Modified phenolic resin-based material for high-temperature-resistant brake pad and preparation method thereof

Abstract

The invention belongs to the technical field of friction materials and phenolic resin modification, and discloses a modified phenolic resin-based material for a high-temperature-resistant brake pad and a preparation method of the modified phenolic resin-based material. The modified phenolic resin-based material is prepared from the following components in parts by weight: 80 to 120 parts of novolac type phenolic resin, 8 to 30 parts of low-free hydroxymethyl resol type phenolic prepolymer containing a rigid bisphenol structural unit and 6 to 9 parts of hexamethylenetetramine, wherein the rigid bisphenol structural unit exists in the low-free hydroxymethyl resol type phenolic prepolymer in the form of a copolycondensation structural unit, and the obtained material still retains part of functional structural units which are not completely reacted after primary molding. The preparation method comprises the following steps: preparing a low-free hydroxymethyl resol type phenolic prepolymer through prepolymerization, mixing the low-free hydroxymethyl resol type phenolic prepolymer with novolac type phenolic resin, adding hexamethylenetetramine, and carrying out hot press molding and staged curing. The material has better high-temperature structural stability and heat fading resistance, and is suitable for a resin-based binding phase in a brake pad friction material.

Description

Technical Field

[0001] This invention belongs to the field of friction materials and phenolic resin modification technology, specifically relating to a modified phenolic resin-based material for high-temperature brake pads and its preparation method. Background Technology

[0002] Phenolic resins have long been used in friction materials such as brake pads due to their good heat resistance, adhesion, moldability, and cost advantages. They play a crucial role as a resin-based binder phase in the braking process, binding reinforcing fibers, fillers, and friction-modifying components. Novolac-type phenolic resins, cured with hexamethylenetetramine to form a cross-linked network, are among the most commonly used systems. However, with the increasing demands on braking performance in automobiles, construction machinery, and high-speed equipment, the heat load on brake pads under continuous braking, heavy-load braking, and high-temperature conditions has significantly increased. Traditional phenolic resin-based materials have gradually revealed problems such as insufficient heat resistance, significant thermal fading, easy powdering of the surface structure, and insufficient frictional stability. Particularly under high-temperature friction conditions, the resin binder phase is prone to thermal decomposition or structural instability, leading to discontinuous friction layers, accelerated wear, and further affecting braking safety and service life.

[0003] Existing technologies typically enhance the high-temperature performance of phenolic resins by introducing heat-resistant components, increasing the degree of crosslinking, or improving the resin matrix structure, but some shortcomings remain. On the one hand, while some modification schemes can improve initial heat resistance, they can easily lead to increased material brittleness, affecting toughness and wear resistance in actual use. On the other hand, some multi-component composite systems suffer from mismatched reaction temperature zones and insufficient synergy between components, making it difficult to balance processability and high-temperature service stability. Furthermore, existing technologies do not adequately consider the design of retaining some functional structural units that can continue to react after the initial molding and further stabilizing the surface structure under high-temperature conditions, thus failing to effectively improve sustained stability under high-temperature braking conditions. Summary of the Invention

[0004] To address the shortcomings mentioned in the background art, the present invention aims to provide a modified phenolic resin-based material for high-temperature brake pads and its preparation method. This material uses novolac-type phenolic resin as the main body, introduces a low-free hydroxymethyl resol-type phenolic prepolymer containing rigid bisphenol structural units, and is cured with hexamethylenetetramine, so that the resulting material retains some incompletely reacted functional structural units after the initial molding. By controlling the prepolymer composition, raw material ratio, and staged curing process, the structural stability, surface continuity, and heat fade resistance of the material under high-temperature conditions are improved, making it suitable for the resin-based binder phase in brake pad friction materials.

[0005] The objective of this invention can be achieved through the following technical solutions: A modified phenolic resin-based material for high-temperature brake pads comprises the following raw materials in parts by weight: 80-120 parts of novolac-type phenolic resin, 8-30 parts of low-free hydroxymethyl resol-type phenolic prepolymer containing rigid bisphenol structural units, and 6-9 parts of hexamethylenetetramine. In the low-free hydroxymethyl resol phenolic prepolymer containing rigid bisphenol structural units, the rigid bisphenol structural units exist in the form of co-condensation structural units in the low-free hydroxymethyl resol phenolic prepolymer, and the modified phenolic resin-based material retains some incompletely reacted functional structural units after the initial molding.

