Rapidly-cured flame-retardant epoxy resin and preparation method thereof

Abstract

The invention relates to the field of high polymer materials, and discloses a fast-curing flame-retardant epoxy resin, which comprises the following components by mass: 55-60 parts of a phosphorus-containing epoxy monomer; 18-22 parts of a self-triggering curing agent; 10-14 parts of a nanocellulose reinforcing material with the surface modified with a polyphenol coating; 7-9 parts of a dynamic boric acid ester cross-linking agent; the phosphorus-containing epoxy monomer is phosphonated epoxy resin generated by reaction of cardanol and phosphorus oxychloride, the epoxy value is 0.48-0.55 mo l / 100g, the self-triggering curing agent is an epoxypropane grafted product of thioacetamide and 2-ureido-4 [1H]-pyrimidone, the active hydrogen equivalent is 200-220 g / eq, and the diluent is a diluent. The dynamic boric acid ester cross-linking agent is an epoxy glyceryl ether cross-linking product of phenylboronic acid and catechol, the dynamic bond exchange activation energy is 40-50 kJ / mol, the diluent is cardanol glycidyl ether or neopentyl glycol diglycidyl ether, and the viscosity of the diluent is 50-100 mPa.s. According to the invention, through molecular collaborative design of phosphonate groups and dynamic borate bonds, flame-retardant and self-repairing functions are fused in a single material system.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, specifically to a fast-curing flame-retardant epoxy resin and its preparation method. Background Technology

[0002] Epoxy resins are widely used in electronic packaging, aerospace, and architectural coatings due to their excellent adhesion, chemical resistance, and mechanical strength. However, traditional epoxy resins are flammable and lack self-healing capabilities, making them prone to performance degradation due to microcrack propagation during long-term service. As industrial equipment evolves towards lightweight and high reliability, the development of epoxy resin materials with both flame-retardant and self-healing functions has become an urgent industry need.

[0003] Current technologies largely rely on physical blending modifications to achieve flame retardancy and self-healing functions. For example, flame retardancy is improved by adding halogenated / phosphorus-based flame retardants, or self-healing capabilities are imparted by introducing microencapsulated repair agents. While these methods can improve material properties to some extent, the poor physical compatibility between functional components and the resin matrix easily leads to problems such as interface weakening and uneven dispersion. Furthermore, the independent addition of flame retardants and repair agents increases the complexity of the material system, narrows the processing window, and makes it difficult to meet the demands of high-performance integration.

[0004] However, the aforementioned physical blending technology has a fundamental flaw: flame retardants and repair agents are difficult to form stable chemical bonds in the resin matrix, and phase separation or functional component migration easily occurs during long-term use. This bottleneck causes the flame retardant efficiency of the material to decrease with the extension of service time, and the self-healing function fails prematurely due to the depletion of the repair agent. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a fast-curing flame-retardant epoxy resin and its preparation method. Traditional epoxy resins suffer from unstable performance and insufficient long-term effectiveness due to uneven dispersion of functional components and weakened interfaces caused by physical blending of flame retardants and self-healing agents.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a fast-curing flame-retardant epoxy resin, comprising the following components by weight:

[0007] 55-60 parts of phosphorus-containing epoxy monomer;

[0008] 18-22 parts of self-triggered curing agent;

[0009] 10-14 parts of nanocellulose-reinforced material with surface-modified polyphenol coating;

[0010] 7-9 parts of dynamic borate ester crosslinking agent;

[0011] 1-3 parts diluent.

[0012] Preferably, the phosphorus-containing epoxy monomer is a phosphonate esterified epoxy resin generated by the reaction of cashew phenol and phosphorus oxychloride, with an epoxy value of 0.48-0.55 mol / 100g.

[0013] Preferably, the self-triggered curing agent is a propylene oxide graft product of thioacetamide and 2-ureido-4[1H]-pyrimidinone, with an active hydrogen equivalent of 200-220 g / eq.

[0014] Preferably, the dynamic borate ester crosslinking agent is a crosslinking product of phenylboronic acid and catechol with an epoxy glycerol ether, and the dynamic bond exchange activation energy is 40-50 kJ / mol.

[0015] Preferably, the diluent is cashew phenol glycidyl ether or neopentyl glycol diglycidyl ether, with a viscosity of 50-100 mPa·s.

[0016] This invention also provides a method for preparing a rapidly curing flame-retardant epoxy resin, comprising the following steps:

[0017] Step 1: A phosphorus-containing intermediate is synthesized through phosphonate esterification reaction, and then a phosphorus-containing epoxy monomer is prepared by epoxy grafting.

[0018] Step 2: Prepare a self-triggered curing agent by combining thioamides with supramolecular units;

[0019] Step 3: Vertically grow polydopamine nanofibers on the surface of nanocellulose through dopamine oxidative polymerization;

[0020] Step 4: Prepare dynamic borate ester crosslinking agent via dynamic bond crosslinking reaction of borate ester;

[0021] Step 5: Mix the mixture obtained in steps 1 to 4 with a diluent, and prepare a resin prepolymer by microwave-assisted in-situ composite.

[0022] Step 6: Curing is completed by triggering the cross-linking reaction through low-temperature self-exothermic reaction.

[0023] Preferably, step one further includes:

[0024] Cashew phenol reacts with phosphorus oxychloride at 60-70℃ to form a phosphorus-containing intermediate;

[0025] The phosphorus-containing intermediate was reacted with epichlorohydrin at 85-95°C for 5-6 hours.

[0026] Preferably, step two further includes:

[0027] Thioacetamide and UPy were refluxed in toluene for 10-12 hours;

[0028] Grafting with propylene oxide is carried out at a temperature of 45-55℃ for 4-5 hours.

[0029] Preferably, step three further includes:

[0030] Bio-based cellulose whiskers were dispersed in Tris buffer.

[0031] Add dopamine hydrochloride at a mass ratio of 1.8-2.2:1 and react with oxygen for 22-24 hours.

[0032] Preferably, step four further includes:

[0033] Benzylboronic acid reacts with catechol at 70-80℃ to form a borate ester prepolymer;

[0034] The prepolymer is crosslinked with glycidyl ether at 75-85℃ for 3-4 hours, and the dynamic bond exchange activation energy is 40-50kJ / mol.

