High-heat-resistant melamine resin composite material and preparation method thereof

Abstract

The application relates to the technical field of melamine resin 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 modified melamine resin prepolymer, crosslinking accelerant, curing agent and first dispersant; the modified melamine resin prepolymer is prepared by modifying melamine resin prepolymer through lignosulfonate and modified nanocellulose whisker; the modified nanocellulose whisker is prepared by modifying nanocellulose whisker through a silane coupling agent; the second component comprises composite reinforcing material; the composite reinforcing material comprises carboxylated carbon nanotube, montmorillonite-graphene hybrid, second dispersant and nucleating agent; the carboxylated carbon nanotube is prepared by reacting carbon nanotube and strong acid; and the montmorillonite-graphene hybrid is prepared by reacting montmorillonite and graphene oxide. The melamine resin is modified by lignosulfonate and modified nanocellulose whisker, and the composite reinforcing material is combined to improve the heat resistance and mechanical properties of the melamine resin by using a gradient curing process.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of melamine resin polymer materials, in particular to a high-heat-resistance melamine resin composite material and a preparation method thereof. BACKGROUND

[0002] As a thermosetting plastic, melamine resin (melamine-formaldehyde resin) 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 hindered its application in high-temperature environments. The thermal decomposition temperature of traditional melamine resin is usually lower than 260℃, and above 120℃, the molecular chain may break and release formaldehyde and melamine monomers, posing a risk to food safety and the environment. 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, and long-term use may also exacerbate formaldehyde migration due to repeated heating.

[0003] Existing modification methods mostly use single nano fillers (such as graphene oxide, titanium dioxide nanotubes) or chemical crosslinking agents, but generally have poor dispersion and weak interfacial bonding. For example, a Chinese patent with publication number CN119264599A discloses a preparation method of a cellulose melamine resin composite material, which increases the thermal decomposition temperature to 300℃ by introducing a diphenyl sulfone structure. However, the compatibility problem between inorganic fillers and melamine resin leads to the formation of agglomerates in the matrix, significantly reducing the dispersion uniformity, and the ultimate mechanical property improvement is limited. In addition, although single nano fillers (such as carbon fibers, sericite) can improve heat resistance through physical filling, they lack chemical bonding with the resin matrix, have weak interfacial bonding, low load transfer efficiency, low bending strength improvement rate, and are prone to crack due to stress concentration at high temperatures.

[0004] In summary, existing technologies still have bottlenecks in improving the heat resistance, optimizing the dispersion, strengthening the interfacial bonding, and improving the environmental friendliness of melamine resin, and there is an urgent need for a solution to high-heat-resistance melamine resin. SUMMARY

[0005] To overcome the shortcomings of the prior art, the application provides a high-heat-resistance melamine resin composite material and a preparation method thereof. The application modifies melamine resin with lignosulfonate and modified nanocellulose whiskers (CNC, Cellulose Nanocrystal), and simultaneously composites with reinforcing materials, combining with gradient curing process to improve the heat resistance and mechanical properties of melamine resin.

[0006] To achieve the above-mentioned purposes, the application adopts the following technical solutions:

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

[0011] Preferably, the lignin sulfonate has a molecular weight of 2000-5000 Da and a sulfonation degree 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, the lignin sulfonate and the modified CNC is (16-19):(2-4):1; the addition amount of the cross-linking accelerator is 0.5%-1.5% of the mass of the melamine resin prepolymer; the addition amount of the curing agent is 2%-6% of the total mass of the first component; the addition amount of the first dispersant is 1%-3% of the mass of the lignin sulfonate; the addition amount of the silane coupling agent is 3%-8% of the mass of the CNC; the mass ratio of the carboxylated carbon nanotube to the montmorillonite-graphene hybrid is 1:(2-3); the addition amount of the second dispersant is 1.5%-3.5% of the total mass of the composite reinforcing material; the addition amount of the nucleating agent is 2%-5% of the total mass of the second component; the mass ratio of the strong acid to the carbon nanotube is (5-10):1; and the mass ratio of the montmorillonite to the graphene oxide is (3-5):1.

