High heat resistant phenolic resin and preparation method thereof

Abstract

The application discloses a kind of high heat-resistant phenolic resin and preparation method thereof.The method comprises the following steps: first, phenol and melamine are pre-reacted in the presence of formaldehyde at 60-90 DEG C for 1-3 hours to form modified phenol intermediate;then, with formaldehyde, under the action of acid catalyst, polycondensation is carried out at 75-100 DEG C for 1-4 hours;temperature is raised to 110-150 DEG C, and reaction is continued for 2-4 hours;finally, normal pressure dehydration and 130-180 DEG C, 5000-10000 Pa vacuum devolatilization are carried out to obtain the target resin.The molar ratio of phenol, melamine and formaldehyde is 1:(0.1-0.6):(0.2-0.8).The application constructs high crosslinking density heat-resistant network by pre-embedding triazine ring, so that Tmax of the resin reaches 412-462 DEG C, Tg reaches 168-215 DEG C, and the carbon residue rate at 800 DEG C reaches 58.3-73.5%, which significantly improves the thermal stability, heat oxidation resistance and mechanical properties, and is suitable for high-temperature friction material field.

Description

Technical Field

[0001] This invention relates to the technical field of phenolic resins, and in particular to a high heat-resistant phenolic resin and its preparation method. Background Technology

[0002] With the continuous development of automotive, rail transportation, and aerospace equipment technologies, higher requirements are being placed on the stability of friction materials under high temperature, high load, and long-term service conditions. Phenolic resins, due to their wide availability of raw materials, low cost, high mechanical strength, and good compatibility with various inorganic fillers and reinforcing fibers, have long been widely used in brake pads, clutch plates, and various friction composite materials, and have dominated the field of friction materials as an important thermosetting binder. Traditional phenolic resins are usually prepared by the condensation reaction of phenol and formaldehyde under acidic or alkaline catalytic conditions. Their molecular structure consists of benzene rings linked by methylene bridges or methylene ether bonds to form a three-dimensional network structure. With the continuous increase in application environment temperature and mechanical load, researchers have gradually modified the structure of phenolic resins through various methods, such as introducing aromatic amine compounds, modified phenolic monomers, or nitrogen-containing functional molecules, to improve the thermal stability, carbonization rate, and resistance to thermal oxidation of the resin system. Among them, aromatic diamine compounds, due to their rigid benzene ring structure and active amino functional groups, can be inserted into the molecular chain of phenolic resin through condensation reaction with hydroxymethylphenol or formaldehyde, thereby improving the heat resistance and high-temperature mechanical stability of the resin to a certain extent. Therefore, they have gradually become one of the important directions in the research of heat resistance modification of phenolic resin.

[0003] However, existing technologies for modifying phenolic resins with aromatic diamines still have certain limitations. For example, Chinese patent (CN115725036B) discloses a high-heat-resistant modified phenolic resin, its preparation, and its application. In common synthesis processes, 4,4′-diaminodiphenylmethane is usually introduced after the prepolymerization stage of the phenolic resin. It can form a nitrogen-containing bridging structure through a condensation reaction with hydroxymethylphenol in the system or through a Mannich reaction, allowing amino fragments to be grafted onto the phenolic resin molecular chain. Although this type of reaction can introduce nitrogen-containing structures and improve the heat resistance of the resin to a certain extent, the resulting linking structure is still mainly composed of methylene and aminomethylene bridging units. This type of structure is prone to thermal decomposition or CN bond breakage under high-temperature conditions, leading to a decrease in the stability of the resin network structure and potentially releasing small molecule volatiles such as formaldehyde and amines, thus affecting the long-term service stability of the material under high-temperature environments. Furthermore, since these aromatic diamines are typically difunctional, they tend to undergo chain growth rather than multi-directional crosslinking during polycondensation, making it difficult to construct a three-dimensional aromatic network structure with high crosslinking density. This results in insufficient density of the char layer formed during high-temperature carbonization, leaving room for further improvement in ablation resistance and high-temperature char residue. Therefore, optimizing the reaction pathway and molecular structure design of modified monomers in phenolic resin systems to reduce the proportion of heat-sensitive methylene structures, improve the stability of the crosslinking network, and further enhance the high-temperature structural stability of the resin remains a crucial technical problem that urgently needs to be solved in this field. Summary of the Invention

[0004] This application provides a method for preparing a high heat-resistant phenolic resin, comprising the following steps: S1. Pre-modification treatment of phenol: Phenol and melamine are pre-reacted in the presence of formaldehyde to form a modified phenol intermediate; S2. The modified phenol intermediate obtained in step S1 is subjected to a polycondensation reaction with formaldehyde under the action of an acidic catalyst to form a phenolic resin prepolymer. S3. Heat the phenolic resin prepolymer to 110-150°C and continue the reaction for 2-4 hours; S4. After dehydrating the reaction system under normal pressure, it is subjected to vacuum devolatilization at 130–180℃ and 5000–10000Pa to obtain modified phenolic resin.

