient temperature curing molding in step S4 includes a first gradient temperature curing molding and a second gradient temperature curing molding; the first gradient temperature curing molding heats the temperature to 120-150℃ at a heating rate of 5-20℃ / min and holds it at a pressure of 10-20MPa for 15-30 minutes; the second gradient temperature curing molding heats the temperature to 160-190℃ at a heating rate of 3-10℃ / min and holds it at a pressure of 20-30MPa for 20-40 minutes.

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