[0013] In a second aspect, the present application provides a high-heat-resistant melamine resin composite material and a preparation method, including the following steps:

[0014] S1, composite modification liquid preparation: dissolve the lignin sulfonate in deionized water, adjust the pH to 8-9, add the first dispersant, stir until completely dissolved, to obtain a lignin sulfonate solution; mix the CNC with deionized water, ultrasonically disperse, then add the silane coupling agent, stir uniformly, to obtain a modified CNC; mix the above lignin sulfonate solution and the modified CNC, to obtain a first mixture;

[0015] S2, composite reinforced material dispersion preparation: mixing strong acid with carbon nanotubes at a set temperature, washing with deionized water to neutral after cooling to room temperature, vacuum drying to obtain carboxylated carbon nanotubes; dispersing montmorillonite and graphene oxide in deionized water to obtain a montmorillonite and graphene oxide solution, adding ascorbic acid, freeze-drying to obtain a montmorillonite-graphene hybrid; mixing the carboxylated carbon nanotubes and the montmorillonite-graphene hybrid, adding deionized water to prepare a suspension, adding a second dispersant, and then adding a nucleating agent, high-speed shearing to obtain a second mixture;

[0016] S3, blending reaction: adding melamine prepolymer to the first mixture and stirring; then adding the second mixture and a crosslinking promoter and stirring to obtain a blending system;

[0017] S4, gradient curing molding: adding a curing agent to the blending system and stirring to make the system uniform, and then transferring into a mold for gradient temperature curing molding.

[0018] In the present application, the gradient curing molding process is used in step S4. In the low-temperature stage, hydrogen bonds are preferentially formed due to the thermodynamic properties of hydrogen bonds and covalent bonds, fixing the spatial distribution of the modified CNC and lignin; in the high-temperature stage, the crosslinking degree is improved by rapid polycondensation of the melamine prepolymer, reducing unreacted groups and internal stress, and the modified CNC fully contacts the melamine resin matrix, hindering thermal deformation, and 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 the CNC to deionized water is 1:(30-50), and the ultrasonic dispersion power is 400-600W and the time is 40-60min.

[0020] Preferably, in step S2, the set temperature is 50-70℃, the vacuum drying process has a vacuum degree of 0.05-0.1MPa, a temperature of 60-100℃, and a time of 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 the montmorillonite, and the freeze-drying temperature is -40~-20℃ and the time is 20-24h.

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

[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 a temperature of 60-80℃.

[0023] Preferably, the gradient curing molding comprises a first gradient curing molding and a second gradient curing molding; the first gradient curing molding is heated to 120-150℃ at a heating rate of 5-20℃ / min, and the pressure range is 10-20MPa, and the holding time is 15-30min; the second gradient curing molding is heated to 160-190℃ at a heating rate of 3-10℃ / min, and the pressure range is 20-30MPa, and the holding time is 20-40min.

[0024] Beneficial technical effects:

[0025] In the present application, after the CNC is dispersed by ultrasonic, the surface hydroxyl group is exposed and condensation reaction occurs with the silane coupling agent. Under the condition of an acidic pH of 4-4.5, the silane coupling agent is first hydrolyzed to silanol (Si-OH) under this condition, and the silanol and the siloxane oligomers generated by the condensation of the silanol are adsorbed on the surface of the CNC through hydrogen bonding to obtain modified CNC. The introduction of low-surface-energy organic groups reduces the surface energy of the modified CNC, enabling it to disperse uniformly in the system and construct a three-dimensional nanometer network, which not only limits the thermal motion of the melamine resin molecular chain to improve the material's heat resistance, but also blocks the crack propagation when stressed. The polar groups of lignin sulfonate are combined with the residual siloxane groups or silane organic functional groups (such as amino groups) of the modified CNC through hydrogen bonding or electrostatic interaction. The aromatic ring of lignin and the melamine triazine ring undergo π-π stacking, and the interface compatibility is enhanced through the charge transfer effect of the conjugated system, reducing the phase separation of lignin sulfonate and melamine prepolymer and thus reducing the heat resistance defects caused by phase separation; at the same time, the long-chain alkyl fragments of lignin sulfonate are embedded in the melamine prepolymer through molecular chain entanglement, further relieving the brittleness of the resin.