[0035] This invention provides a fast-curing flame-retardant epoxy resin and its preparation method. It has the following beneficial effects:

[0036] 1. This invention integrates flame retardancy and self-healing functions into a single material system through the molecular synergistic design of phosphonate groups and dynamic borate bonds. Compared with existing technologies that rely on physically blended flame retardants or exogenous repair agents, this invention completely solves the defects of uneven dispersion of functional components and weakened interfaces, achieving stability and long-lasting performance of the material.

[0037] 2. This invention is based on the controllable dissociation characteristics of supramolecular hydrogen bond networks, enabling the curing reaction to be autonomously triggered and completed rapidly at low temperatures. Traditional processes rely on high-temperature heating or complex catalytic systems. This invention replaces harsh external conditions with molecular dynamics, overcoming the stringent limitations of traditional curing technologies on energy consumption and equipment.

[0038] 3. This invention achieves vertically oriented growth and interface strengthening of nanocellulose through the synergistic effect of dopamine oxidative polymerization and oxygen flow rate. In existing technologies, nanofillers tend to agglomerate or be randomly distributed, resulting in low mechanical reinforcement efficiency. This invention overcomes this bottleneck with a dual chemical-process control strategy.

[0039] 4. This invention avoids structural defects in materials caused by process runaway by setting critical thresholds for key parameters such as microwave power and oxygen flow rate. Traditional methods often lead to sudden performance changes or batch instability due to parameter exceeding limits. This invention achieves process robustness through boundary adaptation, significantly improving industrial feasibility. Attached Figure Description

[0040] Figure 1 This is a schematic diagram of the method flow of the present invention. Detailed Implementation

[0041] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0042] This invention provides a fast-curing flame-retardant epoxy resin, comprising the following components by weight:

[0043] 55-60 parts of phosphorus-containing epoxy monomer;

[0044] The product is prepared by reacting cashew phenol with phosphorus oxychloride to generate a phosphonate intermediate, which is then grafted with epichlorohydrin to form epoxy groups. During combustion, the phosphonate groups decompose into phosphoric acid compounds, catalyzing the carbonization of the resin to form a dense char layer (condensed-phase flame retardancy), while simultaneously releasing PO· free radicals (gas-phase flame retardancy), achieving a two-phase synergistic flame retardancy.

[0045] 18-22 parts of self-triggered curing agent;

[0046] Thioacetamide was grafted onto 2-ureido-4[1H]-pyrimidinone (UPy) via propylene oxide. The supramolecular hydrogen bond network of UPy dissociates directionally at 60-80°C, releasing highly active thioacetamide groups, which triggers rapid crosslinking of the epoxy resin without the need for external high-temperature input.

[0047] 10-14 parts of nanocellulose-reinforced material with surface-modified polyphenol coating;

[0048] Vertical nanofibers (80-100 nm in length) are formed on the surface of nanocellulose through dopamine oxidative polymerization. The dopamine coating provides phenolic hydroxyl and amino groups, enhancing hydrogen bonding with the resin matrix; the vertical nanofibers improve tensile strength and crack resistance through mechanical interlocking.

[0049] 7-9 parts of dynamic borate ester crosslinking agent;

[0050] Benzylboronic acid reacts with catechol in epoxy glycerol ether to form dynamic borate ester bonds (BO bonds). These bonds can be reversibly broken and reformed at 80-100℃, endowing the material with self-healing capabilities and a repair efficiency >90%.

[0051] 1-3 parts diluent;

[0052] Cashew phenol glycidyl ether or neopentyl glycol diglycidyl ether are selected. Their low viscosity (50-100 mPa·s) reduces the viscosity of resin mixing while retaining reactivity, avoiding curing defects caused by non-reactive diluents.

[0053] The phosphorus-containing epoxy monomer is a phosphonate esterified epoxy resin produced by the reaction of cashew phenol and phosphorus oxychloride, with an epoxy value of 0.48-0.55 mol / 100g.

[0054] The self-triggered curing agent is a propylene oxide graft product of thioacetamide and 2-ureido-4[1H]-pyrimidinone, with an active hydrogen equivalent of 200-220 g / eq.

[0055] The dynamic borate ester crosslinking agent is a crosslinking product of phenylboronic acid and catechol with an epoxyglycerol ether, and the dynamic bond exchange activation energy is 40-50 kJ / mol.

[0056] The diluent is cashew phenol glycidyl ether or neopentyl glycol diglycidyl ether, with a viscosity of 50-100 mPa·s.

[0057] The preparation method of a fast-curing flame-retardant epoxy resin described below can be referred to in correspondence with the fast-curing flame-retardant epoxy resin described above.

[0058] See appendix Figure 1 A method for preparing a fast-curing flame-retardant epoxy resin includes the following steps:

[0059] Step 1: A phosphorus-containing intermediate is synthesized through phosphonate esterification reaction, and then a phosphorus-containing epoxy monomer is prepared by epoxy grafting.

[0060] Step 2: Prepare a self-triggered curing agent by combining thioamides with supramolecular units;

[0061] Step 3: Vertically grow polydopamine nanofibers on the surface of nanocellulose through dopamine oxidative polymerization;

[0062] Step 4: Prepare dynamic borate ester crosslinking agent via dynamic bond crosslinking reaction of borate ester;

[0063] Step 5: Mix the mixture obtained in steps 1 to 4 with a diluent, and prepare a resin prepolymer by microwave-assisted in-situ composite.

[0064] Step 6: Curing is completed by triggering the cross-linking reaction through low-temperature self-exothermic reaction.

[0065] In this embodiment, the synthesis of phosphorus-containing epoxy monomers includes two key stages: the preparation of phosphonate intermediates and the grafting of epoxy groups, which are implemented as follows:

[0066] Preparation of phosphonate intermediates

[0067] In some embodiments, the phosphonate esterification reaction of cashew phenol with phosphorus oxychloride (POCl3) is carried out under the following conditions:

[0068] Reactant ratio: The molar ratio of cashew phenol to POCl3 is 1:1.1-1.3, preferably 1:1.2;

[0069] Catalyst: Add triethylamine (TEA) as an acid absorbent, at a concentration of 5%-8% of the molar weight of POCl3;

[0070] Reaction temperature: 80-85℃, reaction time: 4-5 hours;

[0071] Post-processing: After the reaction is completed, unreacted POCl3 and byproduct HCl are removed by vacuum distillation. The distillation pressure is controlled at 10-15 kPa and the temperature is ≤60℃.

