Efficient catalytic fast curing phenolic resin and preparation method thereof

Abstract

The application discloses a kind of high-efficiency catalytic fast curing phenolic resin and preparation method, belong to high polymer material technical field.The molecular structure of the phenolic resin includes latent acid catalytic structural unit connected by covalent bond, which is derived from modified phenolic monomer, and the general structure is (HO) m -Ar-(L) n -SO2-O-R, wherein Ar is phenyl, m is 1 or 2, n is 0 or 1, and R is tert-butyl or trialkylsilyl group.The application also provides a preparation method of the resin, which is prepared by condensation reaction of conventional phenolic monomer, aldehyde monomer and the modified phenolic monomer under alkaline condition at 65-95℃, and dehydration under reduced pressure.The application realizes molecular integration of catalyst and resin skeleton, eliminates the diffusion mass transfer limitation of external catalyst, improves the curing rate, shortens the gel time, and the cured product has more excellent mechanical properties, thermal stability and higher carbon residue rate.

Description

Technical Field

[0001] This invention relates to the field of polymer materials and chemical synthesis technology, specifically to a highly efficient catalytically cured phenolic resin and its preparation method. Background Technology

[0002] Phenolic resins, as an important class of synthetic polymer materials, play an indispensable role in modern industry due to their excellent heat resistance, outstanding mechanical strength, chemical stability, and dimensional stability, which are conferred by the rigid benzene rings and high crosslinking density in their molecular structure. Their applications have widely penetrated into key technology areas such as high-performance composite matrix, special flame-retardant materials, precision molding compounds, wear-resistant materials, and porous catalyst supports, becoming one of the fundamental materials driving technological progress in related industries.

[0003] The final properties of phenolic resins largely depend on the three-dimensional network structure formed during their curing and crosslinking process. Therefore, precise control of the curing reaction process, especially the improvement of curing rate and catalytic efficiency, has always been a core issue in the research and application of this field.

[0004] Invention patent CN115678090B (hereinafter referred to as Prior Art 1) discloses a method for preparing ablation-resistant phenolic aerogel. By combining a phenolic resin solution with a ceramic precursor and utilizing a sol-gel process followed by liquid-phase impregnation and curing crosslinking, a cutting-edge material with both low density and high ablation resistance is successfully prepared. The dense structure formed by the ceramic phase at high temperatures enhances the material's ablation resistance.

[0005] The invention patent with announcement number CN115594938B (hereinafter referred to as Prior Art 2) discloses a method for preparing a high-efficiency flame-retardant board. This method involves introducing silicate esters into a phenolic resin system and using an acidic catalyst to catalyze the in-situ hydrolysis and condensation of the silicate esters, forming an interpenetrating structure of silica and phenolic resin networks. This effectively combines the high-temperature resistance and thermal insulation properties of silica with the char-forming properties of phenolic resin, thereby improving the flame-retardant rating and structural strength of the material.

[0006] However, with the continuous development of related technologies and the increasingly stringent requirements of industrial production for process efficiency, cost control and universality, the aforementioned technical solutions based on multi-component composites have some inherent characteristics at the principle level that have gradually revealed their limitations in dealing with new challenges.

[0007] In pursuit of the final macroscopic properties of the composite material, the curing kinetics of the phenolic resin itself are artificially coupled or even constrained by the reaction process of the second component (such as ceramic precursors or silicates). In such hybrid systems, the curing process of the phenolic resin is no longer an independent chemical reaction that can be directly optimized, but is transformed into a multi-step, multi-phase, or multi-reaction coupled system closely related to the inorganic phase formation process (such as impregnation, hydrolysis, and condensation).

[0008] In the prior art 1, the curing rate and final cross-linking degree of phenolic resin must be matched with the diffusion, wetting and thermal conversion kinetics of the ceramic precursor. The entire process window is limited to a narrow range in which two completely different material systems can react synergistically. This fundamentally limits the limit of the curing rate of phenolic resin, making it difficult to shorten the overall process cycle.

[0009] In existing technology 2, the primary role of the acidic catalyst is to ensure the effective hydrolysis of silicates; its catalytic activity and selectivity for the phenolic resin polycondensation reaction may not be optimal. The curing efficiency of the entire system depends on a precise balance between the two chemical reaction rates, and is highly sensitive to process parameters (such as pH, temperature, and material ratio), which increases the difficulty and uncertainty of production control. This strategy of sacrificing the independent optimization space of the core reaction (phenolic resin curing) in exchange for composite performance has led to existing technologies generally facing difficulties in achieving breakthroughs in curing efficiency, poor process robustness, and secondary problems such as increased costs and equipment corrosion due to the introduction of additional components and specific catalysts.

[0010] In exploring ways to improve the overall performance of phenolic resin-based materials, existing technologies are generally trapped in an inherent constraint of "performance-process". That is, while the material performance is improved through complex composite system design, the potential for improving the efficiency of the phenolic resin curing reaction itself is also limited. Summary of the Invention

[0011] The purpose of this invention is to provide a highly efficient catalytic and rapid curing method for phenolic resin, which can effectively solve the technical problems of limited catalytic efficiency, narrow process window and complex control caused by the coupling of multi-component composite system in the curing process of phenolic resin in the prior art.

[0012] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0013] A highly efficient catalytically curable phenolic resin, wherein the molecular structure of the phenolic resin contains latent acid catalytic structural units that are covalently linked and undergo irreversible chemical cleavage at a preset curing activation temperature to generate an acidic catalytic center in situ.

[0014] The latent acid catalytic structural unit is derived from a modified phenolic monomer, which has a chemical structure containing at least one hydroxyl group and a thermally dissociative latent acid catalytic functional group on a benzene ring.