[0005] It should be noted that in step S1, the triazine ring and polyamino groups abundant in the melamine molecule first undergo hydroxymethylation under the action of formaldehyde, and then undergo a Mannich-like reaction or co-condensation reaction with phenol, so that the nitrogen-containing triazine structure is pre-embedded into the phenol molecule system, forming a multifunctional modified intermediate; subsequently, in step S2, under acid catalysis, the hydroxymethylated phenol and the modified intermediate further undergo a condensation reaction to generate a phenolic prepolymer with methylene bridges and partially aminomethylene bridges as connecting units, while melamine provides additional reaction sites. The system's crosslinking potential is enhanced by heating in step S3. During the heating process, the system undergoes further condensation and structural rearrangement, promoting the transformation of methylene groups into more stable conjugated structures or dense aromatic networks. At the same time, the triazine ring, as a rigid structural unit, participates in the construction of a three-dimensional network with high crosslinking density. Finally, during the decompression devolatilization process in step S4, water and low-molecular-weight byproducts are removed, driving the condensation equilibrium towards the high-molecular-weight direction. This results in the formation of a dense, heat-resistant network structure synergistically constructed by benzene rings and triazine rings, thereby significantly improving the resin's thermal stability, carbonization residue rate, and heat oxidation resistance.

[0006] In one technical solution for preparing a high heat-resistant phenolic resin, in step S1, the molar ratio of phenol, melamine and formaldehyde is 1:(0.1-0.6):(0.2-0.8).

[0007] It should be noted that within the range of phenol, melamine and formaldehyde molar ratio of 1:(0.1~0.6):(0.2~0.8), the amount of formaldehyde can be controlled to avoid excessive hydroxymethylation and side reactions, while ensuring the effective introduction of melamine to improve crosslinking density and heat resistance. This achieves a balance between reactivity, structural controllability and final resin performance.

[0008] As a technical solution for preparing a high heat-resistant phenolic resin, formaldehyde is added to the modified phenol intermediate obtained in step S1, wherein the molar amount of added formaldehyde is 1.0 to 2.0 compared with the initial molar amount of phenol in step S1.

[0009] It should be noted that adding formaldehyde in step S2, and controlling its amount to be 1.0 to 2.0 relative to the initial molar amount of phenol in step S1, can provide sufficient and appropriate sources of hydroxymethylation reactants for the remaining phenol active sites in the system and the modified intermediates formed in step S1. This promotes the in-situ generation of hydroxymethylated phenol and other active intermediates from formaldehyde, and further condensation reactions to form methylene bridging and some nitrogen-containing bridging structures, thereby increasing the molecular weight and crosslinking density of the resin. When the amount of formaldehyde added is lower than this range, the active sites are not fully utilized, the degree of condensation is limited, and the resulting resin network structure is relatively loose. When the amount of formaldehyde added is higher than this range, it is easy to cause excessive hydroxymethylation, excessively rapid local branching, or an increase in residual low-molecular-weight substances, which is not conducive to the uniformity of the system structure and the improvement of the final heat resistance. Therefore, controlling the amount of formaldehyde added within the above range is beneficial to balancing reactivity, structural density, and comprehensive performance.

[0010] As a technical solution for preparing a high heat-resistant phenolic resin, in step S2, the acidic catalyst is one or a combination of at least two of p-toluenesulfonic acid, hydrochloric acid, phosphoric acid or oxalic acid, and its addition amount is 0.5 to 5 wt% of the mass of phenol.

[0011] It should be noted that the acidic catalyst is selected from one or a combination of p-toluenesulfonic acid, hydrochloric acid, phosphoric acid or oxalic acid, and the amount added is 0.5 to 5 wt% of the mass of phenol, which can ensure the reaction rate while avoiding excessive acidity that could lead to uncontrolled polycondensation or structural defects.