[0072] Specifically, the general formula for phosphonate esterification reactions can be expressed as:

[0073]

[0074] Where Δ represents the heating condition, the reaction progress can be monitored by titration to check the concentration of free chloride ions, and the reaction endpoint is determined when the residual chloride ion content is <0.1%.

[0075] In one possible implementation, if the moisture content in the reaction system is too high (>0.5%), POCl3 will hydrolyze to generate phosphoric acid impurities. In this case, an additional dehydrating agent (such as a molecular sieve) needs to be added, or nitrogen bubbling can be used to remove water.

[0076] As an alternative, the epoxy grafting reaction uses epichlorohydrin (ECH) as the epoxidizing agent, and the reaction conditions include:

[0077] Material ratio: The molar ratio of phosphonate intermediate to ECH is 1:1.4-1.6;

[0078] Alkali catalysis: Add a 20%-25% NaOH solution, the amount of which is 1.05-1.10 times the molar amount of the phosphonate intermediate;

[0079] Reaction parameters: temperature 60-65℃, time 5-6 hours, stirring speed 200-300 rpm;

[0080] Purification process: After the reaction is complete, wash with hot water until neutral (pH 6.8-7.2), then dehydrate under reduced pressure at 70-75℃ until the water content is <0.3%.

[0081] In one possible implementation, if the epoxy value exceeds 0.55 mol / 100g, ECH can be added and the reaction time extended by 1-2 hours; if the epoxy value is below 0.48 mol / 100g, the NaOH solution concentration needs to be increased to 28%-30% to accelerate the ring-closing reaction.

[0082] The epoxy value and phosphorus content of the phosphorus-containing epoxy monomers obtained in this step directly affect the crosslinking efficiency and flame retardant properties of the subsequent self-triggered curing agent. For example, when the epoxy value is >0.55mol / 100g, the prepolymer is prone to gelation in the microwave-assisted composite stage of step five; while insufficient phosphorus content (such as cashew nut shell powder: POCl3 <1:1.1) will result in an LOI value of less than 30% after curing in step six.

[0083] The composite reaction of thioacetamide and UPy: Thioacetamide and UPy are mixed at a molar ratio of 1:1.0-1.2, dissolved in a polar solvent (e.g., acetone or tetrahydrofuran), and propylene oxide is added as a linker for grafting. Preferably, the molar amount of propylene oxide is 1.1-1.3 times the total molar amount of thioacetamide and UPy.

[0084] Reaction conditions control: Under inert gas protection, heat to 50-55℃ and stir continuously for 6-8 hours. It should be noted that temperatures below 50℃ may result in insufficient grafting rate, while temperatures above 60℃ may easily trigger the thermal decomposition of thioacetamide.

[0085] Specifically, UPy forms a quadruple hydrogen bond network through its urea group and pyrimidinone group, while the epoxy group of propylene oxide undergoes a ring-opening reaction with the amino group of thioacetamide, generating a dynamically responsive grafted structure. For example, the general reaction formula can be represented as:

[0086] Thioacetamide + UPy + propylene oxide → thioacetamide-UPy graft compound;

[0087] In one possible implementation, the presence of trace amounts of water (>0.2%) in the reaction system can cause premature dissociation of the UPy hydrogen bond network. In this case, an additional dehydrating agent (such as anhydrous sodium sulfate) or solvent azeotropic dehydration is required.

[0088] Understandably, after the reaction is complete, the solvent needs to be removed by evaporation, followed by the removal of unreacted thioacetamide and UPy monomers using a Soxhlet extractor with diethyl ether as the washing agent. Preferably, the washing time is 6-8 hours, until the extract shows no characteristic absorption peak of thioacetamide (λ = 265 nm) when detected by ultraviolet spectroscopy.

[0089] It should be noted that the active hydrogen equivalent of the curing agent is determined by titration: the curing agent sample is reacted with an excess of epoxy resin prepolymer, and the unreacted epoxy groups are back-titrated using the hydrochloric acid-acetone method to calculate the molar amount of active hydrogen consumed. Preferably, the active hydrogen equivalent is controlled within the range of 200-220 g / eq to ensure the efficiency of low-temperature self-exothermic triggering crosslinking in step six.

[0090] For example, if the grafting reaction yield is less than 80% (calculated by 1H NMR spectrum), propylene oxide can be added and the reaction time extended by 2-3 hours; if insoluble gel appears in the product (due to the self-polymerization of thioacetamide), a polymerization inhibitor (such as hydroquinone) needs to be added at the beginning of the reaction, with the amount added being 0.05%-0.1% of the total mass of the reaction system.

[0091] The dynamic hydrogen bond network of the self-triggered curing agent plays a crucial role in the low-temperature curing stage of step six: when heated to 60-80℃, the UPy hydrogen bonds preferentially dissociate, releasing the thioacetamide active groups and triggering rapid crosslinking of the epoxy resin. It should be noted that if the UPy ratio is insufficient (molar ratio < 1:1.0), the hydrogen bond network dissociation temperature will significantly increase, leading to a prolonged curing time; while an excess of thioacetamide (molar ratio > 1:1.2) may shorten the prepolymer's shelf life.

[0092] As an alternative, surface finishing processes include the following key steps:

[0093] Nanocellulose pretreatment: Disperse nanocellulose in a buffer solution and adjust the pH to weakly alkaline. Preferably, use Tris-HCl buffer (pH 8.5-9.0) at a concentration of 0.01-0.05M to ensure a suitable rate for the auto-oxidative polymerization of dopamine.

[0094] Dopamine oxidative polymerization: Dopamine hydrochloride is added to the dispersion system, and the mass ratio of dopamine to nanocellulose is controlled at 1.8:1-2.2:1. It should be noted that too low a dopamine ratio (<1.8:1) will result in insufficient coverage of nanofibers, while too high a ratio (>2.2:1) may cause dopamine to self-aggregate into spherical particles.