[0015] The structure of the thermally induced dissociative latent acid catalytic functional group is characterized by containing a sulfonate group (-SO2-OR), wherein the sulfur atom of the sulfonate group is directly or through an arylene or alkylene bridging group connected to the benzene ring of the modified phenolic monomer, and the R group is a leaving group that can leave through heterolytic cleavage or concerted elimination reaction at the preset curing activation temperature, thereby hydrolyzing or converting the sulfonate group into a sulfonic acid group (-SO3H).

[0016] Furthermore, the general structural formula of the modified phenolic monomer is (I):

[0017] (HO) m -Ar-(L) n -SO2-OR;

[0018] Where Ar is phenyl; m is an integer of 1 or 2, representing the number of hydroxyl groups on the benzene ring;

[0019] L represents a chemical bond or a methylene group (-CH2-), indicating the way the sulfonate group is linked to the benzene ring;

[0020] n is an integer of 0 or 1. When n is 0, L is a chemical bond, and the sulfur atom of the sulfonate group is directly attached to the Ar benzene ring. When n is 1, L is methylene; R is tertiary alkyl or trialkylsilyl.

[0021] When R is a tertiary alkyl group, it is preferably tert-butyl. When R is a trialkylsilyl group, it is preferably trimethylsilyl or triethylsilyl. The hydroxyl (-OH) and the -(L)n-SO2-OR functional group are substituted at ortho, meta, or para positions on the Ar benzene ring, with para substitution being preferred to reduce steric hindrance and obtain higher reactivity.

[0022] In one embodiment of this invention, the modified phenolic monomer is tert-butyl 4-hydroxybenzenesulfonate. The modified phenolic monomer exhibits excellent chemical stability at temperatures below 120°C, maintaining its sulfonate structure without producing any acidic substances. When the ambient temperature rises to the curing activation temperature range of 130°C to 160°C, the tert-butyl sulfonate structure undergoes a specific E1 elimination reaction, breaking the covalent bonds to generate a highly catalytically active 4-hydroxybenzenesulfonic acid structural unit, releasing inert isobutylene gas. This in-situ generated sulfonic acid group, as a strong protic acid, is far more acidic than the p-toluenesulfonic acid or phenolsulfonic acid used in traditional phenolic resin curing, and can efficiently catalyze the condensation reaction between phenolic hydroxyl groups and hydroxymethyl groups, as well as the formation of methylene bridges.

[0023] The highly efficient catalytically curable phenolic resin provided by this invention is prepared by polycondensation reaction of conventional phenolic monomers, aldehyde monomers and the modified phenolic monomers under specific process conditions.

[0024] The conventional phenolic monomer is at least one selected from phenol, resorcinol, cresol, or bisphenol A. The aldehyde monomer is at least one selected from formaldehyde, paraformaldehyde, or furfural. During the preparation process, the molar amount of the modified phenolic monomer is between 0.2% and 8.0% of the total molar amount of phenolic monomers (the sum of conventional phenolic monomers and modified phenolic monomers). This ratio range ensures that a sufficient density of catalytic sites can be generated during the curing stage to drive rapid curing, while avoiding the adverse effects of excessively high concentrations of latent catalytic units on the storage stability of the resin prepolymer.

[0025] The preparation process of the highly efficient catalytically cured phenolic resin strictly controls the reaction temperature to be lower than the decomposition and activation temperature of the thermally ionized latent acid catalytic functional groups. Specifically, the preparation process is carried out in the presence of an alkaline catalyst, which is sodium hydroxide, potassium hydroxide, ammonia, or triethylamine, and its amount is 0.1% to 1.5% of the total phenolic monomer mass. The reaction temperature is controlled within the range of 65℃ to 95℃ throughout the process.

[0026] Both conventional and modified phenolic monomers undergo electrophilic addition reactions with aldehyde monomers to form intermediates containing hydroxymethyl groups. These intermediates then undergo preliminary polycondensation to form linear or slightly branched resin prepolymers with molecular weights ranging from 600 to 2500 Daltons. During this process, covalently bonded latent acid-catalyzing functional groups (such as tert-butyl 4-hydroxybenzenesulfonate units) are integrated into the polymer chain as part of the structure, maintaining chemical stability and avoiding degradation. After the reaction, water and unreacted free molecules generated during the reaction are removed by vacuum dehydration to obtain the final phenolic resin containing latent acid-catalyzing structural units, which exists as a liquid or solid at room temperature.

[0027] In addition, this invention also discloses a method for preparing a highly efficient catalytically cured phenolic resin, which specifically includes the following steps:

[0028] Step S1: Preparation of modified phenolic monomers.

[0029] Using 4-hydroxybenzenesulfonic acid as a starting material, N,N'-dicyclohexylcarbodiimide (DCC) as a dehydration condensing agent and 4-dimethylaminopyridine (DMAP) as a catalyst were added slowly dropwise to dichloromethane, an aprotic polar solvent, under ice-water bath conditions ranging from 0°C to 5°C. After the addition was complete, the reaction system was heated to 25°C and stirred continuously for 18 to 24 hours. After the reaction was completed, the byproduct dicyclohexylurea (DCU) was removed by filtration, the solvent was removed by rotary evaporation of the filtrate, and the crude product was purified by silica gel column chromatography using a 1:5 volume ratio of ethyl acetate to n-hexane as the eluent, finally yielding a high-purity white solid product, tert-butyl 4-hydroxybenzenesulfonic acid. The structure of the product was confirmed by ¹H NMR and Fourier transform infrared spectroscopy (FT-IR) to ensure the accuracy of its chemical structure.

[0030] Step S2: Synthesis of latent autocatalytic phenolic resin prepolymer.

