High-thermal-conductivity light-weight composite friction material and warm-pressing forming process thereof

Abstract

This invention belongs to the technical field of automotive braking friction materials, and discloses a high thermal conductivity lightweight composite friction material and its warm pressing molding process. The process first involves coupling expanded graphite, flake graphite, graphitized chopped carbon fibers, and sheet boron nitride, pre-coating them with resin, and granulating them to form a thermally conductive masterbatch. Then, thermosetting resin, reinforcing fibers, and friction-modifying components are mixed to form a friction surface base material. The thermally conductive masterbatch is added to the friction surface base material in a low-shear manner to form a friction surface side material, while a backing side material containing lightweight fillers is prepared. Subsequently, the friction surface side material and the backing side material are layered and laid, and then subjected to vibration leveling, pre-pressure venting, main pressure curing, depressurization venting, and gradient post-curing to obtain the high thermal conductivity lightweight composite friction material.

Description

Technical Field

[0001] This invention belongs to the field of friction material technology, specifically relating to a high thermal conductivity lightweight composite friction material and its warm pressing molding process, which is suitable for the preparation of resin-based composite friction materials such as brake pads, clutch plates, and industrial friction linings. Background Technology

[0002] Friction materials are typically manufactured by hot pressing or warm pressing, after being compounded from bonding resin, reinforcing fibers, friction modifiers, lubricants, and fillers. During braking or transmission, they need to withstand frictional heat, mechanical loads, and repeated shearing. With the development of lightweight vehicles and high-load braking conditions, friction materials, in addition to maintaining a stable coefficient of friction and wear resistance, also need to have good thermal conductivity and diffusion capabilities to reduce the risk of localized heat accumulation and thermal degradation on the friction surface.

[0003] In existing technologies, copper-free or low-metal friction materials have been prepared by adding components such as artificial graphite, coke, steel fiber, alumina, metal sulfides, and phenolic resin to improve thermal conductivity and high-temperature friction stability. For example, CN117700920A discloses a high-wear-resistant copper-free or low-metal friction material and its preparation method, whose components include composite reinforcing fibers, artificial graphite, coke, steel fiber, alumina, metal sulfide composite, fillers, and phenolic resin, and proposes to improve friction stability under high-temperature and high-load conditions.

[0004] However, existing friction materials typically involve directly mixing graphite, fibers, fillers, and resins. The sheet-like thermally conductive components are prone to agglomeration or breakage under high shear, making it difficult to form continuous thermal conduction pathways on the friction surface. Adding hollow or porous lightweight fillers to reduce density may disrupt these pathways and affect the material's compactness. Furthermore, conventional single-layer mixed hot-pressing processes struggle to simultaneously achieve thermal diffusion on the friction surface, lightweight support on the backing plate, and in-layer venting and densification. Therefore, it is necessary to provide a composite friction material and its molding process that utilizes a synergistic control of thermally conductive masterbatch, layered plying, and temperature-pressure venting. Summary of the Invention

[0005] The technical objective of this invention is to provide a high thermal conductivity and lightweight composite friction material and its warm pressing process that takes into account the thermal diffusion on the friction surface side, the lightweight support on the back plate side, and the denseness of the warm pressing.

[0006] To achieve the above-mentioned technical objectives, the present invention provides the following technical solutions.

[0007] In a first aspect, the present invention provides a warm pressing process for forming a high thermal conductivity, lightweight composite friction material, comprising the following steps:

[0008] S1. Expanded graphite, flake graphite, graphitized short-cut carbon fibers and sheet boron nitride are pretreated with a coupling agent, pre-coated with a portion of thermosetting resin and granulated to obtain thermally conductive masterbatch with a particle size of 0.3 to 1.5 mm.

[0009] S2. Thermosetting resin, reinforcing fiber and friction modifier are mixed to form friction surface base material, and then the thermally conductive masterbatch is added to the friction surface base material in a low-shear manner to obtain friction surface side material; thermosetting resin, reinforcing fiber, elastic toughening component and lightweight filler are mixed to obtain back plate side material.

[0010] S3. The friction surface side material and the back plate side material are laid in layers with a thickness ratio of 1:0.4 to 1.2. After vibration leveling, pre-compression is performed to exhaust air, so that the air trapped in the layer and some volatiles are discharged.

[0011] S4. Perform main pressure curing at 120-160℃ and 8-18MPa, and depressurize and vent air at least once during the main pressure curing process, and then perform gradient post-curing to obtain a high thermal conductivity lightweight composite friction material.

[0012] Specifically, by weight, the friction surface side material comprises: 8-16 parts of thermosetting resin, 10-24 parts of reinforcing fiber, 8-22 parts of thermally conductive masterbatch, 18-38 parts of friction regulating component, 3-10 parts of lubricating component, and 8-25 parts of inorganic filler.

[0013] The backplate side material comprises: 10-18 parts of thermosetting resin, 8-20 parts of reinforcing fiber, 3-8 parts of elastic toughening component, 8-22 parts of lightweight filler, 15-35 parts of inorganic filler, and 2-8 parts of thermally conductive transition component.

[0014] Specifically, the thermosetting resin is one or more of boron-modified phenolic resin, cashew nut shell oil-modified phenolic resin, and benzoxazine-modified phenolic resin.

[0015] The reinforcing fiber includes two or more of basalt fiber, aramid pulp, ceramic fiber, and graphitized chopped carbon fiber, and the length of the graphitized chopped carbon fiber is 0.5 to 4 mm.

[0016] Specifically, in the thermally conductive masterbatch, the mass ratio of expanded graphite, flake graphite, graphitized short-cut carbon fiber, and sheet boron nitride is 2-5:1-3:1-2:0.5-1.5.