[0026] At the same time, the carboxyl groups of the carboxylated carbon nanotubes are combined with the amino groups of the melamine resin prepolymer through electrostatic interaction, and at the same time, the sp 2 The carbon domain undergoes π-π stacking with the melamine triazine ring, forming an effective stress transfer network and a heat conduction path through the 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 lamellar structure is entangled with the melamine molecular chain through van der Waals force, constructing a planar high-temperature barrier network; the second dispersant forms steric hindrance by adsorbing on the surface of the reinforcing material, avoiding agglomeration and promoting interfacial bonding with the melamine resin matrix. The crosslinking promoter and the curing agent induce the second component and the resin molecules to form a network structure with high local crosslinking density during the gradient heating process, and the highly dense crosslinking network effectively limits the thermal motion of the molecular chain segments, thereby improving the heat resistance of the composite material. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 A flow chart of a preparation method of a high-heat-resistant melamine resin composite material. DETAILED DESCRIPTION

[0028] In order to make the technical problems, technical solutions and beneficial effects of the present application clearer, the present application will be further described in detail below in conjunction with embodiments. However, it should not be understood as a limitation on the scope of the present application only to the following examples. Without departing from the method idea of the present application, all other embodiments obtained by those of ordinary skill in the art without creative labor are within the scope of protection of the present application.

[0029] In the present application, the terms used in the present application are only for the purpose of describing specific embodiments, and are not intended to limit the present application.

[0030] The singular forms "is", "or", "a", "an", and "the" used in the present application are intended to include the plural forms, unless the context clearly indicates otherwise.

[0031] In addition, if the terms "first", "second" appear, they are only for the purpose of description, and cannot be understood as indicating or implying relative importance.

[0032] The following will specifically describe a high-heat-resistant melamine resin composite material and a preparation method provided by the present application in conjunction with different embodiments.

[0033] Embodiment 1

[0034] The present embodiment provides a preparation method of a high-heat-resistant melamine resin composite material, comprising the following steps:

[0035] S1, preparation of a composite modification solution: dissolve sodium lignosulfonate with a molecular weight of 2000 Da and a sulfonation degree of 1.7 mmol / g in deionized water to prepare a 20% mass concentration solution, adjust the pH to 8, and add 1% of the mass of sodium lignosulfonate glyceryl stearate, and stir to dissolve; mix CNC with a diameter of 20 nm and a length of 100 nm with deionized water at a mass ratio of 1:30, ultrasonically disperse for 40 min at a power of 400 W, add 3% of the mass of CNC KH550, and stir uniformly to obtain modified CNC; mix the above two solutions, wherein the mass ratio of sodium lignosulfonate and modified CNC is 2:1, to obtain a first mixture;

[0036] S2, dispersion of composite reinforcing material: mix concentrated sulfuric acid and carbon nanotubes at a mass ratio of 5:1 at a temperature of 50°C, wash with deionized water to neutralize after cooling to room temperature, and dry at a vacuum degree of 0.05 MPa and a temperature of 60°C for 8 h to obtain carboxylated carbon nanotubes;

[0037] The 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% of the mass of the montmorillonite was added ascorbic acid, and -40°C freeze-drying for 20h to obtain a montmorillonite-graphene hybrid;

[0038] The carboxylated carbon nanotubes and the montmorillonite-graphene hybrid were mixed at a mass ratio of 1:2, and a 2% mass concentration suspension was prepared by adding deionized water, 1.5% of the total mass of the composite reinforcing material was added as polyethylene glycol, and 2% of the total mass of the second component was added as sodium benzoate, and a second mixture was obtained by high-speed shearing at 8000 rpm for 20 min;

[0039] S3, blending reaction: first, melamine and formaldehyde were reacted at a mass ratio of 1:0.7 at pH 8 and a temperature of 60°C to prepare a melamine resin prepolymer, wherein the mass ratio of the melamine resin prepolymer and the modified CNC was 16:1, 0.5% of the mass of the melamine prepolymer was added as triethanolamine in the first mixture, and the second mixture was added after stirring, to obtain a blending system, wherein the mass ratio of the first mixture and the second mixture is the mass ratio of the first component and the second component, which is 6:1.

[0040] S4, gradient curing molding: 2% of the mass of the first component was added as ammonium sulfate in the blending system, and the system was stirred uniformly, and then transferred into a mold, first heated to 120°C at a heating rate of 5°C / min, and then heated to 160°C at a heating rate of 3°C / min under a pressure of 20 MPa for 20 min.

