Friction material

Abstract

[Problem] To provide a friction material which is used for a disk brake pad and is obtained by molding a friction material composition which is an NAO material and which contains a binder, a fiber base material, an organic friction adjusting material, an inorganic friction adjusting material, and a lubricant, wherein the occurrence of abnormal noise and squeal at the time of braking after being left under low temperature conditions such as outdoors and during the winter months is suppressed without using a fluororesin such as polytetrafluoroethylene. [Solution] As the friction material composition, a friction material composition which contains, as the organic friction adjusting material, ultra-high-molecular-weight polyethylene particles in an amount of 0.1-3 wt% with respect to the whole amount of the friction material composition is used, and it is preferable that the average particle diameter of the ultra-high-molecular-weight polyethylene particles is 10-70 µm.

Description

Friction material

[0001] The present invention relates to a friction material, and more particularly to a friction material used in disc brake pads, which is molded from a friction material composition of a non-asbestos-organic (NAO) material containing a binder, a fibrous base material, an organic friction modifier, an inorganic friction modifier, and a lubricant.

[0002] Conventionally, disc brakes have been used as braking devices for passenger vehicles, and disc brake pads have been used as their friction members, with a metal base member to which a friction material made of NAO material that does not contain steel-based fibers such as steel fibers or stainless steel fibers is attached as a fiber substrate.

[0003] Such NAO friction materials have the problem of generating abnormal noises and squeals when braking after being left outdoors or under low-temperature conditions such as in winter, and improvements in this area are needed.

[0004] Patent Document 1 describes a friction material obtained by hot-press molding a friction material composition that contains a fibrous substance, a binder, and a friction modifier, and that contains 0.08% by weight to 10.5% by weight of a fluororesin powder such as polytetrafluoroethylene in the total composition.

[0005] According to the invention of Patent Document 1, it is possible to provide a friction material suitable for reducing the generation and sound pressure of goo noise when the absolute humidity is high or when the vehicle is parked overnight.

[0006] Patent Document 2 describes a friction material composition containing a fibrous base material, a binder, and a friction modifier, in which the copper content in the friction material composition is 0.5 mass % or less, and which is obtained by molding a friction material composition containing fluorine-based polymer particles such as polytetrafluoroethylene.

[0007] According to the invention of Patent Document 2, it is possible to provide a friction material that can suppress squealing after being left in a cold state.

[0008] However, fluororesins such as polytetrafluoroethylene described in Patent Documents 1 and 2 are difficult to decompose and are likely to remain in the environment for a long time, and therefore, restrictions on them are being considered.

[0009] In light of this background, there is a demand for friction materials that do not use fluororesins such as polytetrafluoroethylene and that suppress the generation of abnormal noise and squealing when braking after being left outdoors or under low-temperature conditions such as in winter.

[0010] JP 2000-256650 A JP 2015-93936 A

[0011] The present invention aims to provide a friction material that is used for disc brake pads and is molded from a friction material composition of an NAO material containing a binder, a fiber base material, an organic friction modifier, an inorganic friction modifier, and a lubricant, and that suppresses the generation of abnormal noise and squealing when braking after being left outdoors or under low-temperature conditions in winter, etc., without using a fluororesin such as polytetrafluoroethylene.

[0012] As a result of extensive research, the present inventors have discovered that in a friction material molded from a friction material composition of an NAO material containing a binder, a fiber base material, an organic friction modifier, an inorganic friction modifier, and a lubricant, which is used in disc brake pads, by adding a specific amount of ultra-high molecular weight polyethylene particles as an organic friction modifier to the friction material composition, it is possible to obtain a friction material in which the generation of abnormal noise and squealing when braking after being left outdoors or under low temperature conditions in winter, etc., is suppressed, even without using a fluororesin such as polytetrafluoroethylene, and thus completed the present invention.

[0013] The present invention relates to a friction material used in disc brake pads, which is formed from a friction material composition of an NAO material containing a binder, a fiber base material, an organic friction modifier, an inorganic friction modifier, and a lubricant, and is based on the following technology.