[0017] The coupling agent is a silane coupling agent or a titanate coupling agent, and the thermosetting resin accounts for 8-20% of the mass of the thermally conductive masterbatch.

[0018] Specifically, the preparation of the thermally conductive masterbatch includes: drying expanded graphite, flake graphite, graphitized chopped carbon fibers and sheet boron nitride at 60-90°C for 1-3 hours; adding a coupling agent ethanol aqueous solution for surface treatment; adding a thermosetting resin ethanol solution for pre-coating; and collecting particles with a particle size of 0.3-1.5 mm as thermally conductive masterbatch after low-temperature drying, crushing and sieving.

[0019] Specifically, the low-shear method is as follows: first, the thermosetting resin, reinforcing fiber, friction modifier, lubricant and inorganic filler are mixed at 300-800 r / min for 3-8 min to obtain the friction surface base material; then, thermally conductive masterbatch is added and mixed at 60-180 r / min for 2-6 min to obtain the friction surface side material; the sheet-like thermally conductive component and the fibrous thermally conductive component in the thermally conductive masterbatch are kept in an overlapping state.

[0020] Specifically, the lightweight filler is one or more of hollow glass microspheres, hollow ceramic microspheres, expanded vermiculite, and closed-cell porous alumina particles, and the true density of the lightweight filler is 0.25–0.80 g / cm³. 3 The particle size is 20–120 μm;

[0021] The lightweight filler is pre-wetted with a thermosetting resin diluent before being added to the backplate side material.

[0022] Specifically, the layered paving includes: first laying the back plate side material in the mold cavity and vibrating it to level it, then laying the friction surface side material and vibrating it to level it; or first laying the friction surface side material and then laying the back plate side material.

[0023] The vibration leveling frequency is 20-60Hz and the time is 10-60s, so that the sheet-like heat-conducting components in the friction surface side material form an overlapping arrangement along the friction surface direction.

[0024] Specifically, the pre-compression exhaust includes: pre-compressing at 2-6 MPa for 20-90 seconds at room temperature to 90°C, and then depressurizing to 0.2-1 MPa for 5-30 seconds;

[0025] The main pressure curing includes: heating to 120-160℃ and holding at 8-18MPa for 3-12 minutes, and releasing pressure and venting 1-3 times during the pressure holding process, with the pressure after each release being 10-35% of the main pressure;

[0026] The gradient post-curing includes: holding at 90–110℃ for 0.5–1.5 h, holding at 130–150℃ for 0.5–1.5 h, and holding at 160–190℃ for 1–3 h.

[0027] Secondly, the present invention also discloses a high thermal conductivity lightweight composite friction material, which is prepared by the warm pressing molding process described in the first aspect.

[0028] This invention utilizes expanded graphite, flake graphite, graphitized chopped carbon fibers, and sheet boron nitride, coupled with resin pre-coating and granulation to form a thermally conductive masterbatch. This creates a relatively stable overlap between the sheet-like and fibrous thermally conductive components before they enter the friction surface side material. A low-shear addition method reduces excessive fragmentation of the masterbatch, allowing it to expand along the friction surface during thermocompression and form a continuous thermally conductive path. Layering the friction surface side material and the backing plate side material ensures that the thermally conductive components are mainly concentrated on the friction surface side, while the lightweight filler is mainly distributed on the backing plate side, reducing the impact of the lightweight filler on the thermally conductive channels. Pre-compression venting, depressurization and venting during main pressure curing, and gradient post-curing gradually expel air and curing volatiles from the layers, reducing porosity and improving interlayer bonding stability. Therefore, the resulting composite friction material can reduce overall density while maintaining the thermal diffusion capacity of the friction surface and improving thermal decay and wear stability during high-temperature friction. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the warm pressing process for the high thermal conductivity lightweight composite friction material of the present invention. Detailed Implementation

[0030] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the protection scope of the present invention.

[0031] I. Overview of the Overall Technical Approach

[0032] like Figure 1 As shown, the warm pressing molding process of the high thermal conductivity lightweight composite friction material provided in this embodiment includes the following steps: First, expanded graphite, flake graphite, graphitized chopped carbon fibers, and sheet boron nitride are coupled, pre-coated with resin, and granulated to form a thermally conductive masterbatch; then, thermosetting resin, reinforcing fibers, and friction modifiers are mixed to form a friction surface base material, and the thermally conductive masterbatch is added to the friction surface base material in a low-shear manner to obtain a friction surface side material; at the same time, a backing side material containing thermosetting resin, reinforcing fibers, elastic toughening components, and lightweight fillers is prepared; then, the friction surface side material and the backing side material are layered and laid, and vibration leveling, pre-pressure venting, main pressure curing, depressurization venting, and gradient post-curing are performed in sequence to obtain the composite friction material.

[0033] II. Raw Material Description

[0034] In this embodiment, the thermosetting resin may be one or more of boron-modified phenolic resin, cashew nut shell oil-modified phenolic resin, and benzoxazine-modified phenolic resin. Optionally, the softening point of the thermosetting resin is 90–115°C, and the gelation time at 150°C is 60–150 s.

[0035] The reinforcing fibers can be two or more of the following: basalt fiber, aramid pulp, ceramic fiber, and graphitized chopped carbon fiber. The length of basalt fiber can be 1–6 mm; the beating degree of aramid pulp can be 20–45°SR; the length of ceramic fiber can be 0.5–3 mm; and the length of graphitized chopped carbon fiber can be 0.5–4 mm.