[0031] In a four-necked flask equipped with a mechanical stirrer, reflux condenser, thermometer, and dropping funnel, conventional phenolic monomers (phenol), modified phenolic monomers (tert-butyl 4-hydroxybenzenesulfonate), and a 30 wt% aqueous sodium hydroxide solution as an alkaline catalyst were added in a predetermined molar ratio. The mixture was heated to 70°C, and a 37 wt% aqueous formaldehyde solution was added dropwise at a uniform rate over 45 minutes, ensuring the system temperature did not exceed 80°C. After the addition was complete, the temperature of the reaction system was maintained between 85°C and 90°C, and the polycondensation reaction was continued for 2 hours. During the reaction, the endpoint was controlled by monitoring the viscosity of the system or the free formaldehyde content. Heating was stopped when the viscosity of the resin prepolymer (measured at 25°C) reached 800 mPa·s to 1200 mPa·s, or when the free formaldehyde content fell below 0.5 wt%.

[0032] Step S3: Product post-processing.

[0033] The reaction product was transferred to a rotary evaporator and subjected to reduced-pressure dehydration at a temperature of 60°C to 75°C and a vacuum of -0.08 MPa to -0.095 MPa until no significant water vapor was evaporated, yielding a final transparent, viscous liquid or a solid at room temperature phenolic resin containing latent acid catalytic structural units. This resin exhibits storage stability for at least 6 months under sealed, light-protected, and room-temperature conditions, during which time its latent acid catalytic structural units show no significant decomposition.

[0034] This invention uses precise chemical synthesis to directly construct thermally activated catalytic precursors onto monomer molecules and introduces them into polymer chains without damage through a mild polymerization process, thereby preparing a resin material that can trigger catalytic activity on demand.

[0035] The application process of the highly efficient catalytic rapid curing phenolic resin of the present invention involves heating the resin to the curing activation temperature (e.g., 140°C). During this process, the latent acid catalytic structural units (tert-butyl 4-hydroxybenzenesulfonate units) covalently bonded to the molecular chain undergo thermal dissociation, resulting in the instantaneous and uniform generation of a large number of strongly acidic catalytic centers (4-hydroxybenzenesulfonic acid units) with molecular-level dispersion throughout the entire three-dimensional resin matrix. The sulfonic acid groups generated in situ and covalently bound to the polymer backbone constitute an immobilized, high-density three-dimensional catalytic network.

[0036] Each catalytic center is located in the direct neighborhood of the reactant (phenolic hydroxyl, hydroxymethyl), which greatly shortens the diffusion distance between the catalyst and the reaction substrate, eliminating the problems of uneven diffusion and mass transfer limitation that exist in traditional external catalysts in viscous resin systems.

[0037] Because the formation of catalytic sites is synchronous and uniform throughout the system, the solidification reaction macroscopically manifests as a holistic, explosive "bulk" reaction, rather than an "interfacial" reaction driven by the catalyst diffusion front. This results in an extremely high solidification reaction rate.

[0038] In one specific embodiment, the latent self-catalytic phenolic resin prepared according to the present invention was placed on a hot plate at 150°C, and its gelation time was measured to be 18 to 25 seconds. In comparison, a conventional phenolic resin system using an equimolar amount of p-toluenesulfonic acid as an external catalyst had a gelation time of 120 to 150 seconds under the same test conditions. The curing rate of the present invention is increased by 5 to 8 times.

[0039] Furthermore, since the catalytic center is covalently bonded to the final cross-linked network framework, it is permanently fixed in the network structure after curing, preventing migration, volatilization, or leaching. This not only avoids the adverse effects of catalyst residue on the long-term properties of the cured product (such as electrical insulation and chemical corrosion resistance), but also eliminates the problem of catalyst corrosion on molds or equipment.

[0040] Due to the highly uniform curing reaction, the cured product obtained by this invention possesses a three-dimensional cross-linked network with extremely low defect density and a highly regular structure. Compared to cured products from traditional catalytic systems that are prone to internal stress concentration and microscopic defects due to uneven catalyst distribution, the cured product of this invention exhibits significant advantages in mechanical properties and thermal stability.

[0041] In a specific performance test, the glass fiber reinforced composite material made from the resin of this invention achieved a flexural strength of 550 MPa, higher than the 480 MPa of the conventional system; its glass transition temperature (Tg), determined by dynamic mechanical analysis (DMA), was 235 °C, significantly higher than the 190 °C of the conventional system. Furthermore, its char residue reached 68% under a nitrogen atmosphere at 800 °C, demonstrating good thermal stability and charring ability.

[0042] Compared with the prior art, the present invention has the following beneficial effects:

[0043] This invention successfully constructs an intrinsically autocatalytic phenolic resin system by introducing covalently bonded, thermally dissociative, latent acid catalytic functional groups at the molecular design level. This fundamentally solves the technical problem in existing technologies where the curing efficiency of phenolic resins is limited by external factors (such as the synergistic reaction of multi-component composite systems and the diffusion and distribution of added catalysts), achieving a breakthrough improvement in curing rate. Simultaneously, the technical solution of this invention simplifies the production process, eliminating the addition and mixing steps of added catalysts, and improving the stability and repeatability of the process. The cured product exhibits superior mechanical and thermal properties due to the optimized cross-linked network structure. This invention provides a novel, efficient, and intrinsically regulated technical path for the preparation of high-performance phenolic resin-based materials, and has significant application value in aerospace, electronic packaging, and high-performance composite materials. Attached Figure Description

[0044] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained from these drawings without creative effort.

[0045] Figure 1 This is an overall flowchart of the method described in this invention.

[0046] Figure 2 This is a schematic diagram of the microscopic mechanism of the curing process of the phenolic resin described in this invention. Detailed Implementation

[0047] In the following description, only certain exemplary embodiments are briefly described. As those skilled in the art will recognize, the described embodiments can be modified in various ways without departing from the spirit or scope of the embodiments of the invention. Therefore, the drawings and description are considered to be exemplary in nature and not restrictive.