[0095] Specifically, dopamine undergoes oxidative self-polymerization under weakly alkaline conditions to form polydopamine nanofibers. For example, the reaction is carried out in an environment with a continuous oxygen flow rate of 0.8-1.0 L / min, a reaction temperature controlled at 35-45°C, a stirring rate of 200-300 rpm, and a reaction duration of 20-24 hours. It is understood that oxygen acts as an oxidant in the formation of the quinone structure of dopamine, while a flow rate that is too low (<0.5 L / min) will lead to incomplete polymerization and a significant reduction in the length of the nanofibers.

[0096] In one possible implementation, after the reaction, the surface-modified nanocellulose is obtained by centrifugation, followed by freeze-drying to remove residual moisture. Preferably, the centrifugation rate is 8000-10000 rpm for 10-15 minutes; the freeze-drying conditions are pre-freezing at -50°C to -40°C for 4-6 hours, followed by drying under a vacuum of <10 Pa for 24-36 hours.

[0097] It should be noted that the length and distribution density of the vertical nanofibers can be characterized using scanning electron microscopy (SEM). For example, after gold sputtering of the dried sample, the nanofibers were observed to be radially and vertically arranged on the cellulose surface, with a length distribution of 80-120 nm and a diameter of 10-20 nm. If nanofibers are observed to be agglomerated or flattened, it may be due to structural damage caused by ice crystal growth during freeze-drying. In this case, the pre-freezing rate needs to be optimized or critical point drying should be used as an alternative.

[0098] For example, if the solution rapidly turns black during the reaction (due to excessive oxidation of dopamine), it indicates that the oxygen flow rate is too high or the temperature is out of control. In this case, the oxygen supply should be immediately reduced to below 0.5 L / min and the system cooled to below 30°C. If necessary, ascorbic acid (0.1%-0.3% by mass) should be added to slow down the oxidation rate.

[0099] Understandably, if the nanocellulose is unevenly dispersed (e.g., visible agglomerates appear), the ultrasonic time needs to be extended (e.g., 30-60 minutes) or the ultrasonic power increased to 300-400W during the pretreatment stage. Preferably, the ultrasonic frequency is selected at 20-40kHz to avoid cellulose fiber breakage.

[0100] As an alternative, the synthesis of dynamic borate crosslinking agents includes the following key processes:

[0101] The condensation reaction of phenylboronic acid and catechol: phenylboronic acid and catechol are mixed in a stoichiometric ratio, dissolved in a polar aprotic solvent, and epoxy glycerol ether is added as a crosslinking bridge agent. Preferably, the molar ratio of phenylboronic acid to catechol is 1:1.0-1.1, and the molar amount of epoxy glycerol ether is 1.2-1.5 times that of phenylboronic acid.

[0102] Catalyst selection: A weak acidic catalyst (such as p-toluenesulfonic acid) is used to accelerate the formation of borate ester bonds, with an addition amount of 0.1%-0.3% of the total mass of the reaction system. It should be noted that strong acidic catalysts (such as concentrated sulfuric acid) easily lead to ring-opening side reactions of epoxyglycerol ethers and should be avoided.

[0103] Specifically, the reaction is carried out under nitrogen protection, with the temperature controlled at 60-70℃, the stirring rate maintained at 300-400 rpm, and the reaction time at 8-12 hours.

[0104] In one possible implementation, after the reaction is complete, the solvent is removed by vacuum distillation, followed by column chromatography (silica gel packing, eluent is an ethyl acetate / petroleum ether mixture) to separate unreacted phenylboronic acid and byproducts. Preferably, the eluent ratio is 1:3 (v / v), and the main product fraction is collected and vacuum dried at 40-50°C to constant weight.

[0105] For example, if gelation occurs during the reaction (due to excessive glycidyl ether or excessively high temperature), the reaction must be terminated immediately and the system diluted with solvent. Preferably, tetrahydrofuran or dimethyl sulfoxide is used as the diluent, and the amount added is 2-3 times the volume of the reaction system, after which the reaction parameters are readjusted.

[0106] In one possible implementation, if unreacted catechol remains in the product (detected by thin-layer chromatography), it can be further purified by secondary column chromatography or recrystallization (using an ethanol / water mixture as the solvent).

[0107] As an alternative, microwave-assisted composite processes include the following key operations:

[0108] Multi-component sequential mixing: Add phosphorus-containing epoxy monomer, dynamic borate ester crosslinking agent, and diluent to a high-speed disperser in sequence, and stir at low speed for 5-10 minutes to form a homogeneous matrix; then add surface-modified nanocellulose reinforcing material, gradually increase the stirring speed to 800-1000 rpm, and continue to disperse for 20-30 minutes; finally add self-triggered curing agent to avoid premature contact with epoxy groups to prevent pre-crosslinking.

[0109] Microwave irradiation treatment: The mixture is transferred to a microwave reactor, and the microwave power is preferably set to 180-220W, the frequency to 2.45GHz, and the irradiation time to 8-12 minutes. It should be noted that the non-thermal effect of microwaves can promote the hydrogen bonding association between polydopamine nanofibers and epoxy groups on the surface of nanocellulose, and at the same time initiate local pre-crosslinking of dynamic borate ester bonds.

[0110] Specifically, the system temperature needs to be monitored in real time during microwave treatment, with a temperature control range of 50-65℃. For example, when the temperature exceeds 70℃, the supramolecular hydrogen bonds of the self-triggered curing agent may dissociate prematurely. In this case, intermittent microwave radiation (on / off ratio of 3:1) or an external circulating water cooling system needs to be started.

[0111] For example, if localized gelation occurs during microwave treatment (manifested as a sharp increase in viscosity to >8000 mPa·s), irradiation must be stopped immediately and a diluent (1%-2%) added, while simultaneously cooling the system to below 40°C for redispersibility. Preferably, the type of diluent should be consistent with that used in steps one to four to avoid phase separation caused by compatibility differences.

[0112] It should be noted that if the prepolymer is stored for more than 48 hours (at 25°C), the dynamic borate ester bonds may slowly recombine, leading to viscosity fluctuations. In this case, the fluidity can be restored by ultrasonic treatment (200W power, 5 minutes).

[0113] As an alternative, the low-temperature self-exothermic curing process includes the following key operations:

[0114] Prepolymer mold loading: Inject the resin prepolymer obtained in step five into a mold preheated to 60-70°C. The surface of the mold cavity is preferably coated with a release agent (such as silicone oil compound). The filling rate is controlled at 95%-98% to avoid curing shrinkage causing bubble defects.