[0006] More preferably, the free hydroxymethyl content of the low-free hydroxymethyl resol type phenolic prepolymer is 3.0 to 10.0 wt%.

[0007] More preferably, the rigid bisphenol structural unit is 9,9-bis(4-hydroxyphenyl)fluorene.

[0008] More preferably, the unreacted functional structural unit includes one or both of the hydroxymethyl and benzyl ether structures.

[0009] More preferably, the low-free hydroxymethyl resol-type phenolic prepolymer containing rigid bisphenol structural units is prepared by prepolymerization of phenol, formaldehyde, and rigid bisphenol compounds under alkaline catalytic conditions.

[0010] A method for preparing a modified phenolic resin-based material for high-temperature resistant brake pads includes the following steps: S1. Phenol, formaldehyde and rigid bisphenol type compounds are prepolymerized to obtain a low free hydroxymethyl resol type phenolic prepolymer containing rigid bisphenol structural units; S2. Mix the novolac type phenolic resin with the low free hydroxymethyl resol type phenolic prepolymer to obtain a resin mixture system; S3. Add hexamethylenetetramine to the resin mixture system and mix evenly to obtain the mixture to be molded; S4. The mixture to be molded is hot-pressed and then cured in stages, so that the resulting material retains some unreacted functional structural units after the initial molding, thus obtaining the modified phenolic resin-based material for high-temperature brake pads.

[0011] More preferably, in the prepolymerization step S1, formaldehyde is added in the form of formaldehyde aqueous solution or paraformaldehyde, the prepolymerization reaction temperature is 60-90°C, and the prepolymerization reaction time is 1-4 hours.

[0012] More preferably, the mixing temperature of the novolac type phenolic resin and the low free hydroxymethyl resol type phenolic prepolymer is 80-100°C, and the mixing temperature of the added hexamethylenetetramine is 65-80°C.

[0013] More preferably, the staged curing includes the following steps: holding at 110-125℃ for 10-20 min, holding at 140-155℃ for 20-40 min, holding at 165-175℃ for 10-25 min, and then post-treating at 175-182℃ for 0.5-1.5 h.

[0014] Application of modified phenolic resin-based materials for high-temperature brake pads in brake pad friction materials.

[0015] The beneficial effects of this invention are: This invention uses novolac-type phenolic resin as the main body, introduces a low-free hydroxymethyl resol-type phenolic prepolymer containing rigid bisphenol structural units, and combines it with a hexamethylenetetramine curing system to construct a modified phenolic resin-based material that combines main network stability, residual reactivity, and rigid structural reinforcement. Compared with conventional phenolic resin-based materials, this invention introduces rigid bisphenol structural units into the prepolymer in the form of co-condensation structural units, allowing them to enter the continuous resin phase in a reactive manner rather than existing in a simple physically dispersed form. This is beneficial for improving the skeletal stability and structural retention of the material under high-temperature conditions. Simultaneously, by controlling the composition, free hydroxymethyl content, and ratio of the low-free hydroxymethyl resol-type phenolic prepolymer with novolac-type phenolic resin and hexamethylenetetramine, the material retains some incompletely reacted functional structural units after initial molding. These units can continue to undergo further condensation and densification under subsequent high-temperature braking conditions, thus helping to maintain the continuity and integrity of the surface structure and reducing structural loosening, pulverization, and local instability under thermal effects. Since the formation of the main network, the retention of the residual active structure, and the introduction of the rigid structure in this invention are all completed synergistically within similar phenolic curing temperature ranges, the problems of mismatched reaction windows and asynchronous structure formation in existing multi-system modification schemes are avoided. Therefore, it can not only maintain good processability, but also take into account high-temperature structural stability, frictional stability, and resistance to thermal degradation. Attached Figure Description

[0016] The invention will now be further described with reference to the accompanying drawings.

[0017] Figure 1 Thermogravimetric analysis curves for Example 3 and Comparative Example 2; Figure 2 The bar chart shows the tribological performance test results of Examples 1-3 and Comparative Examples 1-2. Detailed Implementation

[0018] The technical solutions of 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.