[0036] The thermally conductive components include expanded graphite, flake graphite, graphitized chopped carbon fibers, and flake boron nitride. The expansion ratio of the expanded graphite can be 150–300 mL / g; the particle size of the flake graphite can be 50–200 μm; and the average flake diameter of the flake boron nitride can be 5–30 μm. The thermally conductive components are dried before preparing the thermally conductive masterbatch to reduce the release of moisture during the subsequent curing process.

[0037] Friction modifiers may include one or more of barium sulfate, alumina, iron oxide, zirconium silicate, mica powder, coke powder, and friction powder. Lubricating components may include one or more of molybdenum disulfide, antimony sulfide, and graphite powder. Inorganic fillers may include one or more of calcium carbonate, wollastonite, talc, and kaolin.

[0038] The elastic toughening component can be one or more of the following: nitrile rubber powder, carboxylated nitrile rubber powder, cashew nutshell oil friction powder, and rubber-modified phenolic resin powder. The lightweight filler can be one or more of the following: hollow glass microspheres, hollow ceramic microspheres, expanded vermiculite, and closed-cell porous alumina particles, with a true density of 0.25–0.80 g / cm³ and a particle size of 20–120 μm. Before being added to the backing plate side material, the lightweight filler can be pre-wetted with a thermosetting resin dilution to reduce breakage and floating stratification during mixing.

[0039] The coupling agent can be a silane coupling agent or a titanate coupling agent. The silane coupling agent can be one of γ-aminopropyltriethoxysilane or γ-glycidoxypropyltrimethoxysilane; the titanate coupling agent can be isopropyltriisostearoyl titanate. The amount of coupling agent used can be 0.5% to 2.0% of the total mass of the thermally conductive component.

[0040] III. Preparation of Thermally Conductive Masterbatch

[0041] Thermally conductive masterbatch can be prepared according to the following steps.

[0042] The first step involves placing expanded graphite, flake graphite, graphitized chopped carbon fibers, and sheet boron nitride separately in a forced-air drying oven and drying them at 60–90°C for 1–3 hours. After drying, they are cooled to room temperature in a sealed container.

[0043] The second step involves adding the coupling agent to an ethanol-water solution, with an ethanol-to-water volume ratio of 8:2 to 9:1. The pH is adjusted to 4.5 to 6.5, and the mixture is stirred and hydrolyzed for 10 to 30 minutes to obtain the coupling agent-treated solution. The dried thermally conductive component is then placed in a mixer and stirred at 100 to 300 rpm, while the coupling agent-treated solution is sprayed in to allow the coupling agent to adhere to the surface of the thermally conductive component. Subsequently, it is dried at 60 to 80°C to obtain the coupling-treated thermally conductive component.

[0044] The third step involves dissolving or dispersing a portion of the thermosetting resin in ethanol to form a resin solution or dispersion with a resin mass fraction of 10–30%. The coupling-treated thermally conductive component is then added to the resin solution and mixed at 60–150 r / min, allowing the resin to form a pre-coating layer on the surface of the thermally conductive component. High-speed shearing is avoided during the pre-coating process to reduce excessive breakage of the expanded graphite sheets and chopped carbon fibers.

[0045] The fourth step involves drying the pre-coated mixture at low temperature until the volatile matter content is no higher than 2.0%. After crushing and sieving, particles with a diameter of 0.3–1.5 mm are collected as thermal conductive masterbatch. Fine powder with a diameter less than 0.3 mm can be returned to the pre-coating step for regranulation, while particles with a diameter greater than 1.5 mm can be lightly crushed and sieved again.

[0046] In the thermally conductive masterbatch, the mass ratio of expanded graphite, flake graphite, graphitized chopped carbon fibers, and sheet boron nitride can be 2–5:1–3:1–2:0.5–1.5; some thermosetting resin accounts for 8–20% of the mass of the thermally conductive masterbatch. Through the above method, the thermally conductive components have formed a resin pre-fixed structure before entering the friction surface side material, making it easier to maintain the overlap relationship between the sheet-like thermally conductive components and the fibrous thermally conductive components during subsequent mixing.

[0047] IV. Preparation of Friction Surface Side Material and Back Plate Side Material

[0048] The friction surface base material can be prepared as follows: First, thermosetting resin, reinforcing fibers, friction modifiers, lubricants, and inorganic fillers are added to a mixer and mixed at 300–800 r / min for 3–8 min to disperse the powder and fibers, thus obtaining the friction surface base material. Then, thermally conductive masterbatch is added to the friction surface base material and mixed at 60–180 r / min for 2–6 min to obtain the friction surface side material. After low-shear mixing, the thermally conductive masterbatch is distributed in the friction surface side material, with some masterbatch surface resin in contact with the matrix resin. During subsequent pressing, the thermally conductive masterbatch is compressed and unfolds, and the sheet-like thermally conductive components overlap and arrange along the friction surface direction.

[0049] The backsheet side material can be prepared as follows: First, add the thermosetting resin, reinforcing fibers, elastic toughening components, inorganic fillers, and thermally conductive transition components to a mixer and mix at 200–600 r / min for 3–8 min; then add the lightweight filler pre-wetted with thermosetting resin dilution and mix at 60–160 r / min for 2–5 min. Pre-wetting treatment reduces breakage of hollow microspheres or closed-cell porous particles during the mixing process.

[0050] The thermally conductive transition component can be one or more of fine-grained graphite, thermally conductive carbon black, and flake boron nitride, and its dosage is 2 to 8 parts. It is used to form a thermally conductive transition zone near the interlayer bonding area of ​​the backsheet side layer to reduce the abrupt change in heat transfer between the friction surface side layer and the backsheet side layer.