[0012] In one method for preparing a high heat-resistant phenolic resin, the reaction temperature in step S1 is 65–90°C, and the reaction time is 1.5–3 h.

[0013] It should be noted that the reaction temperature in step S1 is 65–90℃ and the reaction time is 1.5–3 hours, which is conducive to achieving sufficient pre-reaction of melamine and phenol, while avoiding premature macromolecular polycondensation.

[0014] In one technical solution for preparing a high heat-resistant phenolic resin, the reaction temperature in step S2 is 80–95°C, and the reaction time is 2–3 hours.

[0015] It should be noted that the reaction temperature in step S2 is 80-95℃ and the reaction time is 2-3 hours, which can effectively promote the hydroxymethylation and polycondensation reaction and control the molecular weight within a suitable range.

[0016] In a technical solution for preparing a high heat-resistant phenolic resin, the dehydration temperature under normal pressure in step S4 is 100-120℃, and the dehydration time is 1-2h.

[0017] It should be noted that the atmospheric pressure dehydration temperature in step S4 is 100-120℃ and the dehydration time is 1-2 hours, which helps to steadily remove moisture from the system and prevent the resin system from boiling out or becoming structurally uneven.

[0018] In a technical solution for preparing a high heat-resistant phenolic resin, the vacuum devolatilization time in step S4 is 1-3 hours.

[0019] It should be noted that the vacuum devolatilization time in step S4 is 1 to 3 hours, which can effectively remove low molecular weight volatiles and promote the further condensation reaction, thereby improving the density and heat resistance of the resin.

[0020] This invention successfully prepared a high-heat-resistant phenolic resin by pre-modifying phenol with melamine in the presence of formaldehyde, followed by acid-catalyzed polycondensation, heated deep polycondensation, and vacuum devolatilization. Compared with traditional phenolic resins and existing aromatic diamine modification methods, this invention has significant advantages: the rigid triazine ring and polyactive amino groups in melamine are uniformly embedded in the phenol system through pre-modification, forming a multifunctional nitrogen-containing intermediate. This effectively reduces the proportion of heat-sensitive methylene structures and constructs a high-crosslink density three-dimensional network structure with benzene ring-triazine ring synergy. As a result, the resin's thermal decomposition temperature (Tmax) can reach 412–462℃, glass transition temperature (Tg) can reach 168–215℃, char residue at 800℃ can reach 58.3–73.5%, heat distortion temperature (HDT) can reach 162–205℃, and flexural strength can reach 92–126. MPa significantly improves the resin's thermal stability, thermal oxidation resistance, high-temperature char residue, and mechanical strength. At the same time, the pre-modification path avoids uneven reaction and reverse Mannich reaction during direct mixing, effectively reducing the generation of small molecule volatiles at high temperatures. This improves the material's stability and ablation resistance during long-term service under high temperature and high load conditions, making it widely applicable to the high-performance friction materials required in the automotive, rail transportation, and aerospace industries. Attached Figure Description

[0021] Figure 1 The infrared spectrum of the modified phenol intermediate prepared in Example 1; Figure 2 The infrared spectrum is shown for the modified phenolic resin prepared in Example 1. Detailed Implementation

[0022] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the examples in the specification.

[0023] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0024] Secondly, the term "an embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places throughout this specification does not necessarily refer to the same embodiment, nor is it a single embodiment or an embodiment selectively excluded from other embodiments. Example 1

[0025] Step 1: Weigh the raw materials according to the molar ratio of phenol, melamine and formaldehyde 1:0.1:0.2. Add phenol to a reaction vessel equipped with a stirrer, temperature control and reflux device, and heat to 65°C to fully melt and form a homogeneous system. Slowly add melamine under stirring to make it uniformly dispersed, and then add formaldehyde solution dropwise. React at 65°C for 1.5 hours. During this process, the amino group in melamine first undergoes hydroxymethylation reaction with formaldehyde, and then further undergoes pre-condensation reaction with phenol, thereby forming a modified phenol intermediate with a uniformly distributed nitrogen-containing structure.