[0115] Stepped temperature control triggering: In the initial stage, the mold is placed in an environment of 60-65℃ and kept at that temperature for 5-8 minutes to partially dissociate the UPy hydrogen bond network of the self-triggered curing agent; then the temperature is raised to 80-85℃ and maintained for 10-15 minutes to drive the thioacetamide groups and epoxy groups to fully crosslink.

[0116] Specifically, the self-exothermic effect originates from the coupling of the endothermic dissociation of UPy hydrogen bonds and the exothermic reaction of thioacetamide-epoxy. For example, when the degree of dissociation of UPy hydrogen bonds reaches 30%-40%, the system temperature automatically rises to 85-90℃ due to the exothermic reaction of thioacetamide, triggering the synchronous recombination of dynamic borate ester bonds (activation energy adaptation window).

[0117] For example, if the internal temperature gradient of the mold is greater than 10℃ / cm (detected by infrared thermal imaging), it may cause uneven curing. In this case, it is necessary to adjust the heat conduction design of the mold (such as embedding copper heat-conducting sheets) or reduce the heating rate to 2-3℃ / min.

[0118] Understandably, if the degree of curing does not reach 80% during the heat preservation stage (as determined by the gel time test), an accelerator (such as imidazole compounds, added at a rate of 0.05%-0.1%) needs to be added or the heat preservation time extended by 5-10 minutes. Preferably, the accelerator should be added during the premixing stage in step five to avoid excessively high local concentrations that could lead to explosive polymerization.

[0119] Step one also includes:

[0120] Cashew phenol reacts with phosphorus oxychloride at 60-70℃ to form a phosphorus-containing intermediate;

[0121] The phosphorus-containing intermediate was reacted with epichlorohydrin at 85-95°C for 5-6 hours.

[0122] Step two also includes:

[0123] Thioacetamide and UPy were refluxed in toluene for 10-12 hours;

[0124] Grafting with propylene oxide is carried out at a temperature of 45-55℃ for 4-5 hours.

[0125] Step three also includes:

[0126] Bio-based cellulose whiskers were dispersed in Tris buffer.

[0127] Add dopamine hydrochloride at a mass ratio of 1.8-2.2:1 and react with oxygen for 22-24 hours.

[0128] Step four also includes:

[0129] Benzylboronic acid reacts with catechol at 70-80℃ to form a borate ester prepolymer;

[0130] The prepolymer is crosslinked with glycidyl ether at 75-85℃ for 3-4 hours, and the dynamic bond exchange activation energy is 40-50kJ / mol.

[0131] Example 1:

[0132] Formula (parts by weight);

[0133] Phosphorus-containing epoxy monomer: 58 parts (epoxy value 0.50 mol / 100g);

[0134] Self-triggered curing agent: 20 parts (active hydrogen equivalent 210 g / eq);

[0135] Nanocellulose reinforcing material: 12 parts (dopamine:cellulose = 2.0:1);

[0136] Dynamic borate ester crosslinking agent: 8 parts (activation energy 45kJ / mol);

[0137] Diluent: Neopentyl glycol diglycidyl ether, 2 parts (viscosity 80 mPa·s).

[0138] Preparation process parameters:

[0139] Synthesis of phosphorus-containing epoxy monomers;

[0140] Phosphonate esterification: Cashew nut powder and phosphorus oxychloride were reacted at a molar ratio of 1:1.3 at 65°C for 3.5 hours;

[0141] Epoxy grafting: epichlorohydrin and cashew phenol in a molar ratio of 1.5:1, reacted at 90°C for 5.5 hours, with tetrabutylammonium bromide catalyst (0.5wt%).

[0142] Preparation of self-triggered curing agent:

[0143] Supramolecular composite: Thioacetamide to UPy molar ratio 1:1.1, refluxed in toluene at 115℃ for 11 hours;

[0144] Propylene oxide grafting: 0.7 molar amount of propylene oxide was added, and the reaction was carried out at 50°C for 4.5 hours.

[0145] Nanocellulose reinforcement:

[0146] Dopamine oxidation: oxygen flow rate 0.8 L / min, reaction at 50 °C for 23 hours, followed by centrifugation and freeze drying.

[0147] Synthesis of dynamic crosslinking agents:

[0148] Esterification: phenylboronic acid and catechol in a molar ratio of 1:1.05 were reacted in ethanol solution at 75°C for 4.2 hours.

[0149] Crosslinking: Add 0.65 molar amounts of glycidyl ether and react at 80°C for 3.5 hours.

[0150] Resin prepolymerization and curing:

[0151] Microwave reaction: 200W, 50℃ for 30 minutes, resin viscosity 480mPa·s;

[0152] Curing: Preheat at 62℃ for 2 minutes, and self-exothermic to 83℃ within 7 minutes to complete cross-linking.

[0153] Example 2:

[0154] Formula adjustments:

[0155] Phosphorus-containing epoxy monomer: 60 parts (epoxy value 0.55mol / 100g, cashew phenol: POCl3 = 1:1.5);

[0156] Nanocellulose reinforcing material: 14 parts (dopamine:cellulose = 2.2:1).

[0157] Other components are the same as in Example 1:

[0158] Key process adjustments:

[0159] Phosphonate esterification reaction: react at 70℃ for 4 hours (to improve phosphorus conversion);

[0160] Dopamine oxidation: oxygen flow rate 1.0 L / min, reaction at 55 °C for 24 hours (nanofibers length increased to 90 nm);

[0161] Curing temperature: Preheat at 65℃ for 1.5 minutes, then self-exothermic to 85℃ after triggering.

[0162] Example 3:

[0163] Formula adjustments:

[0164] Self-triggered curing agent: 22 parts (active hydrogen equivalent 200g / eq, thioacetamide:UPy = 1:1.2)

[0165] Diluent: Cashew nut shell glycidyl ether, 3 parts (viscosity 50 mPa·s)

[0166] The other components are the same as in Example 1.

[0167] Key process adjustments:

[0168] Propylene oxide grafting: Reaction at 55℃ for 5 hours (to improve grafting rate);

[0169] Microwave reaction: 220W power, 45℃ for 25 minutes (viscosity reduced to 450mPa·s);

[0170] Curing trigger: Preheat at 60℃ for 3 minutes, and the self-exothermic peak temperature of 85℃ can be reached in just 6 minutes.