[0051] V. Layered paving, pre-pressure degassing, main pressure curing, and gradient post-curing

[0052] The material is laid in layers within the mold cavity. Alternatively, the back plate side material can be laid first and vibrated to level, followed by the friction surface side material, which is also vibrated to level. Or, depending on the mold structure, the friction surface side material can be laid first, followed by the back plate side material. The thickness ratio of the friction surface side material to the back plate side material is controlled at 1:0.4–1.2. The vibration leveling frequency is 20–60 Hz, and the time is 10–60 s.

[0053] Pre-compression and venting can be carried out at temperatures ranging from room temperature to 90°C, with a pre-compression pressure of 2–6 MPa and a pre-compression time of 20–90 s; subsequently, the pressure is released to 0.2–1 MPa, and venting is performed for 5–30 s. Pre-compression and venting initially densify the material after layered paving and release trapped air within the layers.

[0054] Main pressure curing is carried out at 120–160℃ and 8–18 MPa, with a holding time of 3–12 minutes. During main pressure curing, pressure is released and vented 1–3 times, with the pressure after each release being 10–35% of the main pressure, and the pressure release and venting time being 3–20 seconds. After pressure release, the pressure is restored to the main pressure to continue curing. This step allows the volatiles released by the resin during softening, flow, and curing to be discharged, and promotes the formation of a bond between the friction surface side layer and the backing plate side layer in the interlayer bonding area.

[0055] After the main pressure curing is completed, the product is demolded and subjected to gradient post-curing. Gradient post-curing may include: holding at 90–110℃ for 0.5–1.5 h, holding at 130–150℃ for 0.5–1.5 h, and holding at 160–190℃ for 1–3 h. Gradient post-curing is used to further cure the thermosetting resin and reduce residual stress inside the product.

[0056] VI. Examples

[0057] 1. Example 1

[0058] In this embodiment, a high thermal conductivity lightweight composite friction material is prepared, and the friction surface side material and the back plate side material are laid in layers with a thickness ratio of 1:0.8.

[0059] First, a thermally conductive masterbatch was prepared. By weight, 4 parts expanded graphite, 2 parts flake graphite, 1.5 parts graphitized chopped carbon fiber, 1 part flake boron nitride, and 1.5 parts boron-modified phenolic resin were weighed. The expanded graphite, flake graphite, graphitized chopped carbon fiber, and flake boron nitride were placed in an 80℃ forced-air drying oven and dried for 2 hours. After removal, they were placed in a sealed container and cooled to room temperature. γ-glycidyl etheroxypropyltrimethoxysilane was used as a coupling agent, with the amount of coupling agent being 1.0% of the total mass of the thermally conductive components. The coupling agent was added to an ethanol-water solution, stirred and hydrolyzed, and then sprayed into the above thermally conductive components. Surface treatment was completed under low-speed stirring, followed by drying at 70℃. The boron-modified phenolic resin was then prepared into an ethanol solution and added to the coupled thermally conductive component. The mixture was then mixed under low shear conditions to form a pre-coating layer on the surface of the thermally conductive component. After low-temperature drying, light crushing and sieving, particles with a diameter of 0.5–1.2 mm were collected to obtain the thermally conductive masterbatch.

[0060] Then, the base material for the friction surface is prepared and the side material for the friction surface is formed. By weight, the base material for the friction surface includes 12 parts boron-modified phenolic resin, 8 parts basalt fiber, 4 parts aramid pulp, 4 parts ceramic fiber, 15 parts barium sulfate, 6 parts alumina, 5 parts zirconium silicate, 6 parts coke powder, 4 parts molybdenum disulfide, 3 parts graphite powder, and 8 parts wollastonite. These components are added to a mixer and mixed at 500 rpm for 5 minutes to initially disperse the powder and fibers. Then, 15 parts of the aforementioned thermally conductive masterbatch are added, and mixing continues at 120 rpm for 4 minutes to obtain the side material for the friction surface. This low-shear addition method is used to reduce excessive breakage in the thermally conductive masterbatch, ensuring that the sheet-like and fibrous thermally conductive components maintain an overlap during subsequent pressing.

[0061] Simultaneously, the backplate side material is prepared. By weight, the backplate side material comprises 14 parts cashew nut shell oil-modified phenolic resin, 6 parts basalt fiber, 5 parts ceramic fiber, 5 parts nitrile rubber powder, 10 parts hollow glass microspheres, 5 parts expanded vermiculite, 16 parts calcium carbonate, 8 parts talc, 3 parts thermally conductive carbon black, and 3 parts fine-grained graphite. During preparation, the cashew nut shell oil-modified phenolic resin, basalt fiber, ceramic fiber, nitrile rubber powder, calcium carbonate, talc, thermally conductive carbon black, and fine-grained graphite are first added to a mixer and mixed at 400 rpm for 5 minutes. Then, the hollow glass microspheres and expanded vermiculite are pre-wetted with a cashew nut shell oil-modified phenolic resin dilution solution and added to the mixer, mixed at 100 rpm for 3 minutes to obtain the backplate side material. The pre-wetting treatment is used to reduce the breakage and floating stratification of lightweight fillers during the mixing process.

[0062] During molding, the back plate side material is first laid into the mold cavity and vibrated to level it. Then, the friction surface side material is laid on top of the back plate side material and vibrated to level it. The vibration frequency is 40Hz and the vibration time is 30s to ensure that the thickness distribution of the two layers of material is uniform. Subsequently, pre-compression and degassing are performed. The material is pre-compressed at 4MPa for 60s at 70℃, and then depressurized to 0.5MPa for 15s to remove trapped air and some volatiles from the layers.