[0019] The novolac-type phenolic resin used in this invention is a thermoplastic linear phenolic resin obtained by polycondensation of phenol and formaldehyde under acidic catalytic conditions.

[0020] Example 1: Verification that the present invention can still achieve material molding and structure construction under lower ratios and milder curing conditions.

[0021] I. Preparation of Low-Free-Hydroxymethyl Resol-Type Phenolic Prepolymers Containing Rigid Bisphenol Structural Units 84.7 g of phenol and 35.0 g of 9,9-bis(4-hydroxyphenyl)fluorene were added to a four-necked flask and heated to 70°C with stirring to ensure homogeneous mixing. Then, 3.2 g of 30 wt% sodium hydroxide aqueous solution was added, and stirring continued for 10 min. While maintaining the system temperature at 78°C, 64.9 g of 37 wt% formaldehyde aqueous solution was slowly added dropwise. After the addition was complete, the system temperature was maintained at 80°C for 2 h to obtain a preliminary prepolymer reaction solution. After the reaction was completed, the system temperature was lowered to 68°C, and some water and a small amount of volatile small molecules were removed under reduced pressure to control the resin system to a flowable viscous state. The system was further concentrated under reduced pressure until the solid content was approximately 75 wt%. Heating was stopped, and the material was cooled and discharged to obtain a low-free hydroxymethyl resol-type phenolic prepolymer containing rigid bisphenol structural units, with the free hydroxymethyl content controlled at 3.0 wt%.

[0022] II. Preparation of Modified Phenolic Resin-Based Materials for High-Temperature Resistant Brake Pads The modified phenolic resin-based material for high-temperature brake pads comprises the following raw materials in parts by weight: 80 parts of novolac phenolic resin, 8 parts of low-free hydroxymethyl resol phenolic prepolymer containing rigid bisphenol structural units, and 6 parts of hexamethylenetetramine. The preparation steps are as follows: 80.0 g of novolac-type phenolic resin and 8.0 g of low-free hydroxymethyl resol-type phenolic prepolymer containing rigid bisphenol structural units are added to a mixing vessel equipped with a mechanical stirrer, thermometer, and heating device. Under stirring conditions, the temperature is raised to 80℃, the stirring speed is controlled at 200 r / min, and the mixture is kept at this temperature for 20 min to ensure that the novolac-type phenolic resin and the prepolymer are fully melted and mixed evenly, thus obtaining a resin mixture system. Subsequently, the system is cooled to 65℃, and 6.0 g of hexamethylenetetramine is added. Stirring is continued for 10 min to ensure that the hexamethylenetetramine is evenly dispersed in the resin mixture system, thus obtaining the mixture to be molded. The obtained mixture to be molded is removed, cooled to room temperature, pulverized, and passed through a 20-40 mesh sieve to obtain molding powder. The molding powder was loaded into a preheated mold and hot-pressed at 10 MPa on a flatbed hot press. The molded material was then cured in stages according to the following schedule: 110℃ for 10 min, then heated to 140℃ for 20 min, and finally heated to 165℃ for 10 min. After curing, the molded sample was demolded to obtain the initial molded sample. The sample was then placed in a forced-air drying oven and post-treated at 175℃ for 0.5 h, followed by cooling to room temperature to obtain the modified phenolic resin-based material for high-temperature brake pads.

[0023] Example 2: Verification that the present invention can still achieve material molding and structure construction under higher ratio and stronger curing conditions.

[0024] I. Preparation of Low-Free-Hydroxymethyl Resol-Type Phenolic Prepolymers Containing Rigid Bisphenol Structural Units 84.7 g of phenol and 35.0 g of 9,9-bis(4-hydroxyphenyl)fluorene were added to a flask, and the mixture was heated to 80°C with stirring to ensure thorough mixing. Then, 3.2 g of 30 wt% sodium hydroxide aqueous solution was added, and stirring continued for 10 min. While maintaining the system temperature at 80°C, 81.1 g of 37 wt% formaldehyde aqueous solution was slowly added dropwise. After the addition was complete, the system temperature was maintained at 80°C for 2 h to obtain a preliminary prepolymer reaction solution. After the reaction was complete, the system temperature was lowered to 68°C, and some water and a small amount of volatile small molecules were removed under reduced pressure to control the resin system to a flowable viscous state. The solution was further concentrated under reduced pressure until the solid content was approximately 80 wt%. Heating was stopped, and the solution was cooled and discharged to obtain a low-free hydroxymethyl resol-type phenolic prepolymer containing rigid bisphenol structural units. The free hydroxymethyl content of the obtained prepolymer was controlled to be 10.0 wt%.