[0041] Example 2

[0042] The present embodiment provides a method for preparing a melamine resin composite material with high heat resistance, comprising the following steps:

[0043] S1, preparation of a composite modification 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% of the mass of the calcium lignosulfonate was added as polyethylene glycol stearate, and stirred to dissolve; CNC with a diameter of 30 nm and a length of 200 nm was mixed with deionized water at a mass ratio of 1:40, and ultrasonic dispersion was performed at a power of 500 W for 50 min, and 6% of the mass of the CNC was added as KH550, and stirred uniformly to obtain modified CNC; the above two solutions were mixed, wherein the mass ratio of sodium lignosulfonate and modified CNC was 3:1, to obtain a first mixture;

[0044] S2, composite reinforcing material dispersion: concentrated nitric acid and carbon nanotubes are mixed in a mass ratio of 7:1 at a temperature of 60°C, cooled to room temperature, washed with deionized water until neutral, dried at a vacuum degree of 0.07 MPa and a temperature of 80°C for 10 hours, and carboxylated carbon nanotubes are obtained;

[0045] Montmorillonite and graphene oxide are dispersed in deionized water in a mass ratio of 4:1 to obtain a montmorillonite and graphene oxide solution, 7% of ascorbic acid by mass of montmorillonite is added, and the mixture is freeze-dried at -30°C for 22 hours to obtain a montmorillonite-graphene hybrid;

[0046] Carboxylated carbon nanotubes and montmorillonite-graphene hybrid are mixed in a mass ratio of 1:2.5, deionized water is added to prepare a 3% mass concentration suspension, composite reinforcing material is added, 2% of total mass of sodium dodecylbenzenesulfonate is added, and 3% of total mass of nano calcium carbonate is added, and the mixture is sheared at a high speed of 10,000 rpm for 25 minutes to obtain a second mixture;

[0047] S3, blending reaction: melamine and formaldehyde are first reacted in a mass ratio of 1:0.8 at a pH of 9 and a temperature of 70°C to prepare a melamine resin prepolymer. In the first mixture, 1% of diethanolamine by mass of the melamine prepolymer is added, stirred, and then the second mixture is added to obtain a blending system, wherein the mass ratio of the first mixture to the second mixture is the mass ratio of the first component to the second component, which is 7:1.

[0048] S4, gradient curing molding: 4% of ammonium dihydrogen phosphate by mass of the first component is added to the blending system, stirred to make the system uniform, and then transferred into a mold. First, the temperature is raised to 130°C at a temperature rise rate of 10°C / min, and then the system is kept at 15 MPa for 20 minutes. Second, the temperature is raised to 170°C at a temperature rise rate of 5°C / min, and then the system is kept at a pressure range of 25 MPa for 30 minutes.

[0049] Example 3

[0050] The present embodiment provides a method for preparing a melamine resin composite material with high heat resistance, comprising the following steps:

[0051] S1, composite modification liquid preparation: 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% of the mass of ammonium lignosulfonate sorbitan monostearate was added and stirred to dissolve; CNC with a diameter of 50 nm and a length of 300 nm was mixed with deionized water at a mass ratio of 1:50, ultrasonic dispersion was carried out at a power of 600 W for 60 min, 8% of the mass of CNC was added, and stirring was uniform to obtain modified CNC; the above two solutions were mixed, and the mass ratio of sodium lignosulfonate and modified CNC was 4:1 to obtain a first mixture;

[0052] S2, composite reinforcing material dispersion: concentrated sulfuric acid and carbon nanotubes were mixed at a mass ratio of 10:1 at a temperature of 70°C, cooled to room temperature, washed with deionized water to neutral, dried at a vacuum degree of 0.1 MPa and a temperature of 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, 10% of the mass of montmorillonite was added ascorbic acid, and freeze-drying was carried out at -20°C for 24 h to obtain a montmorillonite-graphene hybrid;

[0054] Carboxylated carbon nanotubes and montmorillonite-graphene hybrid were mixed at a mass ratio of 1:3, deionized water was added to prepare a 4% mass concentration suspension, 3.5% of the total mass of the composite reinforcing material was added as polyvinylpyrrolidone, and 5% of the total mass of the second component was added as zinc stearate, and high-speed shearing was carried out at 12000 rpm for 30 min to obtain a second mixture;

[0055] S3, blending reaction: first, melamine and formaldehyde were reacted at a mass ratio of 1:0.9 at pH 10 and a temperature of 80°C to prepare a melamine resin prepolymer, and the mass ratio of the melamine resin prepolymer and the modified CNC was 19:1. In the first mixture, 1.5% of the mass of the melamine prepolymer was added as N,N-dimethylethanolamine, and after stirring, the second mixture was added to obtain a blending system, and the mass ratio of the first mixture and the second mixture was the mass ratio of the first component and the second component, which was 8:1.