[0063] After pre-pressure venting, main pressure curing is performed. The mold is heated to 145℃ and main pressure cured at 14MPa for 8 minutes. Depressurization and venting are performed twice during the 3rd and 6th minutes of main pressure curing, with each depressurization at a pressure of 3MPa and a venting time of 10 seconds. After venting, the pressure is restored to 14MPa for continued curing. After main pressure curing is completed, the mold is demolded to obtain the pre-cured composite friction material.

[0064] Finally, the pre-cured composite friction material underwent gradient post-curing, specifically: holding at 100℃ for 1 hour, at 140℃ for 1 hour, and at 180℃ for 2 hours. After post-curing, it was allowed to cool naturally to room temperature to obtain a high thermal conductivity, lightweight composite friction material.

[0065] The resulting composite friction material comprises a friction surface side layer and a backing plate side layer bonded together. In the friction surface side layer, thermally conductive masterbatch is expanded under pressure during thermo-pressing, and expanded graphite, flake graphite, graphitized chopped carbon fibers, and sheet boron nitride form thermally conductive pathways distributed along the friction surface direction. In the backing plate side layer, hollow glass microspheres, expanded vermiculite, and reinforcing fibers together form a lightweight support structure. An interlayer bonding region is formed between the friction surface side layer and the backing plate side layer.

[0066] The sample was tested and found to have a density of 1.92 g / cm³. 3 The in-plane thermal conductivity is 2.64 W / (m·K), the thickness-direction thermal conductivity is 1.08 W / (m·K), the average coefficient of friction at room temperature is 0.42, the average coefficient of friction after thermal decay is 0.38, the coefficient of friction retention rate after thermal decay is 90.5%, and the volumetric wear rate is 0.28 × 10⁻⁶. -7 cm 3 / (N·m), porosity 4.8%, interlaminar shear strength 6.7MPa.

[0067] 2. Example 2

[0068] The difference between this embodiment and Embodiment 1 is as follows: the thickness ratio of the friction surface side material to the back plate side material is 1:1.0; the mass ratio of expanded graphite, flake graphite, graphitized chopped carbon fiber, and flake boron nitride in the thermally conductive masterbatch is 3:1.5:1:0.8; the amount of thermally conductive masterbatch added is 10 parts; the hollow glass microspheres in the back plate side material are 14 parts, and the expanded vermiculite is 6 parts. The main pressure curing temperature is 135℃, the pressure is 12MPa, the holding time is 10min, and the pressure is released and vented twice during the holding process. The remaining steps are the same as in Embodiment 1.

[0069] 3. Example 3

[0070] The difference between this embodiment and Embodiment 1 is as follows: the thickness ratio of the friction surface side material to the back plate side material is 1:0.5; the mass ratio of expanded graphite, flake graphite, graphitized chopped carbon fiber, and flake boron nitride in the thermally conductive masterbatch is 5:3:2:1.2; the amount of thermally conductive masterbatch added is 20 parts; the lightweight filler in the back plate side material consists of 8 parts hollow ceramic microspheres and 6 parts closed-cell porous alumina particles. The main pressure curing temperature is 155℃, the pressure is 16MPa, the holding time is 6min, and the pressure is released and vented once during the holding time. Gradient post-curing is performed at 110℃ for 1h, 150℃ for 1h, and 185℃ for 2h. The remaining steps are the same as in Embodiment 1.

[0071] 4. Example 4

[0072] This embodiment describes the implementation using different resin systems. The pre-coated resin in the thermally conductive masterbatch is benzoxazine-modified phenolic resin, and the thermosetting resin in the friction surface side material is a mixture of boron-modified phenolic resin and benzoxazine-modified phenolic resin at a mass ratio of 2:1. The amount of thermally conductive masterbatch added is 16 parts. The main pressure curing temperature is 150℃, the pressure is 15MPa, and the holding time is 7min; the gradient post-curing is 100℃ for 1h, 145℃ for 1h, and 180℃ for 2.5h. The remaining steps are the same as in Example 1.

[0073] VII. Comparative Example

[0074] To illustrate the effects of factors such as conventional processes, thermally conductive masterbatch, low-shear mixing, layered layup, lightweight backing plate side structure, and pressure relief and venting during main pressure curing on the structure and properties of composite friction materials, the following comparative examples were set up. Except for explicitly stated differences, the raw material sources, mold dimensions, post-curing conditions, and test methods in each comparative example were consistent with those in Example 1.

[0075] 1. Comparative Example 1: Friction Materials for Ordinary Processes

[0076] This comparative example uses a conventional dry-mixing and one-time hot-pressing process to prepare friction materials, serving as a basic reference sample. Its formulation is based on conventional resin-based friction materials and, by weight, includes: 14 parts phenolic resin, 8 parts basalt fiber, 3 parts aramid pulp, 4 parts ceramic fiber, 8 parts flake graphite, 4 parts expanded graphite, 18 parts barium sulfate, 6 parts alumina, 8 parts coke powder, 10 parts wollastonite, 12 parts calcium carbonate, 4 parts molybdenum disulfide, and 3 parts rubber powder.

[0077] During preparation, all the above components are added to a high-speed mixer at once and mixed at 600 r / min for 8 min to obtain a general mixture. The general mixture is then laid into the mold cavity in one go, without distinguishing between the friction surface side material and the back plate side material. No thermally conductive masterbatch is prepared, no layered laying is performed, and no lightweight support structure is set on the back plate side. Subsequently, it is hot-pressed and cured at 145℃ and 14MPa for 8 min, without depressurization or venting during the hot-pressing process. After demolding, post-curing is performed by holding at 100℃ for 1 h, 140℃ for 1 h, and 180℃ for 2 h.

[0078] 2. Comparative Example 2: The thermally conductive component is not made into a thermally conductive masterbatch.