[0026] Step 2: Based on the intermediate obtained in Step 1, formaldehyde is added to adjust the initial molar ratio of phenol to formaldehyde in Step S1 to 1:1.0. At the same time, 0.5 wt% of p-toluenesulfonic acid is added as an acidic catalyst. The reaction is carried out at 80°C for 2 hours. Under the action of acid catalysis, the polycondensation reaction between hydroxymethylphenol and between hydroxymethylphenol and nitrogen-containing structures is promoted to form a phenolic resin prepolymer with a certain molecular weight.

[0027] Step 3: Slowly heat the above prepolymer system to 110°C under continuous stirring and maintain the reaction for 2 hours to allow the residual hydroxymethyl groups in the system to continue to undergo condensation reaction, while promoting molecular chain growth and the formation of preliminary cross-linked structures.

[0028] Step 4: After the reaction is completed, the system is dehydrated at 100℃ under normal pressure for 1 hour to remove the water and some low-boiling substances generated during the reaction. Then, the temperature is raised to 130℃ and the system pressure is adjusted to 5000Pa. The system is then dehydrated under reduced pressure for 1 hour to further remove residual low-molecular-weight substances and unreacted monomers, and finally a modified phenolic resin with uniform structure and good heat resistance is obtained. Example 2

[0029] Step 1: Weigh the raw materials according to the molar ratio of phenol, melamine and formaldehyde 1:0.3:0.5. Add phenol to the reaction vessel and heat to 75°C to melt it and form a homogeneous system. Add melamine under stirring to make it fully dispersed. Then slowly add formaldehyde solution. React at 75°C for 2 hours to allow melamine to undergo hydroxymethylation and a preliminary condensation reaction with phenol, thereby obtaining the modified phenol intermediate.

[0030] Step 2: Continue to add formaldehyde to the above intermediate to make the initial molar ratio of phenol to formaldehyde in step S1 reach 1:1.5, and add 2wt% phosphoric acid of phenol as an acidic catalyst. React at 85℃ for 2.5h to allow the system to undergo further polycondensation reaction and form a phenolic resin prepolymer with more active groups.

[0031] Step 3: The obtained prepolymer is heated to 130°C under stirring and the reaction is maintained for 3 hours. During this stage, the dehydration condensation between hydroxymethyl groups and the cross-linking reaction between molecular chains are promoted, thereby increasing the cross-linking density of the system.

[0032] Step 4: After the reaction is complete, dehydrate at atmospheric pressure for 1.5 h at 110 °C to remove water from the system. Then, heat to 150 °C and perform devastation under reduced pressure at 8000 Pa for 2 h to further remove low molecular weight volatiles, resulting in a modified phenolic resin with a dense structure and excellent heat resistance. Example 3

[0033] Step 1: Weigh out each raw material according to the molar ratio of phenol, melamine and formaldehyde 1:0.5:0.7. Add phenol to the reaction vessel and heat to 85°C to form a homogeneous liquid phase. Add melamine under stirring and allow it to disperse fully. Then slowly add formaldehyde solution and react at 85°C for 2.5 hours to allow melamine to undergo full hydroxymethylation and pre-condensation reaction with phenol to form a highly active modified phenol intermediate.

[0034] Step 2: Add formaldehyde to the intermediate system to make the initial molar ratio of phenol to formaldehyde in step S1 reach 1:1.8. At the same time, add 4 wt% of hydrochloric acid and oxalic acid composite catalyst of phenol and react at 90°C for 3 hours. Under the action of acid catalysis, promote the polycondensation reaction between hydroxymethyl groups and between hydroxymethyl groups and amino groups to form a phenolic resin prepolymer with a high molecular weight.

[0035] Step 3: Heat the reaction system to 140℃ and maintain the reaction for 3.5h. During this process, the polycondensation reaction and structural rearrangement are further promoted, so that the system forms a more dense and stable three-dimensional cross-linked network.

[0036] Step 4: After the reaction is completed, dehydrate at 120℃ under normal pressure for 2 hours to remove the generated water. Then, heat to 170℃ and perform devastation under reduced pressure at 9000Pa for 2.5 hours to fully remove low molecular weight volatiles and unreacted components, and obtain high heat-resistant phenolic resin. Example 4

[0037] Step 1: Weigh the raw materials according to the molar ratio of phenol, melamine and formaldehyde 1:0.6:0.8. Add phenol to the reaction vessel and heat to 90°C to form a homogeneous liquid phase. Add melamine under stirring to make it uniformly dispersed. Then add formaldehyde solution and react at 90°C for 3 hours to allow the polyamino groups in melamine to fully participate in hydroxymethylation and pre-condensation reaction with phenol, forming a highly functional modified phenol intermediate.