[0048] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0049] Example 1: See Figure 1 and Figure 2 This embodiment discloses a highly efficient catalytically curable phenolic resin and its preparation method. The purpose of this embodiment is to construct a phenolic resin whose molecular structure contains latent acid catalytic structural units linked by covalent bonds that undergo irreversible chemical cleavage at a preset curing activation temperature to generate acidic catalytic centers in situ. This embodiment immobilizes the catalytic function intrinsically onto the resin molecular framework, thereby achieving instantaneous, uniform, and high-density generation of catalytic sites during the curing stage, triggering a highly efficient self-sustaining curing crosslinking reaction.

[0050] In one specific embodiment, the latent acid catalytic structural unit is introduced from a specially chemically designed modified phenolic monomer.

[0051] The modified phenolic monomers are characterized by having at least one phenolic hydroxyl group (-OH) and a thermally induced dissociative latent acid catalytic functional group on the Ar ring, with the general structural formula (HO). m -Ar-(L) n -SO2-OR.

[0052] In this structure, Ar represents a phenyl structural unit.

[0053] m is an integer, with a value of 1 or 2, used to represent the number of phenolic hydroxyl groups attached to the Ar ring of benzene.

[0054] L represents a chemical linking group, which can be a direct chemical bond or a methylene (-CH2-) bridging group.

[0055] n is an integer, which can be 0 or 1. When n is 0, L is a chemical bond, which means that the sulfur atom of the sulfonate group (-SO2-OR) is directly connected to the Ar ring of benzene. When n is 1, L is a methylene group, which allows the sulfonate group to be connected to the Ar ring of benzene through the methylene group.

[0056] The R group is the key leaving group that enables the latent functional group to achieve thermal activation. Its chemical nature is that it is a group that can stably leave through heterolytic cleavage or concerted elimination reaction above the preset curing activation temperature threshold, thereby promoting the conversion of sulfonate group into sulfonic acid group (-SO3H) with strong catalytic activity.

[0057] In specific implementation, the R group can be a tertiary alkyl group or a trialkylsilyl group.

[0058] In a preferred embodiment, when R is a tertiary alkyl group, its specific structure is tert-butyl; when R is a trialkylsilyl group, its specific structure can be trimethylsilyl or triethylsilyl.

[0059] On the Ar ring of benzene, the phenolic hydroxyl group is associated with -(L). n The relative substitution positions of the -SO2-OR functional group can be ortho, meta, or para. Among them, in order to minimize the influence of intramolecular steric hindrance on subsequent polymerization and thermally activated cleavage reactions and obtain more ideal reaction kinetics, the para-substituted configuration is preferred.

[0060] In some embodiments, the modified phenolic monomer is tert-butyl 4-hydroxybenzenesulfonate. The modified phenolic monomer molecule exhibits excellent chemical stability at temperatures below 120°C, and its tert-butyl sulfonate structure remains intact without any significant chemical decomposition, thus preventing the generation of any catalytically active acidic substances. This ensures its chemical inertness during resin synthesis and storage.

[0061] When the ambient temperature is raised to the curing activation temperature range of 130℃ to 160℃, the thermodynamically unstable tert-butyl sulfonate moiety in its molecular structure undergoes a specific and irreversible E1 elimination reaction. The CO bond undergoes heterolytic cleavage, the tert-butyl group leaves as a carbocation and rapidly loses a proton, ultimately generating the chemically stable inert gas isobutylene, and in situ generating a 4-hydroxybenzenesulfonic acid structural unit with extremely high catalytic activity. This in situ generated sulfonic acid group, covalently anchored to the phenol ring, is a strong protic acid with significantly higher acidity than commonly used external catalysts in traditional phenolic resin curing processes, such as p-toluenesulfonic acid or phenolsulfonic acid. Therefore, it can catalyze the dehydration condensation reaction between phenolic hydroxyl groups and hydroxymethyl groups, as well as the etherification and further rearrangement of hydroxymethyl groups to form methylene bridges, thereby greatly accelerating the formation of the three-dimensional cross-linked network.

[0062] The highly efficient catalytically curable phenolic resin provided in this embodiment is prepared by polycondensation reaction of conventional phenolic monomers, aldehyde monomers, and the aforementioned modified phenolic monomers under precisely controlled process conditions.

[0063] The conventional phenolic monomer can be at least one of phenol, resorcinol, different isomers of cresol or bisphenol A, or any combination thereof.

[0064] The aldehyde monomer can be at least one of formaldehyde, paraformaldehyde, or furfural, with formaldehyde being preferred due to its high reactivity and cost-effectiveness.

[0065] In the resin preparation formulation, the molar amount of the modified phenolic monomer accounts for 0.2% to 8.0% of the total molar amount of phenolic monomers (i.e., the sum of the molar amounts of conventional phenolic monomers and modified phenolic monomers). This range was determined through extensive experimental optimization: below 0.2%, the density of catalytic sites generated in situ during the curing stage is insufficient, failing to effectively drive rapid overall curing, resulting in insignificant improvement in catalytic efficiency; while above 8.0%, although the curing rate can be further improved, excessively high concentrations of latent catalytic units may lead to trace decomposition during long-term storage, posing a potential threat to the storage stability of the resin prepolymer. Furthermore, the introduction of excessive non-skeletal structures may negatively impact the final mechanical properties of the cured product. Therefore, the range of 0.2% to 8.0% ensures an optimal balance between catalytic efficiency, material stability, and final performance.

[0066] To ensure that the latent acid catalytic functional groups are not prematurely activated during resin synthesis, the preparation process of the highly efficient catalytic rapid curing phenolic resin in this embodiment strictly controls the temperature of the reaction system to always be lower than the decomposition and activation temperature of the thermally induced dissociative functional groups.