[0025] II. Preparation of Modified Phenolic Resin-Based Materials for High-Temperature Resistant Brake Pads The modified phenolic resin-based material for high-temperature brake pads comprises the following raw materials in parts by weight: 120 parts of novolac phenolic resin, 30 parts of low-free hydroxymethyl resol phenolic prepolymer containing rigid bisphenol structural units, and 9 parts of hexamethylenetetramine. The preparation steps are as follows: 120.0 g of novolac-type phenolic resin and 30.0 g of the above-mentioned low-free hydroxymethyl resol-type phenolic prepolymer containing rigid bisphenol structural units are added to a mixing vessel. Under stirring conditions, the temperature is raised to 90℃, the stirring speed is controlled at 200 r / min, and the mixture is kept at this temperature for 20 min to ensure that the novolac-type phenolic resin and the prepolymer are fully melted and mixed evenly, thus obtaining a resin mixture system. Subsequently, the system is cooled to 75℃, 9.0 g of hexamethylenetetramine is added, and the mixture is stirred and mixed for another 10 min to obtain the mixture to be molded. The obtained mixture to be molded is removed, cooled to room temperature, pulverized, and passed through a 20-40 mesh sieve to obtain the molding powder. The molding powder was loaded into a preheated mold and hot-pressed at 10 MPa on a flatbed hot press. The molded material was then cured in stages according to the following schedule: 125°C for 20 min, then heated to 155°C and held for 40 min, followed by heating to 175°C and holding for 25 min. After staged curing, the molded material was demolded to obtain the initial molded sample. The initial molded sample was placed in a forced-air drying oven and post-treated at 182°C for 1.5 h, then cooled to room temperature to obtain the modified phenolic resin-based material for high-temperature brake pads.

[0026] Example 3: Preparation of modified phenolic resin-based materials by selecting representative proportions of each component and process condition. I. Preparation of Low-Free-Hydroxymethyl Resol-Type Phenolic Prepolymers Containing Rigid Bisphenol Structural Units 84.7 g of phenol and 35.0 g of 9,9-bis(4-hydroxyphenyl)fluorene were added to a flask, and the mixture was heated to 75°C with stirring to ensure homogeneous mixing. Then, 3.2 g of 30 wt% sodium hydroxide aqueous solution was added, and stirring continued for 10 min. While maintaining the system temperature at 75°C, 73.0 g of 37 wt% formaldehyde aqueous solution was slowly added dropwise. After the addition was complete, the system temperature was maintained at 75°C, and the reaction continued for 2.5 h to obtain a preliminary prepolymer reaction solution. After the reaction was completed, the system temperature was lowered to 68°C, and some water and a small amount of volatile small molecules were removed under reduced pressure to control the resin system to a flowable viscous state. The solution was further concentrated under reduced pressure until the solid content was approximately 75 wt%. Heating was stopped, and the solution was cooled and discharged to obtain a low-free hydroxymethyl resol-type phenolic prepolymer containing rigid bisphenol structural units. The free hydroxymethyl content of the obtained prepolymer was controlled to be 6.5 wt%.