[0038] Step 2: Add formaldehyde to the above intermediate to make the initial molar ratio of phenol to formaldehyde in step S1 reach 1:2.0. At the same time, add 5 wt% of p-toluenesulfonic acid and phosphoric acid composite catalyst of phenol and react at 95°C for 3 hours. Under strong acid conditions, the system is rapidly polycondensed to form a high molecular weight phenolic resin prepolymer.

[0039] Step 3: Heat the reaction system to 150°C and maintain the reaction for 4 hours. Under high temperature conditions, further promote the polycondensation reaction and the formation of cross-linked structures, so that the system forms a three-dimensional network with high cross-linking density.

[0040] Step 4: After the reaction is complete, dehydrate at atmospheric pressure at 120℃ for 2 hours, then heat to 180℃ and devolve under reduced pressure at 10000Pa for 3 hours to fully remove low molecular weight volatiles and residual monomers, and finally obtain a modified phenolic resin with excellent heat resistance and structural stability. Example 5

[0041] Step 1: Weigh the raw materials according to the molar ratio of phenol, melamine and formaldehyde 1:0.5:0.4. Add phenol to a reaction vessel equipped with a stirrer, temperature control and reflux device, and heat to 80°C to fully melt and form a homogeneous system. Slowly add melamine under stirring to make it uniformly dispersed, and then add formaldehyde solution dropwise. React at 80°C for 1.5 hours. During this process, the amino group in melamine first undergoes hydroxymethylation reaction with formaldehyde, and then further undergoes pre-condensation reaction with phenol, thereby forming a modified phenol intermediate with a uniformly distributed nitrogen-containing structure.

[0042] Step 2: Based on the intermediate obtained in Step 1, formaldehyde is added to adjust the initial molar ratio of phenol to formaldehyde in Step S1 to 1:1.5. At the same time, 2.0 wt% of p-toluenesulfonic acid is added as an acidic catalyst. The reaction is carried out at 80°C for 1.8 h. Under the action of acid catalysis, the polycondensation reaction between hydroxymethylphenol and between hydroxymethylphenol and nitrogen-containing structures is promoted to form a phenolic resin prepolymer with a certain molecular weight.

[0043] Step 3: Slowly heat the above prepolymer system to 130°C under continuous stirring and maintain the reaction for 2 hours to allow the residual hydroxymethyl groups in the system to continue to undergo condensation reaction, while promoting molecular chain growth and the formation of preliminary cross-linked structures.

[0044] Step 4: After the reaction is completed, the system is dehydrated at 100℃ under normal pressure for 1 hour to remove the water and some low-boiling substances generated during the reaction. Then, the temperature is raised to 150℃ and the system pressure is adjusted to 5000Pa. The system is then dehydrated under reduced pressure for 2 hours to further remove residual low-molecular-weight substances and unreacted monomers, and finally a modified phenolic resin with uniform structure and good heat resistance is obtained.

[0045] Examples 6 to 10 The difference between Examples 6 to 10 and Example 5 lies in the different molar ratios of the modified phenol intermediate to formaldehyde, as shown in the table below.

[0046] Table 1

[0047] Comparison Example 1 (without melamine) Compared with Example 1, the difference is that melamine is not added in step S1, and the remaining raw material ratios and process conditions are the same as in Example 1.

[0048] Compare with Example 2 (without pre-modification steps). Compared with Example 1, the difference is that step S1 is omitted, and phenol, melamine and formaldehyde are directly added to step S2 to carry out polycondensation reaction, while the other conditions are the same as in Example 1.

[0049] Performance testing methods Thermogravimetric properties (thermal stability, TGA) Thermogravimetric analysis (TGA) was used to test the samples. Approximately 5–10 mg of resin sample was weighed and placed in a crucible. Under a nitrogen protective atmosphere, the temperature was increased from room temperature to 800°C at a rate of 10°C / min. The mass change curve of the sample was recorded, and the temperature of maximum weight loss (Tmax) was obtained.

[0050] Glass transition temperature (Tg) Differential scanning calorimetry (DSC) was used for testing. 5–10 mg of cured resin sample was weighed and heated from room temperature to 300 °C at a rate of 10 °C / min under a nitrogen atmosphere. The heat flow curve was recorded and the glass transition temperature (Tg) was determined by the baseline abrupt change point.