[0079] Compared to Example 1, this comparative example does not involve coupling agent treatment, resin pre-coating, or granulation of expanded graphite, flake graphite, graphitized chopped carbon fibers, and sheet boron nitride. Instead, the thermally conductive components are directly added to the friction surface base material in powder form, and mixed to form the friction surface side material. The total amount of each thermally conductive component is the same as in Example 1. This comparative example still employs a layered layup of the friction surface side material and the backing plate side material, low-shear final mixing, pre-pressure venting, main pressure curing, pressure release venting, and gradient post-curing process.

[0080] 3. Comparative Example 3: Thermal conductive masterbatch was added using high-speed shearing.

[0081] Compared with Example 1, this comparative example still uses the same method to prepare the thermally conductive masterbatch. However, after adding the friction surface base material to the thermally conductive masterbatch, it is mixed at 800 r / min for 8 min to form the friction surface side material, instead of using the low-shear mixing method of 60-180 r / min. This comparative example still uses the layered paving, pre-pressure venting, main pressure curing, pressure relief venting, and gradient post-curing process.

[0082] 4. Comparative Example 4: Without layered paving and lightweight back panel side structure

[0083] Compared to Example 1, this comparative example combines all components of the friction surface side material and the back plate side material into a single mixture. The thermally conductive masterbatch, lightweight filler, reinforcing fiber, and friction-modifying components are randomly distributed in the same mixing system, without forming friction surface side layers and back plate side layers. This comparative example still uses the processes of thermally conductive masterbatch preparation, pre-pressure venting, main pressure curing, pressure relief venting, and gradient post-curing.

[0084] 5. Comparative Example 5: No pressure relief or venting is performed during the main pressure curing process.

[0085] Compared with Example 1, this comparative example retains the steps of thermally conductive masterbatch, low-shear mixing, layered application, lightweight backing plate side structure, and pre-pressure venting. However, during the main pressure curing process, it maintains a pressure of 14 MPa for 8 minutes without depressurization or venting. The remaining steps are the same as in Example 1.

[0086] 6. Comparative Example 6: No lightweight filler was added to the backplate side.

[0087] Compared to Example 1, this comparative example omits hollow glass microspheres and expanded vermiculite from the backsheet side material and replaces them with equal masses of calcium carbonate and barium sulfate. The preparation of the thermally conductive masterbatch, low-shear mixing, layered application, pre-compression venting, main pressure curing, depressurization, and gradient post-curing conditions are the same as in Example 1.

[0088] VIII. Testing Methods

[0089] To evaluate the performance of the materials prepared in the examples and comparative examples, samples from the same batch were used for testing. At least five samples were prepared for each group, and the test results were taken as the arithmetic mean. The samples were placed in an environment of 23°C and 50% relative humidity for 24 hours before testing.

[0090] 1. Density test

[0091] The density test is performed using standard solid density testing methods. For regular samples, their mass, length, width, and thickness are measured, and the density is calculated by volume. For irregular samples, the volume can be determined using the displacement method. Density is denoted as ρ, and the unit is g / cm³. 3 .

[0092] 2. Thermal conductivity test

[0093] The thermal diffusivity was tested using a laser thermal conductivity meter, measuring both the in-plane and thickness directions. The thermal conductivity was calculated using the following formula:

[0094] λ=α⋅ρ⋅Cp

[0095] In the formula: λ is the thermal conductivity; α is the thermal diffusivity; ρ is the material density; Cp is the specific heat capacity at constant pressure.

[0096] 3. Friction and wear performance test

[0097] The coefficient of friction and volumetric wear rate were tested using a constant-speed friction testing machine. The test pressure, speed, and temperature programs were set according to standard test conditions for automotive brake friction materials. The average coefficient of friction at room temperature and after the thermal decay stage were recorded, and the coefficient of friction retention rate after thermal decay was calculated.

[0098]

[0099] In the formula: η is the friction coefficient retention rate after thermal decay; μ f is the average friction coefficient after the thermal decay stage; μ0 is the average friction coefficient at room temperature.

[0100] Volumetric wear rate is calculated using the following formula:

[0101]

[0102] In the formula: Wv is the volumetric wear rate; Δm is the mass loss before and after the test; ρm is the sample density; F is the applied normal force; and s is the friction distance.

[0103] 4. Porosity test

[0104] Porosity was determined using either the immersion method or image analysis. In the image analysis method, a cross-section of the sample was taken, polished, cleaned, and then an image of the cross-section was acquired using a microscope. The pore area ratio was statistically analyzed by segmenting the sample using a grayscale threshold. At least five fields of view were taken for each sample.

[0105] 5. Interlaminar shear strength test

[0106] Shear specimens of the composite friction material were prepared along the interlaminar direction and loaded using a universal testing machine. The maximum load at which interlaminar shear failure occurred was recorded, and the interlaminar shear strength was calculated based on the area of ​​force application. Comparative Example 1 is a typical single-layer structure sample without clearly defined friction surface and backing plate layers; therefore, interlaminar shear strength testing was not performed. Comparative Example 4 is a single-layer randomly distributed structure, and interlaminar shear strength testing is also not applicable.

[0107] IX. Test Results

[0108] The density and thermal conductivity of the examples and comparative examples are shown in Table 1.

[0109] Table 1. Density and thermal conductivity of the examples and comparative examples

[0110]

[0111] The tribological properties of the examples and comparative examples are shown in Table 2.

[0112] Table 2. Friction and wear performance of the examples and comparative examples

[0113]

[0114] The porosity and interlaminar shear strength of the examples and comparative examples are shown in Table 3.