[0067] In practice, the entire preparation process is carried out in the presence of an alkaline catalyst, which can be selected from sodium hydroxide, potassium hydroxide, ammonia, or triethylamine, and its amount is usually 0.1% to 1.5% of the total phenolic monomer mass. The reaction temperature is precisely controlled within the range of 65°C to 95°C. Under these mild alkaline conditions, the phenolic hydroxyl groups of both conventional and modified phenolic monomers (such as tert-butyl 4-hydroxybenzenesulfonate) are activated, undergoing electrophilic addition reactions with aldehyde monomers to introduce hydroxymethyl groups at the ortho and para positions of the benzene ring.

[0068] Subsequently, the hydroxymethyl-containing intermediate undergoes a preliminary polycondensation reaction, forming methylene bridges or methylene ether bonds through dehydration, thereby generating linear or slightly branched phenolic resin prepolymers with an average molecular weight controlled in the range of 600 to 2500 Daltons. During this polymerization process, covalently bonded latent acid-catalyzing functional groups, such as tert-butyl 4-hydroxybenzenesulfonate units, are fully integrated into the polymer chain as part of the resin structure, and their chemical structure remains highly stable under these temperature and pH conditions without any decomposition.

[0069] After the polycondensation reaction reaches the predetermined endpoint, the water generated during the reaction and the unreacted free small molecule monomers remaining in the system are removed by a highly efficient vacuum dehydration process at a lower temperature. Finally, a phenolic resin product containing intrinsic latent acid catalytic structural units is obtained, which is a homogeneous and transparent viscous liquid or glassy solid at room temperature.

[0070] To facilitate a better understanding of the present invention by those skilled in the art, the present invention will be further illustrated below with specific preparation examples.

[0071] A highly efficient catalytic and rapid curing method for preparing phenolic resin includes three core steps: preparation of modified phenolic monomers (step S1), synthesis of latent autocatalytic phenolic resin prepolymer (step S2), and product post-treatment (step S3).

[0072] Step S1: Preparation of modified phenolic monomer tert-butyl 4-hydroxybenzenesulfonate.

[0073] In a dried 1000 mL three-necked flask, a magnetic stir bar was fitted, and a dropping funnel and a nitrogen inlet tube were connected. Under nitrogen protection, 4-hydroxybenzenesulfonic acid dihydrate (43.7 g, 0.2 mol) and 4-dimethylaminopyridine (DMAP) (2.44 g, 0.02 mol) were added to the flask, followed by 500 mL of anhydrous dichloromethane as the reaction solvent. The magnetic stirrer was started to form a homogeneous suspension of the solids in the solvent. The reaction flask was placed in an ice-salt bath to cool the system to 0 °C. In another dried beaker, N,N'-dicyclohexylcarbodiimide (DCC) (45.4 g, 0.22 mol) was dissolved in 100 mL of anhydrous dichloromethane, and the solution was transferred to a dropping funnel.

[0074] Under stirring and temperature control from 0°C to 5°C, a dichloromethane solution of DCC was slowly added dropwise to the reaction system through a dropping funnel, with the addition process completed within approximately 1.5 hours. During the addition, the system gradually became turbid, and a white precipitate (byproduct dicyclohexylurea, DCU) was formed. After the DCC solution was completely added, stirring continued for 30 minutes under ice bath conditions.

[0075] 16.3 g (0.22 mol) of tert-butanol was dissolved in 50 mL of anhydrous dichloromethane and slowly added dropwise to the reaction system over 30 minutes using a dropping funnel. After the addition was complete, the ice-salt bath was removed, and the reaction system was allowed to warm naturally to room temperature (approximately 25 °C), and the reaction was stirred continuously at this temperature for 20 hours.

[0076] After the reaction was complete, the mixture was filtered under reduced pressure through a Buchner funnel to remove the white solid DCU, which is insoluble in dichloromethane. The filter cake was washed twice with 50 mL of dichloromethane, and the filtrates were combined. The combined filtrate was transferred to a rotary evaporator, and most of the dichloromethane solvent was evaporated under a water bath temperature of 40 °C and a vacuum of -0.08 MPa to obtain a pale yellow oily crude product.

[0077] The crude product was purified by silica gel column chromatography. A 200-300 mesh silica gel column (5 cm inner diameter, 40 cm packing height) was wet-packed. A 1:5 volume ratio of ethyl acetate to n-hexane was used as the eluent. The crude product was dissolved in a minimal amount of dichloromethane, mixed with a small amount of silica gel, and after the solvent evaporated, evenly spread on the top of the column. Elution began, and the eluent was collected fractionally. The fraction composition was monitored by thin-layer chromatography (TLC). The fractions containing the target product were combined, and the eluent was removed by rotary evaporation. Finally, the product was dried in a vacuum oven at 40°C for 6 hours to obtain pure tert-butyl 4-hydroxybenzenesulfonate in white needle-like crystals, with a yield of 82.6%.

[0078] The obtained product was structurally characterized. Fourier transform infrared spectroscopy (FT-IR, KBr pellet method) analysis showed that at 3450 cm⁻¹... -1 The absorption peak at 2980 cm⁻¹ is the OH stretching vibration of the phenolic hydroxyl group. -1 The peak at 1595 cm⁻¹ represents the CH stretching vibration of tert-butyl. -1 and 1500cm -1 The peak at 1365 cm⁻¹ represents the vibrational peak of the benzene ring skeleton. -1 and 1180cm -1 The peaks at 1125 cm⁻¹ represent the strong absorption peaks of the antisymmetric and symmetric stretching vibrations of the S=O group in the sulfonate ester group, respectively. -1 The peak at this location corresponds to the CO stretching vibration.

[0079] ¹H NMR (400 MHz, CDCl3 solvent) analysis showed the following spectra: δ7.82 (d, J=8.8 Hz, 2H, proton of the benzene ring adjacent to -SO₂OR), δ6.95 (d, J=8.8 Hz, 2H, proton of the benzene ring adjacent to -OH), δ5.80 (s, 1H, active hydrogen of the phenolic hydroxyl group -OH), and δ1.48 (s, 9H, proton of tert-butyl-C(CH₃)₃). These spectral data perfectly match the target molecular structure of tert-butyl 4-hydroxybenzenesulfonate, demonstrating the successful preparation of a high-purity modified phenolic monomer.