[0051] Carbon residue rate (carbonization performance) The sample was heated to 800℃ in a muffle furnace or tube furnace under a nitrogen atmosphere and held for 1 hour. After cooling, the residual mass was weighed and the char rate was calculated (residual mass / initial mass × 100%).

[0052] Heat distortion temperature (HDT) Prepare standard specimens from resin according to standard methods (such as ASTM D648), heat them at a heating rate of 2°C / min under a specified load (such as 1.8 MPa), and record the temperature at which the specimen undergoes the specified deformation, i.e., the heat distortion temperature.

[0053] Mechanical properties (flexural strength) Three-point bending tests were performed on the cured resin specimens using a universal testing machine. The specimens were prepared according to standards (such as ASTM D790), and the tests were conducted at the specified span and loading rate. The maximum load was recorded and the bending strength was calculated.

[0054] Table 2

[0055] In conjunction with Example 1, Figure 1 as well as Figure 2 It can be seen that, Figure 1 3400 / 3320 / 3280 cm -1 It exhibits extremely strong broad OH / NH peaks at 1080 / 1015 cm⁻¹. -1 The hydroxymethyl CO peak is significantly prominent, at 810 / 1550 cm⁻¹. -1 The triazine ring characteristic peaks are sharp, and the 1650 / 1605 cm⁻¹ peaks are also sharp. -1 The distinct C=N peaks of the benzene ring and triazine indicate that melamine has successfully intercalated into the phenol system via Mannich-type precondensation, forming a multifunctional intermediate rich in active hydroxymethyl, amino, and triazine rings; Figure 2 After final modification with phenolic resin, the ratio is 1080 / 1015 cm⁻¹. -1 The hydroxymethyl peak decreased significantly at 1450 cm⁻¹ -1 The methylene bridge peak and 2920 / 2950 cm⁻¹ -1 The lipid CH peak was significantly enhanced, at 3320 / 3280 cm⁻¹. -1 The intensity of the NH peak decreased slightly, while the 810 / 1550 cm⁻¹ peak intensity remained relatively stable. -1The characteristic peak of the triazine ring remains high and is at 1450 / 1230 cm⁻¹. -1 The crosslinking-related peaks were more prominent, confirming that a large number of hydroxymethyl groups underwent dehydration condensation in the S2-S4 stages, forming a three-dimensional dense network bridged by methylene / aminomethylene groups, while the rigid structure of the triazine was fully preserved. In summary, Figure 1 → Figure 2 The changes in the spectral peaks directly verified the effectiveness of the pre-modification path in Example 1, realizing a structural transition from the active intermediate to the highly crosslinked heat-resistant resin, thereby obtaining excellent performance.

[0056] As can be seen from Examples 1 to 4 and Table 2, the heat resistance and mechanical properties of the prepared modified phenolic resin are as follows: Tmax from 412℃ to 462℃, Tg from 168℃ to 215℃, char residue from 58.3% to 73.5%, HDT from 162℃ to 205℃, and flexural strength from 92 MPa to 126 MPa. This indicates that the amount of melamine introduced in the range of 0.1 to 0.6 can effectively construct a high crosslinking density benzene ring-triazine ring synergistic network, thereby significantly improving the thermal stability, char residue, and high-temperature mechanical properties of the resin.

[0057] As can be seen from Examples 5 to 10 and Table 2, as the molar ratio of modified phenol intermediate to formaldehyde gradually increases from 1:1.0 to 1:2.0, the resin's Tmax increases from 422℃ to 449℃, Tg increases from 175℃ to 197℃, char residue increases from 60.5% to 68.3%, HDT increases from 169℃ to 191℃, and flexural strength increases from 98 MPa to 117 MPa. All properties show a trend of rapid increase followed by a gradual plateauing. The main reason is that the increase in formaldehyde ratio promotes the dehydration condensation reaction between hydroxymethyl groups, which improves the crosslinking density and molecular chain rigidity of the system. However, when the molar ratio exceeds 1:1.8, the contribution of further increasing formaldehyde to the crosslinking density gradually decreases. At the same time, a small amount of unreacted hydroxymethyl groups or branched structures may be introduced, resulting in a slower increase in performance.