[0115] Table 3 Porosity and interlaminar shear strength of the examples and comparative examples

[0116]

[0117] As shown in Tables 1 to 3, after using thermally conductive masterbatch, low-shear mixing, layered paving, pre-compression venting, main pressure curing with depressurization and venting, and gradient post-curing, the in-plane thermal conductivity of the materials in Examples 1 to 4 remained in the range of 2.21–2.95 W / (m·K), and the density remained in the range of 1.80–2.06 g / cm³. 3 Within the specified range, the coefficient of friction retention rate remained between 87.8% and 90.7% after thermal decay. This result indicates that, under the conditions of the embodiment, the thermally conductive components in the friction surface side layer can form a relatively continuous in-plane heat transfer channel, the lightweight filler in the back plate side layer can reduce the material density, and the interlayer bonding area maintains detectable shear strength.

[0118] Comparative Example 1 used a conventional dry-mixing and one-time hot-pressing molding method, without the addition of thermally conductive masterbatch, low-shear addition, layered paving, lightweight backing side structure, or pressure relief and venting during main pressure curing. Its density was 2.28 g / cm³, its in-plane thermal conductivity was 1.48 W / (m·K), and its friction coefficient retention rate after thermal decay was 72.5%. Compared with Example 1, Comparative Example 1 had lower thermal conductivity and friction coefficient retention rate after thermal decay, but higher density. These results indicate that in conventional processes, the thermally conductive components, reinforcing fibers, inorganic fillers, and resin are randomly distributed. The sheet-like thermally conductive components are difficult to form a continuous in-plane heat transfer path on the friction surface side. Simultaneously, the high proportion of high-density fillers limits the material's weight reduction effect.

[0119] Comparative Example 2, while retaining the layered layup, low-shear mixing, and depressurization venting conditions, only eliminated the thermally conductive masterbatch preparation step. Its in-plane thermal conductivity was 1.73 W / (m·K), lower than that of Example 1. This result indicates that when the thermally conductive components are added directly in powder form, local agglomeration and resin enrichment are more likely to occur, resulting in weaker overlap continuity between expanded graphite, flake graphite, and chopped carbon fibers.

[0120] Comparative Example 3 used the same thermally conductive masterbatch as Example 1, but employed high-speed shear mixing when adding the friction surface base material and forming the friction surface side material. Its in-plane thermal conductivity was 1.69 W / (m·K), and its volumetric wear rate was 0.52 × 10⁻⁻⁻⁶. 7 cm³ / (N·m). This result indicates that the addition of high-speed shear may lead to an increase in the degree of fragmentation of the thermally conductive masterbatch and a reduction in the bridging path between the lamellar and fibrous thermally conductive components.

[0121] Comparative Example 4 did not employ layered paving and a lightweight backplate side structure. Instead, it randomly mixed thermally conductive masterbatch, lightweight filler, reinforcing fibers, and friction modifiers into a single structure. Its density was 1.88 g / cm³, but its in-plane thermal conductivity was 1.82 W / (m·K), and its friction coefficient retention rate after thermal decay was 77.5%. This result indicates that when lightweight filler and thermally conductive components are randomly distributed in the same layer, the lightweight filler creates a gap between the overlapping of the sheet-like thermally conductive components, making it difficult to form relatively concentrated thermal conduction pathways on the friction surface.

[0122] Comparative Example 5 did not undergo pressure relief and venting during the main pressure curing process. Its in-plane thermal conductivity remained at 2.38 W / (m·K), but its porosity was 10.4%, interlaminar shear strength was 4.6 MPa, and volumetric wear rate increased. These results indicate that, even when thermal pathways are already formed, the venting regime during the resin curing stage still affects pore distribution, interlaminar bonding, and tribological stability.

[0123] Comparative Example 6 did not include lightweight fillers, but instead used high-density inorganic fillers. Its in-plane thermal conductivity and the retention rate of the coefficient of friction after thermal decay were close to those of Example 1, but its density was 2.36 g / cm³. 3 The results are higher than those in Example 1. This indicates that the lightweight filler on the backplate side has an effect on reducing the density of the composite friction material; at the same time, since the lightweight filler is confined in the backplate side layer, its impact on the thermal conductivity path on the friction surface side is relatively small.

[0124] The composite friction material prepared in this embodiment includes a friction surface side layer, a backing plate side layer, and an interlayer bonding region located between the two. The friction surface side layer is formed by warm pressing of friction surface side material containing thermally conductive masterbatch, and the backing plate side layer is formed by warm pressing of backing plate side material containing lightweight fillers and reinforcing fibers.

[0125] During the pre-compression and main compression processes, the thermally conductive masterbatch is compacted and expanded under pressure. Expanded graphite, flake graphite, graphitized chopped carbon fibers, and lamellar boron nitride are distributed within the friction surface side layer along with the thermosetting resin matrix. The lamellar thermally conductive components form an overlapping base facing the friction surface, the graphitized chopped carbon fibers bridge adjacent lamellar thermally conductive components, and the lamellar boron nitride supplements the thermal conduction paths within the resin matrix, thereby creating a thermally conductive pathway within the friction surface side layer.

[0126] In the backplate side layer, lightweight fillers and reinforcing fibers are distributed within the thermosetting resin matrix, forming a lightweight support structure. Since the lightweight fillers are mainly located in the backplate side layer and the thermally conductive masterbatch is mainly located in the friction surface side layer, the impact of the lightweight fillers on the thermal conductivity pathways on the friction surface side can be reduced.

[0127] The interlayer bonding region is formed at the contact interface between the friction surface side material and the backing plate side material. During the pre-pressure venting, main pressure curing, and depressurization venting processes, the thermosetting resins in the two layers soften, flow, and solidify, forming a composite bond between the friction surface side layer and the backing plate side layer. The above structural formation process corresponds to the test results of thermal conductivity, density, tribological properties, and interlayer shear strength in the embodiments and comparative examples.