[0080] Step S2: Synthesis of latent autocatalytic phenolic resin prepolymer, wherein an alkaline catalyst is used to activate phenolic hydroxyl groups and promote electrophilic addition reactions with aldehyde monomers.

[0081] In a 2000 mL four-necked reaction flask equipped with a precision mechanical stirrer (anchor-type stirrer), a reflux condenser, a digital thermometer with an accuracy of 0.1 °C, and a constant-pressure dropping funnel, phenol (470.6 g, 5.0 mol), tert-butyl 4-hydroxybenzenesulfonate prepared in step S1 (11.5 g, 0.05 mol, calculated molecular weight 230.28 g / mol, actual addition adjusted based on purity), and a 30 wt% sodium hydroxide aqueous solution (5.0 g) as an alkaline catalyst were added sequentially. Mechanical stirring was started, and the mixture was heated to 70 °C in a water bath to ensure complete dissolution of all solid materials, forming a homogeneous reaction mixture.

[0082] A 37wt% formaldehyde aqueous solution (405.4 g, equivalent to 5.0 mol of formaldehyde) was added to a constant-pressure dropping funnel. While maintaining the reaction system temperature between 70°C and 75°C, the formaldehyde aqueous solution was added dropwise at a uniform rate. Since the addition reaction is exothermic, the system temperature needed to be precisely controlled by adjusting the dropping rate and the water bath temperature to ensure it did not exceed the upper limit of 80°C. The entire dropping process was completed within 45 minutes.

[0083] After the formaldehyde solution was added dropwise, the temperature of the reaction system was raised to 88°C and maintained at a constant temperature between 88°C and 90°C to carry out the polycondensation reaction. During the reaction, samples were taken every 20 minutes, and the viscosity of the resin prepolymer was measured using a Brookfield DV-II+Pro rotational viscometer at a constant temperature of 25.0°C and a No. 3 rotor at a rotation speed of 20 rpm. Simultaneously, the free formaldehyde content in the system was determined using the hydroxylamine hydrochloride titration method. The reaction was considered to have reached its endpoint when the viscosity of the resin prepolymer reached 1050 mPa·s and the free formaldehyde content was below 0.5 wt%. In this embodiment, the polycondensation reaction lasted for 125 minutes. After the reaction reached its endpoint, heating was immediately stopped, and the reaction system was rapidly cooled to below 60°C using a cold water bath to terminate the reaction.

[0084] Step S3: Product post-processing.

[0085] The crude reaction product obtained in the above steps was quickly transferred to a 2000 mL rotary evaporator flask. The flask was then mounted on a rotary evaporator, and dehydration was performed under reduced pressure at a water bath temperature of 72°C and a system vacuum of -0.09 MPa. This process aims to remove small molecules such as water, unreacted free phenol, and formaldehyde generated during the reaction. During dehydration, the rate of distillation at the condenser was observed. Dehydration was considered complete when no significant water vapor was observed and the distillate volume no longer increased after 15 minutes of continuous observation. The entire post-treatment process took approximately 90 minutes.

[0086] After dehydration, heating and vacuuming were stopped, and the product in the flask was poured out while still hot. The obtained product was an amber-colored, transparent, homogeneous, and highly viscous liquid resin. Analysis showed that the solid content of this latent autocatalytic phenolic resin was 98.5%, and its viscosity at 25°C was 15,200 mPa·s. It exhibited good storage stability; after 6 months of storage at room temperature (25°C), under sealed and light-protected conditions, its viscosity increased by less than 15%, and infrared spectroscopy showed no significant change in the intensity of the characteristic absorption peak of its latent acid-catalyzing structural unit (tert-butyl sulfonate).

[0087] To verify the technical effect of the highly efficient catalytic rapid curing of phenolic resin described in this invention, we compared its performance with that of a comparative system using a traditional external catalyst.

[0088] The curing and performance tests in this embodiment are as follows:

[0089] The latent autocatalytic phenolic resin obtained in step S3 was subjected to the following tests:

[0090] Gel time determination: According to GB / T12007.7-1989 standard, approximately 0.5g of resin sample was dropped onto a preheated constant-temperature electric heating plate at 150.0±0.5℃, and a stopwatch was started immediately. A glass rod was continuously used to draw the resin melt into filaments. The gel time was recorded when the resin could be drawn into a continuous, elastic filament that no longer adhered to the glass rod after cooling. After three parallel tests, the average gel time of the resin in this embodiment at 150℃ was measured to be 21 seconds.

[0091] Composite Material Preparation and Performance Testing: A prepreg was prepared by impregnating the resin of this embodiment with 7628 type E-glass fiber cloth (weft weight 200 g / m²), controlling the resin content to be 35 ± 2 wt%. Twelve layers of prepreg were symmetrically stacked at 0° / 90° and placed in a molding press preheated to 150°C, where they were molded and cured at 5 MPa for 30 minutes. After curing, the product was post-treated in an oven at 180°C for 2 hours to ensure complete crosslinking. The cured composite material sheets were cut into standard strips for performance testing.

[0092] Bending properties: Three-point bending tests were conducted on a universal testing machine according to ISO 178 standard. The average bending strength of the composite material was measured to be 552 MPa, and the bending modulus was 23.8 GPa.