[0171] Comparative Example 1:

[0172] Compared with Example 1, the difference is that the phosphorus-containing epoxy monomer is replaced with ordinary bisphenol A type epoxy resin (E51, 58 parts, epoxy value 0.51mol / 100g), and all other aspects are the same.

[0173] Comparative Example 2:

[0174] Compared with Example 1, the difference is that the self-triggered curing agent is replaced with dicyandiamide (20 parts), and the curing conditions are changed to 120°C / 30 minutes, while the rest are the same.

[0175] Comparative Example 3:

[0176] Compared with Example 1, the difference is that the microwave reaction power is adjusted to 250W, and everything else is the same.

[0177] Comparative Example 4:

[0178] Compared with Example 1, the difference is that the mass ratio of dopamine to cellulose is adjusted to 1.5:1, and all other aspects are the same.

[0179] Comparative Example 5:

[0180] Compared to Example 2, the only difference is that the molar ratio of cashew phenol to phosphorus oxychloride is adjusted to 1:1.8, while all other aspects remain the same.

[0181] Comparative Example 6:

[0182] Compared with Example 2, the difference is that the oxygen flow rate for dopamine oxidative polymerization was adjusted to 0.3 L / min, while all other aspects are the same.

[0183] Comparative Example 7:

[0184] Compared with Example 3, the difference is that the molar ratio of thioacetamide to UPy is adjusted to 1:0.8, and all other aspects are the same.

[0185] Comparative Example 8:

[0186] Compared to Example 3, the difference is that the diluent is replaced with acetone (3 parts), otherwise they are the same.

[0187] Test Experiment 1: Verification of Flame Retardant and Self-Healing Functions

[0188] Experimental objective: To verify the necessity of phosphorus-containing epoxy monomers for flame retardant properties (LOI, char layer structure) and self-healing function, as well as the negative impact of excessive phosphorus content.

[0189] Control group:

[0190] Example 1: Phosphorus-containing epoxy monomer (58 parts, epoxy value 0.50 mol / 100g);

[0191] Comparative Example 1: Ordinary epoxy resin (E51, 58 parts, phosphorus-free);

[0192] Comparative Example 5: Phosphorus content exceeds limit (POCl) 3： Cashew phenol = 1:1.8, epoxy value 0.42mol / 100g).

[0193] Experimental steps:

[0194] Sample preparation:

[0195] Resin samples were prepared according to the formulations of Example 1, Comparative Example 1, and Comparative Example 5, and after curing, they were cut to standard test dimensions (100×10×3m). 3 ).

[0196] Limiting Oxygen Index (LOI) Test:

[0197] Using an HC-2C oxygen index meter, the lowest oxygen concentration (volume percentage) of the material in a nitrogen-oxygen mixture was tested according to GB / T2406-2008 standard.

[0198] Self-repair efficiency test:

[0199] A crack (5 mm in length) was pre-made on the sample surface, placed in an 80°C oven for 1 hour, and the crack width was measured after cooling.

[0200] Analysis of carbon layer morphology:

[0201] The samples were sputter-coated with gold after combustion, and the microstructure of the carbon layer was observed using SEM (SU5000, accelerating voltage 5kV).

[0202] Heat release rate test:

[0203] Using a cone calorimeter, the radiant power is 35 kW / m². 2 Record the total heat release (THR).

[0204] Tensile strength retention rate:

[0205] After self-repair, the sample was subjected to tensile testing (ASTM D638), and the percentage of the repaired strength relative to the original strength was calculated.

[0206] Table 1. Test Experiment 1: Comparison Data of Flame Retardant and Self-Healing Performance

[0207]

[0208] According to Table 1 above:

[0209] The molecular design of phosphorus-containing epoxy monomers, through the introduction of phosphonate groups, achieves a synergistic effect of flame retardancy in both the gas and condensed phases. In Example 1, the phosphoric acid substances produced by high-temperature decomposition catalyze the formation of a continuous and dense char layer (82.4 μm thick) in the resin, effectively isolating oxygen and heat transfer, which is directly related to the significant increase in LOI value to 33.7%. Simultaneously, the PO· radicals released by the phosphorus compounds quench the combustion chain reaction in the gas phase, further suppressing heat release (THR 19.3 MJ / m³). 2 In contrast, Comparative Example 1, due to the complete absence of phosphorus, had a loose carbon layer and a heat release as high as 48.6 MJ / m³. 2 This confirms the irreplaceable role of phosphorus in dual-phase flame retardants.

[0210] The reversible bonding mechanism of the dynamic borate ester crosslinking agent is the core driving force for its self-healing function. In Example 1, the recombination ability of the dynamic bonds at 80°C resulted in a crack healing rate of 91.2%, with a post-repair strength retention rate approaching 90%. This process relies on the moderate activation energy (45 kJ / mol) of the borate ester bond, allowing bond breaking and recombination to reach equilibrium under mild heating conditions. However, in Comparative Example 5, the excess phosphorus (POCl₂)... 3： Cashew phenol (1:1.8) causes distortion of the epoxy network structure, disrupts the chemical environment around the dynamic bonds, and reduces the self-repair rate to 63.8%, indicating that the phosphorus content needs to be strictly controlled within the scope of the claims to maintain the functional integrity of the dynamic bonds.

[0211] Although the vertical nanofiber structure of the nanocellulose-reinforced material was not tested independently in this experiment, its synergistic effect with the phosphorus-containing monomer indirectly affected the flame retardant and mechanical properties. In Example 1, the nanofibers, through physical interpenetration and strong interfacial adhesion to the polyphenol coating, improved the crack resistance of the char layer (SEM showed uniform pore size in the char layer), thereby enhancing flame retardant stability. In Comparative Example 5, excessive phosphorus may have interfered with the dispersion of nanocellulose in the resin matrix, leading to localized cracks in the char layer (45.3 μm thick but with structural defects), further verifying the sensitivity of component ratio to multi-scale synergistic effects. This multi-level structural design of molecular-nano-macro is the fundamental innovation of this invention in achieving integrated flame retardancy and self-healing.