[0128] The foregoing description of embodiments of the present invention, through which those skilled in the art are able to implement or use the present invention, will be readily apparent to those skilled in the art. Various modifications to these embodiments will be readily apparent to those skilled in the art. The general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the present invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novelty disclosed herein.

Claims

1. A warm pressing process for a high thermal conductivity, lightweight composite friction material, characterized in that, Includes the following steps: S1. Expanded graphite, flake graphite, graphitized short-cut carbon fibers and sheet boron nitride are pretreated with a coupling agent, pre-coated with a portion of thermosetting resin and granulated to obtain thermally conductive masterbatch with a particle size of 0.3 to 1.5 mm. S2. Thermosetting resin, reinforcing fiber and friction modifier are mixed to form friction surface base material, and then the thermally conductive masterbatch is added to the friction surface base material in a low-shear manner to obtain friction surface side material. Thermosetting resin, reinforcing fibers, elastic toughening components and lightweight fillers are mixed to obtain backsheet side material; S3. The friction surface side material and the back plate side material are laid in layers with a thickness ratio of 1: (0.4~1.2). After vibration leveling, pre-compression is performed to exhaust air, so that the air trapped in the layer and some volatiles are discharged. S4. Perform main pressure curing at 120-160℃ and 8-18MPa, and depressurize and vent air at least once during the main pressure curing process, and then perform gradient post-curing to obtain a high thermal conductivity lightweight composite friction material.

2. The warm pressing molding process according to claim 1, characterized in that, By weight, the friction surface material comprises: 8-16 parts of thermosetting resin, 10-24 parts of reinforcing fiber, 8-22 parts of thermally conductive masterbatch, 18-38 parts of friction conditioning component, 3-10 parts of lubricating component and 8-25 parts of inorganic filler. The backplate side material comprises: 10-18 parts of thermosetting resin, 8-20 parts of reinforcing fiber, 3-8 parts of elastic toughening component, 8-22 parts of lightweight filler, 15-35 parts of inorganic filler, and 2-8 parts of thermally conductive transition component.

3. The warm pressing molding process according to claim 1, characterized in that, The thermosetting resin is one or more of boron-modified phenolic resin, cashew nut shell oil-modified phenolic resin, and benzoxazine-modified phenolic resin. The reinforcing fiber includes two or more of basalt fiber, aramid pulp, ceramic fiber, and graphitized chopped carbon fiber, and the length of the graphitized chopped carbon fiber is 0.5 to 4 mm.

4. The warm pressing molding process according to claim 1, characterized in that, In the thermally conductive masterbatch, the mass ratio of expanded graphite, flake graphite, graphitized short-cut carbon fiber, and sheet boron nitride is 2-5:1-3:1-2:0.5-1.

5. The coupling agent is a silane coupling agent or a titanate coupling agent, and the thermosetting resin accounts for 8-20% of the mass of the thermally conductive masterbatch.

5. The warm pressing molding process according to claim 1, characterized in that, The preparation of the thermally conductive masterbatch includes: drying expanded graphite, flake graphite, graphitized chopped carbon fibers and sheet boron nitride at 60-90°C for 1-3 hours; adding a coupling agent ethanol aqueous solution for surface treatment; adding a thermosetting resin ethanol solution for pre-coating; and collecting particles with a particle size of 0.3-1.5 mm as thermally conductive masterbatch after low-temperature drying, crushing and sieving.

6. The warm pressing molding process according to claim 1, characterized in that, The low-shear method is as follows: first, thermosetting resin, reinforcing fiber, friction modifier, lubricant and inorganic filler are mixed at 300-800 r / min for 3-8 min to obtain friction surface base material; Add thermally conductive masterbatch and mix at 60–180 r / min for 2–6 min to obtain friction surface material; This ensures that the sheet-like and fibrous thermally conductive components in the thermally conductive masterbatch remain in an overlapping state.

7. The warm pressing molding process according to claim 1, characterized in that, The lightweight filler is one or more of hollow glass microspheres, hollow ceramic microspheres, expanded vermiculite, and closed-cell porous alumina particles, and the true density of the lightweight filler is 0.25–0.80 g / cm³. 3 The particle size is 20–120 μm; The lightweight filler is pre-wetted with a thermosetting resin diluent before being added to the backplate side material.

8. The warm pressing molding process according to claim 1, characterized in that, The layered paving includes: first laying the back plate side material in the mold cavity and vibrating it to level it, then laying the friction surface side material and vibrating it to level it; or first laying the friction surface side material and then laying the back plate side material. The vibration leveling frequency is 20-60Hz and the time is 10-60s, so that the sheet-like heat-conducting components in the friction surface side material form an overlapping arrangement along the friction surface direction.

9. The warm pressing molding process according to claim 1, characterized in that, The pre-compression exhaust includes: pre-compressing at 2-6 MPa for 20-90 seconds at room temperature to 90°C, followed by depressurization to 0.2-1 MPa and exhaust for 5-30 seconds; The main pressure curing includes: heating to 120-160℃ and holding at 8-18MPa for 3-12 minutes, and releasing pressure and venting 1-3 times during the pressure holding process, with the pressure after each release being 10-35% of the main pressure; The gradient post-curing includes: holding at 90–110℃ for 0.5–1.5 h, holding at 130–150℃ for 0.5–1.5 h, and holding at 160–190℃ for 1–3 h.

10. A high thermal conductivity, lightweight composite friction material, characterized in that, It is manufactured using the warm pressing process described in any one of claims 1 to 9.

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