[0056] S4, gradient curing molding: 6% of the mass of the first component was added as citric acid in the blending system, and the system was stirred to be uniform, and then transferred into a mold, first heated to 150°C at a heating rate of 20°C / min, and then heated to 190°C at a heating rate of 10°C / min under a pressure of 30 MPa for 40 min.

[0057] Comparative Example 1

[0058] The comparative example provides a preparation method of a high-heat-resistant melamine resin composite, which is different from Example 1 in that only the modified CNC modified melamine resin is added in S1, and no lignin sulfonate is added.

[0059] Comparative Example 2

[0060] The comparative example provides a preparation method of a high-heat-resistant melamine resin composite, which is different from Example 1 in that no modified CNC is added in S1, only the lignin dispersion liquid is added.

[0061] Comparative Example 3

[0062] The comparative example provides a preparation method of a high-heat-resistant melamine resin composite, which is different from Example 1 in that the isothermal curing molding in S4 is under the conditions of 20 MPa pressure and 160°C temperature for 30 minutes.

[0063] Performance test:

[0064] 1. Thermal decomposition temperature test: The high-heat-resistant melamine resin composite sample of Example 1-Example 3 and Comparative Example 1-Comparative Example 3 is tested by a thermal gravimetric analyzer (TGA) under the conditions of a nitrogen atmosphere with a flow rate of 50 mL / min, a temperature rise rate of 10°C / min from room temperature to 600°C, and the temperature (T) at which 5% mass loss occurs is recorded as the thermal decomposition temperature. The thermal decomposition temperature is used to evaluate the heat resistance limit of the material.

[0065] 2. High-temperature sustained load test: The high-heat-resistant melamine resin composite sample of Example 1-Example 3 and Comparative Example 1-Comparative Example 3 is subjected to a static load of 50 MPa at a constant temperature of 200°C for 100 hours, and the residual bending strength retention rate is measured to evaluate the mechanical stability of the material under long-term high temperature.

[0066] 3. Interfacial integrity test after cold and hot cycle: The high-heat-resistant melamine resin composite sample of Example 1-Example 3 and Comparative Example 1-Comparative Example 3 is subjected to a cold and hot cycle of -40°C to 200°C for 100 times (30 minutes each time), and the shear strength drop rate is measured to verify the interfacial fatigue crack resistance under severe temperature changes.

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

[0068] Group Thermal decomposition temperature (°C) High temperature load endurance (%) 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] From Table 1, it can be seen that the thermal decomposition temperature, high-temperature residual strength retention rate, and shear strength reduction rate after cold and hot cycle of Example 1 and Example 3 are better than those of Comparative Example 1-Comparative Example 3. Because the aromatic ring of lignin sulfonate forms a carbon layer to block heat transfer through π-π stacking, the silane group of the modified CNC forms an ether bond with the amino group (-NH2) of the melamine prepolymer, the rigid crystalline region of the modified CNC inhibits the disordered movement of molecular chains at high temperature, and the nanosize effect of the CNC hinders the heat conduction path, the modified CNC and lignin sulfonate synergistically improve the thermal decomposition activation energy of the melamine resin. However, Comparative Example 1 lacks lignin and relies only on the rigid support of CNC, lacking the hydrogen bond network of lignin, and the molecular chain is easy to break at high temperature, so the thermal decomposition temperature is reduced to 275°C. Because the CNC surface is rich in hydroxyl groups (-OH), polar groups are easy to interact with each other through hydrogen bonds to self-aggregate, and the molecular structure of lignin sulfonate contains both polar groups such as phenolic hydroxyl groups and sulfonic acid groups and non-polar aromatic ring structures, which can inhibit the aggregation of CNC. The aggregated CNC cannot form a continuous nanoskeleton and exists only as isolated large particles, which cannot evenly share thermal stress, and the difference in thermal expansion coefficient between the melamine resin matrix and the interface produces microcracks. Comparative Example 3 has uneven crosslinking due to constant temperature curing, and Comparative Example 2 lacks the rigid support of CNC, both of which cause the thermal stability to decrease.