[0212] Test Experiment 2: Low-Temperature Self-Triggered Curing and Process Adaptability Verification

[0213] Experimental objective: To verify the low-temperature self-triggering mechanism of thioacetamide-UPy supramolecular curing agent, and the effect of hydrogen bond density on curing speed and material properties.

[0214] Control group:

[0215] Example 1: Thioacetamide-UPy curing agent (active hydrogen equivalent 210 g / eq);

[0216] Comparative Example 2: Dicyandiamide curing agent (conventional high-temperature curing);

[0217] Example 3: Highly active curing agent (thioacetamide:UPy = 1:1.2, active hydrogen equivalent 200g / eq);

[0218] Comparative Example 7: Insufficient hydrogen bond density (thioacetamide:UPy = 1:0.8, active hydrogen equivalent 250 g / eq).

[0219] Experimental steps

[0220] Sample preparation:

[0221] Resin prepolymers were prepared according to the formulations of Example 1, Comparative Example 2, Example 3, and Comparative Example 7, and injected into a standard mold (size 50×50×2mm). 3 ).

[0222] Curing time and temperature monitoring:

[0223] The time from the start of heating to the resin reaching a fully cured state was recorded using a gel time tester (GT-1000).

[0224] The infrared thermal imager (FLIRT540) monitors the temperature distribution during the curing process in real time and records the self-exothermic peak temperature.

[0225] Curing degree test:

[0226] The degree of curing was determined by differential scanning calorimetry (DSC, TAQ200).

[0227] Mechanical property testing:

[0228] Tensile strength (ASTM D638 standard, Instron 5966 universal testing machine);

[0229] Glass transition temperature (Tg, the midpoint temperature of the second heating curve in DSC).

[0230] Hydrogen bond density characterization:

[0231] Fourier transform infrared spectroscopy (FTIR, Nicoleti S50) analysis of hydrogen bond characteristic peaks (3200-3500 cm⁻¹) -1Integrate the area and calculate the relative hydrogen bond density.

[0232] Table 2 Test Experiment 2: Effect of Low Temperature Curing and Hydrogen Bond Density on Performance

[0233]

[0234] According to Table 2 above:

[0235] Thioacetamide-UPy supramolecular curing agent undergoes directional dissociation under mild heating via a dynamic hydrogen bond network, releasing highly reactive amine groups that drive rapid crosslinking of epoxy resin. In Example 1, the hydrogen bond density (125.4 au) matched a suitable activation energy, shortening the curing time to 7.2 minutes. Simultaneously, the self-exothermic effect (peak temperature 83.5 °C) accelerated the reaction process. In contrast, the traditional dicyandiamide curing agent (Comparative Example 2), lacking hydrogen bond dynamism, required external high-temperature input (120 °C) and took as long as 30.5 minutes. This comparison directly verifies the core role of the supramolecular triggering mechanism in low-temperature, high-efficiency curing.

[0236] The regulation of hydrogen bond density significantly affects curing efficiency and material properties. In Example 3, by increasing the molar ratio of thioacetamide to UPy (1:1.2), the hydrogen bond density increased to 148.2 au, and the curing time was further reduced to 5.9 minutes. However, the excessively high crosslinking rate resulted in limited molecular chain arrangement (tensile strength 84.6 MPa, slightly lower than 87.3 MPa in Example 1). In contrast, Comparative Example 7 (hydrogen bond density 82.7 au) suffered from insufficient network stability and incomplete curing reaction (degree of curing 89.4%), resulting in a sharp drop in tensile strength to 71.2 MPa. This confirms the necessity of the lower limit of hydrogen bond density (thioacetamide:UPy ≥ 1:1) in the claims.

[0237] It is worth noting that while the traditional dicyandiamide curing system (Comparative Example 2) imparts a higher glass transition temperature (Tg 118.7℃) to the material, its reliance on high-temperature heating and lack of self-healing function highlight the breakthrough of this invention in low-temperature self-triggering and performance balance. This design strategy, which balances curing speed, mechanical strength, and processing efficiency through supramolecular dynamics, provides an innovative direction for the industrial application of epoxy resins.

[0238] Test Experiment 3: Verification of Nano-Reinforced Structure and Mechanical Properties

[0239] Experimental objective: To verify the enhancing effect of dopamine-modified vertical cellulose nanostructure on mechanical properties (tensile strength, flexural modulus), and the regulatory boundary of oxygen flow rate on nanofiber growth.

[0240] Control group:

[0241] Example 1: Dopamine:cellulose = 2.0:1, oxygen flow rate 0.8 L / min;

[0242] Comparative Example 4: Dopamine:cellulose = 1.5:1, oxygen flow rate 0.8 L / min;

[0243] Example 2: Dopamine:cellulose = 2.2:1, oxygen flow rate 1.0 L / min;

[0244] Comparative Example 6: Dopamine:cellulose = 2.2:1, oxygen flow rate 0.3L / min.

[0245] Experimental steps:

[0246] Preparation of nanocellulose:

[0247] Dopamine was oxidized and polymerized according to the parameters of the examples and comparative examples, centrifuged, and then freeze-dried to obtain nanocellulose powder.

[0248] Nanostructure characterization:

[0249] SEM (Hitachi SU3900) was used to observe the morphology of the nanofibers and measure their length and distribution density.

[0250] Atomic force microscopy (AFM, Bruker Dimension Icon) is used to analyze surface roughness.

[0251] Composite material preparation:

[0252] Nanocellulose was dispersed in the resin matrix at a ratio of 12-14 parts, ultrasonically treated for 30 minutes (power 300W), and then cut into standard samples after curing.

[0253] Mechanical property testing:

[0254] Tensile strength (ASTM D638, Instron 5966, rate 5 mm / min);

[0255] Three-point bending test (ASTM D790, span 64 mm, rate 2 mm / min);

[0256] Impact strength (simply supported beam, ASTM D6110, pendulum energy 5.5 J).

[0257] Interface-based analysis:

[0258] Dynamic mechanical analysis (DMA, TAQ800) was used to determine the energy storage modulus (E') and loss factor (tanδ).

[0259] Additional flame retardancy test:

[0260] Vertical burning test (UL94V), recording burning time and dripping pattern.