[0027] II. Preparation of Modified Phenolic Resin-Based Materials for High-Temperature Resistant Brake Pads The modified phenolic resin-based material for high-temperature brake pads comprises the following raw materials in parts by weight: 100 parts of novolac phenolic resin, 19 parts of low-free hydroxymethyl resol phenolic prepolymer containing rigid bisphenol structural units, and 7.5 parts of hexamethylenetetramine. The preparation steps are as follows: 100.0 g of novolac-type phenolic resin and 19.0 g of low-free hydroxymethyl resol-type phenolic prepolymer containing rigid bisphenol structural units are added to a mixing vessel. Under stirring conditions, the temperature is raised to 90℃, the stirring speed is controlled at 200 r / min, and the mixture is kept at this temperature for 20 min to obtain a resin mixture system. Subsequently, the system is cooled to 72℃, 7.5 g of hexamethylenetetramine is added, and the mixture is stirred and mixed for another 10 min to obtain the mixture to be molded. The obtained mixture to be molded is taken out, cooled to room temperature, pulverized, and passed through a 20-40 mesh sieve to obtain molding powder. The molding powder is placed into a preheated mold and hot-pressed on a flatbed hot press at a pressure of 10 MPa. The mixture is then cured in stages according to the following regime: first, it is kept at 117℃ for 15 min, then the temperature is raised to 147℃ and kept for 30 min, and then the temperature is raised to 170℃ and kept for 17.5 min. After curing, the mixture is demolded to obtain the initial molded sample. The initial molded sample was placed in a forced-air drying oven and post-treated at 178°C for 1.0 h, then cooled to room temperature to obtain the modified phenolic resin-based material for high-temperature brake pads.

[0028] Comparative Example 1: No rigid bisphenol structural units were introduced to compare the effects of rigid bisphenol structural units on the structural stability and high-temperature performance of resin-based materials.

[0029] I. Preparation of Low-Free-Hydroxymethyl Resol-Type Phenolic Prepolymers Without Rigid Bisphenol Structural Units 94.1 g of phenol was added to a four-necked flask, and the mixture was heated to 75°C with stirring to ensure homogeneity. Then, 3.2 g of a 30 wt% sodium hydroxide aqueous solution was added, and stirring continued for 10 min. While maintaining the system temperature at 75°C, 73.0 g of a 37 wt% formaldehyde aqueous solution was slowly added dropwise. After the addition was complete, the system temperature was maintained at 75°C, and the reaction continued for 2.5 h to obtain a preliminary prepolymer reaction solution. After the reaction was complete, the system temperature was lowered to 68°C, and some water and a small amount of volatile small molecules were removed under reduced pressure to control the resin system to a flowable viscous state. The solution was then concentrated under reduced pressure until the solid content was approximately 75 wt%. Heating was stopped, and the solution was cooled and discharged to obtain a low-free hydroxymethyl resol type phenolic prepolymer without rigid bisphenol structural units. The free hydroxymethyl content of the obtained prepolymer was controlled to be 6.5 wt%.

[0030] II. Preparation of Modified Phenolic Resin-Based Materials for High-Temperature Resistant Brake Pads The modified phenolic resin-based material for high-temperature brake pads comprises the following raw materials in parts by weight: 100 parts of novolac phenolic resin, 19 parts of low-free hydroxymethyl resol phenolic prepolymer without rigid bisphenol structural units, and 7.5 parts of hexamethylenetetramine. The preparation steps are as follows: 100.0 g of novolac-type phenolic resin and 19.0 g of the above-mentioned low-free hydroxymethyl resol-type phenolic prepolymer without rigid bisphenol structural units are added to a mixing vessel. The mixture is heated to 90°C with stirring at a stirring speed of 200 r / min and mixed for 20 min to obtain a resin mixture. The temperature is then lowered to 72°C, and 7.5 g of hexamethylenetetramine is added. The mixture is stirred and mixed for another 10 min to obtain the mixture to be molded. The obtained mixture is removed, cooled to room temperature, pulverized, and passed through a 20-40 mesh sieve to obtain molding powder. The molding powder is placed into a preheated mold and hot-pressed on a flatbed hot press at a pressure of 10 MPa, followed by staged curing: holding at 117°C for 15 min, then heating to 147°C and holding for 30 min, and then heating to 170°C and holding for 17.5 min. After curing, the mold is removed to obtain the initial molded sample. The initial molded sample was post-treated at 178℃ for 1.0 h and cooled to room temperature to obtain the high-temperature resistant modified phenolic resin-based material for brake pads of Comparative Example 1.

[0031] Comparative Example 2: No low-free hydroxymethyl resol-type phenolic prepolymer was added, used to compare the effects of this prepolymer on the structure formation, high-temperature stability and heat degradation resistance of resin-based materials.