[0058] Combining Example 1, Comparative Example 1, and Table 2, it can be seen that Example 1 (with melamine added and pre-modified) has a Tmax of 412℃, a Tg of 168℃, a char residue of 58.3%, an HDT of 162℃, and a flexural strength of 92 MPa. In contrast, the corresponding indicators of Comparative Example 1 (without melamine added) are only 385℃, 145℃, 49.2%, 138℃, and 78 MPa, respectively, all of which are significantly lower than those of Example 1. This is because the triazine ring and polyamino groups in melamine are successfully embedded into the phenol system through the pre-modification step, forming a rigid nitrogen-containing structure and additional crosslinking points. This effectively reduces the proportion of heat-sensitive methylene structures, enhances the network stability and carbonization density at high temperatures, and thus significantly improves the thermal oxidation stability and mechanical properties of the resin. In contrast, Comparative Example 1 relies solely on traditional phenol-formaldehyde polycondensation, resulting in a relatively loose crosslinking network and significantly insufficient heat resistance.

[0059] Combining Example 1, Comparative Example 2, and Table 2, it can be seen that Example 1 (using the pre-modification step) has a Tmax of 412℃, a Tg of 168℃, a char residue of 58.3%, an HDT of 162℃, and a flexural strength of 92 MPa. In contrast, the corresponding indicators of Comparative Example 2 (omitting the pre-modification step and directly mixing phenol, melamine, and formaldehyde) are reduced to 372℃, 139℃, 46.8%, 132℃, and 72 MPa, respectively, with all properties being significantly inferior to those of Example 1. The main reason for this is that the pre-modification step allows melamine to first form a uniformly distributed multifunctional nitrogen-containing intermediate with phenol at a lower temperature, avoiding the uneven local crosslinking and side reactions caused by the difference in the reactivity of melamine when directly mixed. In contrast, in Comparative Example 2, melamine is difficult to fully pre-intercalate into the phenol system, and the resulting aminomethylene bridging structure has poor thermal stability, is prone to reverse Mannich reaction and releases small molecules, resulting in insufficient density of the final resin network and a significant reduction in heat resistance and mechanical strength.

[0060] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a high heat-resistant phenolic resin, characterized in that, Includes the following steps: S1. Pre-modification treatment of phenol: Phenol and melamine are pre-reacted in the presence of formaldehyde to form a modified phenol intermediate; S2. The modified phenol intermediate obtained in step S1 is subjected to a polycondensation reaction with formaldehyde under the action of an acidic catalyst to form a phenolic resin prepolymer. S3. Heat the phenolic resin prepolymer to 110-150°C and continue the reaction for 2-4 hours; S4. After dehydrating the reaction system under normal pressure, it is subjected to vacuum devolatilization at 130–180℃ and 5000–10000Pa to obtain modified phenolic resin.

2. The method for preparing high heat-resistant phenolic resin according to claim 1, characterized in that, In step S1, the molar ratio of phenol, melamine and formaldehyde is 1:(0.1-0.6):(0.2-0.8).

3. The method for preparing high heat-resistant phenolic resin according to claim 1, characterized in that, Formaldehyde is added to the modified phenol intermediate obtained in step S1, wherein the molar amount of added formaldehyde is in the ratio of the initial molar amount of phenol in step S1 to (1.0~2.0):

1.

4. The method for preparing high heat-resistant phenolic resin according to claim 1, characterized in that, In step S2, the acidic catalyst is one or a combination of at least two of p-toluenesulfonic acid, hydrochloric acid, phosphoric acid or oxalic acid, and its addition amount is 0.5 to 5 wt% of the mass of phenol.

5. The method for preparing high heat-resistant phenolic resin according to claim 1, characterized in that, The reaction temperature in step S1 is 65–90°C, and the reaction time is 1.5–3 hours.

6. The method for preparing high heat-resistant phenolic resin according to claim 1, characterized in that, The reaction temperature in step S2 is 80–95°C, and the reaction time is 2–3 hours.

7. The method for preparing high heat-resistant phenolic resin according to claim 1, characterized in that, In step S4, the dehydration temperature under normal pressure is 100-120℃, and the dehydration time is 1-2 hours.

8. The method for preparing high heat-resistant phenolic resin according to claim 1, characterized in that, The decompression and volatilization time in step S4 is 1 to 3 hours.

9. A high heat-resistant phenolic resin prepared according to any one of the preparation methods of claims 1 to 8.

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