[0261] Table 3. Test Experiment 3: Correlation Data between Nanocellulose Structure and Mechanical Properties

[0262]

[0263] According to Table 3 above:

[0264] Vertical nanofibers formed by dopamine oxidative polymerization significantly enhance the mechanical properties of the resin matrix through interfacial hydrogen bonding and mechanical interlocking effects. In Example 1, the dopamine to cellulose mass ratio (2.0:1) ensured sufficient phenolic hydroxyl and amino group density, driving the nanofibers to grow vertically along the cellulose surface (length 84.3 nm), achieving a tensile strength of 87.2 MPa and a flexural modulus of 3.15 GPa. This enhancement mechanism stems from the bridging effect of the nanofibers in stress transmission—the increased aspect ratio allows for more uniform load distribution across the matrix. In contrast, Comparative Example 4 (dopamine ratio 1.5:1) suffered from insufficient interfacial bonding sites, resulting in short and unevenly dispersed nanofibers (length 52.7 nm) and a 21.4% decrease in tensile strength, confirming the critical value for dopamine ratio in regulating interfacial strengthening.

[0265] Oxygen flow rate, as the driving force for dopamine oxidative polymerization, directly determines the growth integrity and size distribution of the nanofibers. In Example 2, by increasing the oxygen flow rate to 1.0 L / min, the dopamine oxidative cross-linking was promoted, increasing the nanofiber length to 92.6 nm and the tensile strength to 89.7 MPa. Simultaneously, the long nanofibers formed a dense three-dimensional char layer (LOI 35.8%) during combustion. Conversely, in Comparative Example 6, due to insufficient oxygen flow rate (0.3 L / min), the dopamine polymerization reaction stalled, the nanofibers were only 38.4 nm in length and exhibited localized agglomeration, resulting in poor mechanical properties (impact strength 3.8 kJ / m²). 2 The simultaneous decline in flame retardancy (LOI 27.5%) reveals the irreversible effect of oxygen flow rate on the self-assembly of nanostructures.

[0266] The multi-level synergy between nanocellulose and phosphorus-containing monomers further amplifies the performance advantages. In Example 1, nanofibers are interspersed in the pores of the char layer formed by phosphorus catalysis (SEM shows pore size of 5-10 μm), delaying oxygen diffusion through physical barrier, thus increasing the LOI to 33.7%. In contrast, in Comparative Examples 4 and 6, due to nanostructural defects, the continuity of the char layer is disrupted (thickness decreases to 29.8 μm), resulting in a significant reduction in flame retardant efficiency. This multi-level design at the molecular scale (phosphorus catalysis), nanoscale (fiber reinforcement), and macroscopic scale (mechanical / flame retardant) constitutes the core logic of the performance breakthrough of this invention and also provides a theoretical basis for the precise matching of component ratios and process parameters.

[0267] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A fast-curing flame-retardant epoxy resin, characterized in that, Includes the following mass components: 55-60 parts of phosphorus-containing epoxy monomer; 18-22 parts of self-triggered curing agent; 10-14 parts of nanocellulose-reinforced material with surface-modified polyphenol coating; 7-9 parts of dynamic borate ester crosslinking agent; 1-3 parts diluent.

2. The rapid-curing flame-retardant epoxy resin according to claim 1, characterized in that, The phosphorus-containing epoxy monomer is a phosphonate-esterified epoxy resin generated by the reaction of cashew phenol and phosphorus oxychloride, with an epoxy value of 0.48-0.55 mol / 100g.

3. The rapid-curing flame-retardant epoxy resin according to claim 1, characterized in that, The self-triggered curing agent is a propylene oxide graft product of thioacetamide and 2-ureido-4[1H]-pyrimidinone, with an active hydrogen equivalent of 200-220 g / eq.

4. The rapid-curing flame-retardant epoxy resin according to claim 1, characterized in that, The dynamic borate ester crosslinking agent is a crosslinking product of phenylboronic acid and catechol with an epoxyglycerol ether, and the dynamic bond exchange activation energy is 40-50 kJ / mol.

5. The rapid-curing flame-retardant epoxy resin according to claim 1, characterized in that, The diluent is cashew phenol glycidyl ether or neopentyl glycol diglycidyl ether, with a viscosity of 50-100 mPa·s.

6. A method for preparing a fast-curing flame-retardant epoxy resin, comprising the fast-curing flame-retardant epoxy resin according to any one of claims 1-5, characterized in that, Includes the following steps: Step 1: A phosphorus-containing intermediate is synthesized through phosphonate esterification reaction, and then a phosphorus-containing epoxy monomer is prepared by epoxy grafting. Step 2: Prepare a self-triggered curing agent by combining thioamides with supramolecular units; Step 3: Vertically grow polydopamine nanofibers on the surface of nanocellulose through dopamine oxidative polymerization; Step 4: Prepare dynamic borate ester crosslinking agent via dynamic bond crosslinking reaction of borate ester; Step 5: Mix the mixtures obtained in steps 1 to 4 with a diluent, and prepare a resin prepolymer by microwave-assisted in-situ composite. Step 6: Curing is completed by triggering the cross-linking reaction through low-temperature self-exothermic reaction.

7. The method for preparing a rapidly curing flame-retardant epoxy resin according to claim 6, characterized in that, Step one also includes: Cashew phenol reacts with phosphorus oxychloride at 60-70℃ to form a phosphorus-containing intermediate; The phosphorus-containing intermediate was reacted with epichlorohydrin at 85-95°C for 5-6 hours.

8. The method for preparing a rapidly curing flame-retardant epoxy resin according to claim 6, characterized in that, Step two also includes: Thioacetamide and UPy were refluxed in toluene for 10-12 hours; Grafting with propylene oxide is carried out at a temperature of 45-55℃ for 4-5 hours.

9. The method for preparing a rapidly curing flame-retardant epoxy resin according to claim 6, characterized in that, Step three also includes: Bio-based cellulose whiskers were dispersed in Tris buffer. Add dopamine hydrochloride at a mass ratio of 1.8-2.2:1 and react with oxygen for 22-24 hours.

10. A method for preparing a rapidly curing flame-retardant epoxy resin according to claim 6, characterized in that, Step four also includes: Benzylboronic acid reacts with catechol at 70-80℃ to form a borate ester prepolymer; The prepolymer is crosslinked with glycidyl ether at 75-85℃ for 3-4 hours, and the dynamic bond exchange activation energy is 40-50kJ / mol.

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