[0032] The modified phenolic resin-based material for the high-temperature resistant brake pads comprises the following raw materials in parts by weight: 100 parts of novolac-type phenolic resin and 7.5 parts of hexamethylenetetramine. The preparation steps are as follows: 100.0 g of novolac-type phenolic resin is added to a mixing vessel, heated to 90℃ under stirring, and the stirring speed is controlled at 200 r / min. The mixture is kept at this temperature for 20 min to ensure complete melting of the novolac-type phenolic resin, thus obtaining a resin system. The system is then cooled to 72℃, and 7.5 g of hexamethylenetetramine is added. The mixture is stirred and mixed for another 10 min to obtain the mixture to be molded. The resulting mixture is removed, cooled to room temperature, pulverized, and passed through a 20-40 mesh sieve to obtain molding powder. The molding powder is placed into a preheated mold and hot-pressed on a flatbed hot press at a pressure of 10 MPa, followed by staged curing: first, the temperature is held at 117℃ for 15 min, then increased to 147℃ and held for 30 min, and then increased to 170℃ and held for 17.5 min. After staged curing, the sample is demolded to obtain the initial molded sample. The initial molded sample was placed in a forced-air drying oven and post-treated at 178°C for 1.0 h, then cooled to room temperature to obtain the resin-based material of Comparative Example 2.

[0033] Performance testing 1. Structural and curing state testing The post-processed final-state samples obtained in Examples 1-3 and Comparative Examples 1-2 were pulverized and dried for later use. Fourier transform infrared spectroscopy (FTIR) was performed using the KBr pellet method, with a scanning range of 4000–400 cm⁻¹, and nitrogen purging was used to eliminate water vapor interference. Characteristic peak assignment and quantification: The characteristic peak of the rigid bisphenol structural unit is located at ~3030 cm⁻¹, the characteristic peak of hydroxymethyl is located at ~1015 cm⁻¹, and the characteristic peak of benzyl ether is located at ~1150 cm⁻¹. Peak area quantification was used, with background interference corrected by a horizontal baseline. The integrated peak area was taken as the absorption intensity, and the absorption intensity of the hydroxymethyl / benzyl ether peak was taken as the ratio of the corresponding characteristic peak areas.

[0034] Differential scanning calorimetry (DSC) was performed using 5–10 mg samples and a blank aluminum crucible as a reference. Under nitrogen protection, the temperature was increased from room temperature to 350 °C at a rate of 10 °C / min, and the residual exothermic peak temperature ( °C) and peak area (mJ) were recorded. The free hydroxymethyl content was determined using periodic acid oxidation-acid-base titration to obtain the free hydroxymethyl content (wt%) in the samples. The results are shown in Table 1 below.

[0035] Table 1 Structure and Curing State

[0036] As shown in Table 1, the FTIR results of Examples 1-3 show high characteristic peak intensities of the rigid bisphenol structural units, indicating that the rigid bisphenol structural units are effectively introduced into these materials. The absorption intensities of hydroxymethyl and benzyl ether in each example are also significant, especially in Example 2, where the absorption intensity of the hydroxymethyl / benzyl ether peak reaches 0.12, indicating a high content of unreacted functional structural units. DSC results show that the residual exothermic peak temperatures of Examples 1-3 are between 205℃ and 218℃, with large peak areas, indicating strong stability and high crosslinking degree at high temperatures. In contrast, the FTIR characteristic peak absorption in Comparative Examples 1 and 2 are significantly lower, especially in Comparative Example 2, where both the rigid bisphenol peak and the hydroxymethyl / benzyl ether peak are below the detection limit, indicating a lack of effective rigid structural units and reaction sites in its structure. In the DSC test, the residual exothermic peak temperatures of Comparative Examples 1 and 2 are significantly lower than those of the examples, at 185℃ and 178℃ respectively, with large peak areas, indicating poor thermal stability and low crosslinking degree.

[0037] 2. Thermal performance test The post-treatment final-state samples obtained in Examples 1-3 and Comparative Examples 1-2 were pulverized, passed through a 200-mesh standard sieve, and dried at 60°C in a vacuum drying oven to constant weight to eliminate moisture content interference before use. Thermogravimetric analysis (TGA) was used to test the thermal properties of the samples. 5-10 mg of sample was weighed and placed in an alumina crucible. Under a nitrogen protective atmosphere, the crucible was purged with nitrogen at a flow rate of 20 mL / min and heated from room temperature to 800°C at a rate of 10°C / min. The 5% and 10% thermogravimetric temperatures in the TGA curves, as well as the temperature corresponding to the maximum rate of thermogravimetric loss in the differential TGA curves, were recorded. The char residue at 800°C was calculated using the integral method. The results are shown in Table 2 below.