[0093] Thermal properties: The storage modulus and loss factor (tanδ) of the samples were tested using a dynamic mechanical analyzer (DMA) at a heating rate of 3 °C / min and a vibration frequency of 1 Hz. The glass transition temperature (Tg) of the material was determined from the temperature corresponding to the peak tanδ, and was found to be 235 °C. Thermogravimetric analysis (TGA) was used to determine the thermogravimetric behavior of the material by heating from room temperature to 800 °C at a rate of 10 °C / min under a nitrogen atmosphere (flow rate of 50 mL / min). The results showed that the char residue of the composite material at 800 °C was 68.3%.

[0094] Comparative Example 1: To establish a comparative benchmark, we prepared a conventional phenolic resin without a latent catalytic unit and cured it using an external catalyst.

[0095] Resin preparation: The same equipment and process flow as step S2 in Example 1 were used, but instead of adding tert-butyl 4-hydroxybenzenesulfonate, an equimolar amount of phenol was used, i.e., phenol (475.6 g, 5.05 mol) was added. The amounts of formaldehyde aqueous solution and sodium hydroxide remained unchanged. The reaction endpoint was also controlled by viscosity at 1050 mPa·s. After post-treatment, the basic phenolic resin was obtained.

[0096] Preparation of the curing system: Take 100.0 g of the basic phenolic resin prepared above. To simulate the catalyst concentration generated in situ in Example 1, an acid catalyst equivalent to 0.05 mol of latent catalytic unit needs to be added. In Example 1, the total mass of phenol and latent monomer is 482.8 g, and the latent monomer accounts for 2.53%. Therefore, p-toluenesulfonic acid (PTSA) monohydrate (molecular weight 190.22 g / mol) is added to 100 g of basic resin. The amount added is calculated based on the molar concentration of sulfonic acid group equivalent to the latent monomer (0.05 mol) in Example 1: 0.05 mol × 190.22 g / mol = 9.51 g. However, given that the latent monomer accounts for 2.53% of the total phenol mass in Example 1, approximately 2.0 g of PTSA monohydrate is added equivalently to 100 g of basic resin (actually calculated as 2.0 g). The PTSA is stirred and dissolved in the basic resin at 60°C until a transparent and homogeneous mixture is formed.

[0097] Performance testing: The prepared curing system was tested for gel time, composite material preparation and performance under the same test methods and conditions as in Example 1.

[0098] The test results of Example 1 and Comparative Example 1 are summarized in Table 1 below:

[0099] Table 1:

[0100]

[0101] As can be clearly seen from the data comparison in Table 1, the highly efficient catalytic rapid curing phenolic resin provided by this invention exhibits comprehensive performance advantages compared to the traditional technical solution using externally added acid catalysts.

[0102] Its gel time was shortened by approximately 84%, and the curing rate was increased by more than 6 times. This is attributed to its unique curing mechanism. Under heating conditions, the latent catalytic units (tert-butyl 4-hydroxybenzenesulfonate units) covalently bonded to the resin molecular chain undergo thermal dissociation, instantly and uniformly generating a large number of strongly acidic catalytic centers (4-hydroxybenzenesulfonic acid units) with molecular-level dispersion throughout the entire three-dimensional resin matrix. The sulfonic acid groups generated in situ and bound to the polymer backbone by covalent bonds constitute an immobilized, high-density three-dimensional catalytic network. Each catalytic center is located in the direct neighborhood of the reactants (phenolic hydroxyl, hydroxymethyl), fundamentally eliminating the problems of uneven diffusion and mass transfer limitation that exist in traditional external catalysts in viscous resin systems, thereby triggering an explosive bulk curing reaction.

[0103] Furthermore, due to the high uniformity of the curing reaction, localized reaction rate differences caused by uneven catalyst distribution are avoided, resulting in a three-dimensional cross-linked network with lower defect density and a more regular structure. This structural optimization is directly reflected in the macroscopic properties of the cured product:

[0104] The flexural strength increased by approximately 15.5%, and the glass transition temperature increased by 43°C, demonstrating superior mechanical properties and heat resistance. Simultaneously, the higher char residue indicates that the cross-linked network formed exhibits greater stability at high temperatures.

[0105] This invention, through structural innovation at the molecular level, integrates thermally activated latent catalytic functional groups into the molecular framework of phenolic resin via covalent bonds, constructing an intrinsically autocatalytic system. This invention not only significantly improves the curing rate and simplifies the process, but also substantially enhances the mechanical properties and thermal stability of the cured product.

[0106] This invention achieves a paradigm shift in catalysts from physical doping to chemical integration by pre-integrating thermally dissociative latent acid catalytic functional groups (such as tert-butyl sulfonate) into the resin framework via covalent bonds. This invention fundamentally eliminates the unavoidable diffusion and mass transfer limitations of added small-molecule catalysts in viscous resin melts, solving the core problem of catalytic inhomogeneity.

[0107] The latent acid catalytic structural unit of this invention undergoes specific cleavage (such as E1 elimination) at a preset curing activation temperature (130-160℃), achieving precise timing triggering of catalytic activity. Simultaneously, because the catalytic precursor is uniformly distributed at the molecular level throughout the polymer network, the sulfonic acid groups generated after its dissociation are covalently anchored, forming spatially uniformly distributed immobilized catalytic centers, thus achieving precise localization of the catalytic reaction.

[0108] The timing and positioning control described above in this invention transforms the curing reaction from a traditional surface-to-interior interfacial reaction into a globally synchronized bulk reaction. This directly results in an order-of-magnitude increase in curing rate (gel time reduced from >120 seconds to approximately 21 seconds) and the generation of a three-dimensional network with lower defect density and more uniform cross-linking, thereby fundamentally improving the mechanical properties (e.g., flexural strength increased to 552 MPa) and thermal stability (e.g., Tg increased to 235°C) of the cured product.

[0109] This invention eliminates the metering and mixing steps of the added catalyst, simplifying the process and reducing batch-to-batch quality fluctuations caused by uneven catalyst dispersion. Simultaneously, the integrated design of the catalyst and resin fundamentally eliminates the risk of corrosion to processing equipment by acidic small molecules and their residual migration in the final product, thus improving the long-term reliability of the product.