[0070] In the high-temperature residual strength test, the residual bending strength retention rate (85%-90%) of Example 1 and Example 3 is better than that of Comparative Example 3 (75%). The core lies in the dense crosslinking network formed by gradient curing and step-by-step formation: the distribution of hydrogen bond pre-fixed lignin and CNC at low temperature, and the formation of three-dimensional network by covalent bond crosslinking at high temperature, which improves the crosslinking density compared with constant temperature curing, and the phenolic hydroxyl group of lignin sulfonate and the hydroxyl group of CNC form a hydrogen bond-covalent bond complex interface with the melamine prepolymer, resisting stress relaxation and interface debonding at high temperature; Comparative Example 1 has interface defects due to CNC aggregation, and stress concentration at high temperature leads to rapid strength decay, and Comparative Example 2 lacks CNC and has a rigid skeleton, and the melamine resin matrix is prone to creep, resulting in low residual strength.

[0071] In the cold and hot cycle test, the shear strength reduction rate (5%-8%) of Example 1-Example 3 is far lower than that of Comparative Example 1-3 (15%-25%), which is due to the buffering effect of hydrogen bond and covalent bond. The hydrogen bond between lignosulfonate and modified CNC absorbs the cold and hot stress by breaking and reconfiguring, and the ether bond between melamine prepolymer and lignosulfonate provides interfacial bonding force to inhibit crack initiation. At the same time, the carboxyl group of carboxylated carbon nanotube forms acid-base coordination bond with the amino group of melamine matrix, and the tubular structure penetrates into the matrix, which can resist load deformation under high temperature. The sheet layer of montmorillonite-graphene hybrid is intertwined with the matrix through hydrogen bond, forming a "barrier screen" to inhibit the slip of molecular chain under load, and the ordered crystalline phase induced by nucleating agent further strengthens the interfacial bonding. In Comparative Example 1, there is no lignosulfonate to rely on physical adsorption to maintain the interface, which is easy to debond under cold and hot alternation. In Comparative Example 2, the shear strength reduction rate of the modified CNC in the cold and hot cycle test is 20%, which is due to the lack of the rigid skeleton of CNC and the interface regulation effect, resulting in that the composite material cannot effectively buffer the internal stress generated by the dramatic change of temperature. In Comparative Example 3, there is local stress concentration in the process of constant temperature curing due to uneven crosslinking, and microcracks expand after cycling, resulting in a decrease in shear strength. In summary, Example 1-Example 3 improve the high heat resistance and interfacial stability of melamine resin through the modification of lignosulfonate and modified CNC and gradient curing process.

[0072] The above results show and describe the basic principles and main features of the present application and the advantages of the present application.

[0073] Those skilled in the art should understand that the present application is not limited to the above examples, and the above examples and descriptions in the specification are only to illustrate the principles of the present application. Without departing from the spirit and scope of the present application, various changes and improvements can be made to the present application, and these changes and improvements all fall within the scope of the claimed present application. The scope of protection claimed by the present 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 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 citric acid; the first dispersant includes any one of glyceryl stearate, polyethylene glycol stearate, and sorbitan monostearate; the silane coupling agent includes KH550; the strong acid 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; and the nucleating agent includes any one of sodium benzoate, nano-calcium carbonate, 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 cellulose nanofibers 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 amount of the silane coupling agent is 3%-8% of the mass of the 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):

1.

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: Lignosulfonate is dissolved in deionized water, pH is adjusted to 8-9, a first dispersant is added, and the mixture is stirred until completely dissolved to obtain a ligninsulfonate solution; nanocellulose whiskers are mixed with deionized water, ultrasonically dispersed, and then a silane coupling agent is added and stirred evenly to obtain modified nanocellulose whiskers; the ligninsulfonate solution and the modified nanocellulose whiskers are mixed to obtain a first mixture; 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; 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 0.05-0.1 MPa, the temperature is 60-100℃, and the time is 8-12 h; the total mass concentration of the montmorillonite and graphene oxide solution is 0.5-2 mg / mL, the ascorbic acid is 5%-10% of the mass of montmorillonite, and the freezing temperature of the freeze-drying process is -40~-20℃, and the time is 20-24 h.

8. The method for preparing a high heat-resistant melamine resin composite material according to claim 5, characterized in that, 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.

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 20-30 MPa for 20-40 minutes.

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