[0038] Table 2 Thermal Performance Tests

[0039] As shown in Table 2, the samples of Examples 1-3 exhibit significantly better thermogravimetric properties than those of Comparative Examples 1-2, particularly in terms of thermogravimetric temperature and char residue. Example 2 demonstrates the best thermal stability, with 5% and 10% thermogravimetric temperatures of 372℃ and 418℃, respectively, and a char residue of 63.5% at 800℃, indicating strong high-temperature resistance and good thermal stability. In contrast, Comparative Examples 1 and 2 show significantly lower thermogravimetric temperatures and lower char residues, at 51.4% and 46.7%, respectively, indicating poorer thermal stability and high-temperature performance. The thermal properties of Examples 1 and 3 also show good stability, indicating the role of the low-free hydroxymethyl resol phenolic prepolymer containing rigid bisphenol structural units in improving the high-temperature stability of the material.

[0040] 3. Mechanical property testing The post-processed final-state specimens obtained in Examples 1-3 and Comparative Examples 1-2 were used to prepare standard mechanical test specimens by molding. Bending strength testing was performed using the three-point bending method. The specimen dimensions were 80 mm × 10 mm × 4 mm, with a span of 64 mm. The testing speed was 2 mm / min. The maximum load at specimen fracture was recorded, and the bending strength was calculated. Compressive strength testing was performed using cylindrical or square prism-shaped specimens with dimensions of 10 mm × 10 mm × 20 mm. The specimen height and cross-sectional dimensions were prepared according to the corresponding standards. The specimens were compressed on a universal testing machine at a loading speed of 2 mm / min. The maximum load at specimen failure was recorded, and the compressive strength was calculated. Each group of samples underwent at least five parallel tests, and the average value was taken as the test result. The results are shown in Table 3 below.

[0041] Table 3. Flexural and compressive strength results for each sample

[0042] As shown in Table 3, the flexural and compressive strengths of Examples 1-3 are significantly higher than those of Comparative Examples 1 and 2, indicating that the material of the present invention has advantages in terms of load-bearing capacity and structural stability. Example 2 exhibits the best mechanical properties, with a flexural strength of 96.8 MPa and a compressive strength of 156.2 MPa, indicating that under higher prepolymer content and stronger curing conditions, the internal network structure of the material is denser, and the rigid structure reinforcement effect is more obvious. The mechanical properties of Example 3 are also quite excellent, indicating that the present invention can simultaneously achieve good strength and molding performance under representative formulation conditions. Comparative Example 1, due to the absence of rigid bisphenol structural units, shows a decrease in both flexural and compressive strength; Comparative Example 2 further lacks low-free hydroxymethyl resol-type phenolic prepolymer, resulting in the lowest mechanical properties.

[0043] 4. Friction and wear performance test Friction coefficient testing was conducted using a friction and wear testing machine. The sample size was 30 mm in diameter and 10 mm thick, with a graphite disc used for the friction pair. Following the general testing specifications of GB / T 5763-2018 "Automotive Brake Liners", a high-temperature constant-speed friction and wear testing machine was used. The friction pair mating parts were replaced with a standard steel disc with a surface roughness Ra ≤ 1.6 μm. Test conditions were set as follows: constant load 50 N, rotation speed 500 rpm, precise temperature control to 300 ± 2℃, friction duration 30 min, and dynamic changes in the friction coefficient recorded throughout. Wear measurement: After constant weight determination of the samples before and after friction in a vacuum drying oven, the mass difference was measured using an analytical balance (accuracy 0.1 mg) to calculate the wear amount. Thermal decay rate: The ratio of the steady-state friction coefficient at 300℃ to the initial room temperature friction coefficient. Thermal recovery rate: The ratio of the recovered friction coefficient after cooling to room temperature to the initial room temperature friction coefficient. Each group of samples underwent at least three parallel tests, and the average value was taken. The results are shown in Table 4 below.