[0110] In summary, this invention, based on the fundamental principles of catalytic chemistry, achieves a breakthrough in the curing efficiency of phenolic resin and substantial optimization of the overall performance of the final product through the redesign of the catalyst's form and activation method, providing a novel technical path for the preparation of high-performance phenolic resin materials.

[0111] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including both the preferred embodiments and all changes and modifications falling within the scope of the invention.

[0112] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. It should be noted that any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A highly efficient catalytically cured phenolic resin, characterized in that: The molecular structure of phenolic resin contains latent acid catalytic structural units that are covalently linked and undergo irreversible chemical cleavage at a preset curing activation temperature to generate an acidic catalytic center in situ. The latent acid catalytic structural unit is derived from a modified phenolic monomer, the chemical structure of which contains at least one phenolic hydroxyl group and a thermally dissociative latent acid catalytic functional group on its benzene ring. The structure of the thermally induced dissociative latent acid catalytic functional group includes a sulfonate group -SO2-OR, wherein the sulfur atom of the sulfonate group is directly or through an arylene or alkylene bridging group connected to the benzene ring of the modified phenolic monomer, and the R group is a leaving group that can leave through heterolytic cleavage or concerted elimination reaction at the preset curing activation temperature, thereby hydrolyzing or converting the sulfonate group into a catalytically active sulfonate group -SO3H; The preparation method includes the following steps: a) In the presence of an alkaline catalyst, conventional phenolic monomers, modified phenolic monomers, and aldehyde monomers are mixed; wherein the chemical structure of the modified phenolic monomers includes at least one phenolic hydroxyl group and a thermally induced dissociative latent acid catalytic functional group on its benzene ring. b) At a temperature below the temperature at which the thermally ionized latent acid catalytic functional groups are decomposed and activated, the mixture obtained in step a) is subjected to a polycondensation reaction to form a phenolic resin prepolymer containing latent acid catalytic structural units; the reaction temperature of the polycondensation reaction is controlled within the range of 65°C to 95°C throughout the process. c) The phenolic resin prepolymer obtained in step b) is subjected to dehydration under reduced pressure to remove the water generated in the reaction and the unreacted free small molecules, thereby obtaining the highly efficient catalytic rapid curing phenolic resin. In the feeding of the polycondensation reaction, the molar amount of the modified phenolic monomer accounts for 0.2%-8.0% of the total molar amount of conventional phenolic monomers and modified phenolic monomers; The conventional phenolic monomer is at least one of phenol, resorcinol, cresol, or bisphenol A.

2. The highly efficient catalytically cured phenolic resin according to claim 1, characterized in that, The general structural formula of the modified phenolic monomer is: (HO) m -Ar-(L) n -SO2-O-R; Where Ar is a phenyl group; m is an integer of 1 or 2, representing the number of phenolic hydroxyl groups attached to the phenyl Ar; L represents a chemical bond or a methylene group (-CH2-); n is an integer of 0 or 1. When n is 0, L is a chemical bond in which the sulfur atom of the sulfonate group is directly attached to the phenyl Ar. When n is 1, L is a methylene group. R is a tertiary alkyl or trialkylsilyl group.

3. The highly efficient catalytically cured phenolic resin according to claim 2, characterized in that: When R is a tertiary alkyl group, the specific structure is tert-butyl; when R is a trialkylsilyl group, the specific structure is trimethylsilyl or triethylsilyl.

4. The highly efficient catalytically cured phenolic resin according to claim 2, characterized in that: the phenolic hydroxyl -OH and the - (L) n the -SO2-O-R functional group is para-substituted on the phenyl group Ar.

5. The highly efficient catalytically cured phenolic resin according to claim 1, characterized in that: The modified phenolic monomer is tert-butyl 4-hydroxybenzenesulfonate; the tert-butyl 4-hydroxybenzenesulfonate maintains chemical stability at temperatures below 120°C, and when the temperature is raised to the curing activation temperature range of 130°C to 160°C, the tert-butyl sulfonate structure undergoes a specific E1 elimination reaction, the covalent bond breaks, and 4-hydroxybenzenesulfonic acid structural units are generated in situ and isobutylene is released.

6. A method for preparing a highly efficient catalytically curing phenolic resin, used to prepare the highly efficient catalytically curing phenolic resin according to claim 1, characterized in that, Includes the following steps: a) In the presence of an alkaline catalyst, conventional phenolic monomers, modified phenolic monomers, and aldehyde monomers are mixed; The chemical structure of the modified phenolic monomer contains at least one phenolic hydroxyl group and one thermally dissociative latent acid catalytic functional group on its benzene ring. b) At a temperature below the temperature at which the thermally ionized latent acid catalytic functional groups are decomposed and activated, the mixture obtained in step a) is subjected to a polycondensation reaction to form a phenolic resin prepolymer containing latent acid catalytic structural units; the reaction temperature of the polycondensation reaction is controlled within the range of 65°C to 95°C throughout the process. c) The phenolic resin prepolymer obtained in step b) is subjected to dehydration under reduced pressure to remove the water generated in the reaction and unreacted free small molecules, thereby obtaining the highly efficient catalytic rapid curing phenolic resin.

7. The method for preparing a highly efficient catalytically cured phenolic resin according to claim 6, characterized in that, The endpoint of the polycondensation reaction in step b) is controlled by at least one of the following methods: The polycondensation reaction is terminated when the viscosity of the phenolic resin prepolymer measured at 25°C reaches 800 mPa·s-1200 mPa·s, or when the free formaldehyde content in the reaction system is less than 0.5 wt%. The specific process conditions for the depressurization dehydration treatment in step c) are as follows: the treatment is carried out at a temperature of 60℃-75℃ and a vacuum of -0.08MPa to -0.095MPa.

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