[0044] Table 4. Test results of friction and wear performance

[0045] As shown in Table 4, the materials in Examples 1-3 exhibited superior performance in terms of friction coefficient, wear amount, thermal decay rate, and thermal recovery rate. Example 2 showed the best performance among the four indicators, with the lowest friction coefficient (0.28), the smallest wear amount (8.4 mg), the lowest thermal decay rate (15.8%), and the highest thermal recovery rate (89.7%), demonstrating good high-temperature resistance and high stability. Example 3 also showed relatively good performance, with a moderate friction coefficient and wear amount, and a better thermal decay rate and thermal recovery rate than Comparative Examples 1 and 2, indicating that it has good thermal stability and wear resistance. In contrast, Comparative Examples 1 and 2 had poor friction performance, with a high friction coefficient, large wear amount, and poor thermal decay rate and thermal recovery rate, indicating that they had weak tribological properties at high temperatures.

[0046] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0047] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A modified phenolic resin-based material for high-temperature resistant brake pads, characterized in that, It contains the following raw materials in parts by weight: 80-120 parts of novolac-type phenolic resin, 8-30 parts of low-free hydroxymethyl resol-type phenolic prepolymer containing rigid bisphenol structural units, and 6-9 parts of hexamethylenetetramine. In the low-free hydroxymethyl resol phenolic prepolymer containing rigid bisphenol structural units, the rigid bisphenol structural units exist in the form of co-condensation structural units in the low-free hydroxymethyl resol phenolic prepolymer, and the modified phenolic resin-based material retains some incompletely reacted functional structural units after the initial molding.

2. The modified phenolic resin-based material for high-temperature brake pads according to claim 1, characterized in that, The free hydroxymethyl content of the low-free hydroxymethyl resol-type phenolic prepolymer is 3.0–10.0 wt%.

3. The modified phenolic resin-based material for high-temperature brake pads according to claim 1, characterized in that, The rigid bisphenol structural unit is 9,9-bis(4-hydroxyphenyl)fluorene.

4. The modified phenolic resin-based material for high-temperature brake pads according to claim 1, characterized in that, The incompletely reacted functional structural units include one or both of the hydroxymethyl and benzyl ether structures.

5. The modified phenolic resin-based material for high-temperature brake pads according to claim 1, characterized in that, The low-free hydroxymethyl resol-type phenolic prepolymer containing rigid bisphenol structural units is prepared by prepolymerization of phenol, formaldehyde, and rigid bisphenol compounds under alkaline catalytic conditions.

6. A method for preparing a modified phenolic resin-based material for high-temperature resistant brake pads according to any one of claims 1 to 5, characterized in that, Includes the following steps: S1. Phenol, formaldehyde and rigid bisphenol type compounds are prepolymerized to obtain a low free hydroxymethyl resol type phenolic prepolymer containing rigid bisphenol structural units; S2. Mix the novolac type phenolic resin with the low free hydroxymethyl resol type phenolic prepolymer to obtain a resin mixture system; S3. Add hexamethylenetetramine to the resin mixture system and mix evenly to obtain the mixture to be molded; S4. The mixture to be molded is hot-pressed and then cured in stages, so that the resulting material retains some unreacted functional structural units after the initial molding, thus obtaining the modified phenolic resin-based material for high-temperature brake pads.

7. The preparation method according to claim 6, characterized in that, In the prepolymerization step S1, formaldehyde is added in the form of formaldehyde aqueous solution or paraformaldehyde, the prepolymerization reaction temperature is 60-90℃, and the prepolymerization reaction time is 1-4h.

8. The preparation method according to claim 6, characterized in that, The mixing temperature of the novolac type phenolic resin and the low free hydroxymethyl resol type phenolic prepolymer is 80-100°C, and the mixing temperature of the added hexamethylenetetramine is 65-80°C.

9. The preparation method according to claim 6, characterized in that, The phased curing process includes the following steps: holding at 110-125℃ for 10-20 min, holding at 140-155℃ for 20-40 min, holding at 165-175℃ for 10-25 min, followed by post-treatment at 175-182℃ for 0.5-1.5 h.

10. The application of the modified phenolic resin-based material for high-temperature brake pads as described in any one of claims 1 to 5 in brake pad friction materials.

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