[0014] (1) A friction material used for a disc brake pad, which is obtained by molding a friction material composition of an NAO material containing a binder, a fiber base material, an organic friction modifier, an inorganic friction modifier, and a lubricant, wherein the friction material composition contains 0.1 to 3% by weight of ultra-high molecular weight polyethylene particles as an organic friction modifier based on the total weight of the friction material composition.

[0015] (2) The friction material according to (1), wherein the ultra-high molecular weight polyethylene particles are ultra-high molecular weight polyethylene particles having a weight average molecular weight of 1,000,000 to 4,000,000.

[0016] (3) The friction material according to (1) or (2), wherein the ultra-high molecular weight polyethylene particles have an average particle size of 10 μm to 70 μm.

[0017] (4) The friction material according to any one of (1) to (3), wherein the friction material composition does not contain a metal element or an alloy, and contains zirconium oxide as an inorganic friction modifier in an amount of 10 to 35% by weight based on the total amount of the friction material composition.

[0018] According to the present invention, in a friction material used in a disc brake pad, which is obtained by molding a friction material composition of an NAO material containing a binder, a fiber base material, an organic friction modifier, an inorganic friction modifier, and a lubricant, it is possible to provide a friction material in which the generation of abnormal noise and squealing during braking after being left outdoors or under low temperature conditions in winter, etc., is suppressed, even without using a fluororesin such as polytetrafluoroethylene.

[0019] In the present invention, in a friction material used for a disc brake pad, which is obtained by molding a friction material composition of an NAO material containing a binder, a fiber base material, an organic friction modifier, an inorganic friction modifier, and a lubricant, the friction material composition contains 0.1% by weight to 3% by weight of ultra-high molecular weight polyethylene particles as an organic friction modifier, based on the total amount of the friction material composition.

[0020] Abnormal noises that occur when braking after being left outdoors or in low-temperature conditions such as winter occur when water is absorbed into the transfer coating of friction material that has formed on the surface of the disc rotor, causing rust to form between the disc rotor and the transfer coating, and when the vehicle is restarted, the transfer coating peels off at the beginning of operation.

[0021] In addition, squealing that occurs when braking after being left outdoors or in low-temperature conditions such as winter is caused by moisture in the air entering the pores of the friction material, increasing the contact area between the disc rotor and the friction material, and causing the friction coefficient to rise at the beginning of driving when the vehicle is restarted.

[0022] Ultra-high molecular weight polyethylene particles generally have non-water-absorbent properties, and adding 0.1 to 3 weight percent of the ultra-high molecular weight polyethylene particles to the total amount of the friction material composition makes it difficult for water to penetrate into the transfer coating of the friction material formed on the surface of the disc rotor, suppressing rust formation between the disc rotor and the transfer coating and thereby suppressing the generation of abnormal noise.Furthermore, it also makes it difficult for water to penetrate into the pores of the friction material, suppressing an increase in the friction coefficient and suppressing the generation of squealing.

[0023] If the amount of ultra-high molecular weight polyethylene particles added is less than 0.1% by weight of the total amount of the friction material composition, a sufficient effect of suppressing abnormal noise and squealing cannot be obtained, and if it exceeds 3% by weight, heat resistance decreases and sufficient wear resistance cannot be obtained.

[0024] The amount of ultra-high molecular weight polyethylene particles added is more preferably 0.5% by weight to 2% by weight based on the total amount of the friction material composition.

[0025] The ultra-high molecular weight polyethylene particles are preferably ultra-high molecular weight polyethylene particles having a weight average molecular weight of 1,000,000 to 4,000,000.

[0026] The average particle size of the ultra-high molecular weight polyethylene particles is preferably 10 μm to 70 μm. By setting the average particle size of the ultra-high molecular weight polyethylene particles within this range, the particles are easily dispersed in the friction material composition, and the effect of suppressing abnormal noise and squealing is further improved.

[0027] The melting point of the ultra-high molecular weight polyethylene particles is approximately 120° C. to 140° C. If a raw material with high thermal conductivity is present near the ultra-high molecular weight polyethylene particles, the polyethylene particles tend to melt when the friction material is exposed to high temperatures, and flow out of the friction material matrix, reducing the effect of suppressing an increase in the friction coefficient and the effect of suppressing squealing.

[0028] It is preferable to avoid adding raw materials with high thermal conductivity to the friction material composition as much as possible within the range that does not impair the friction characteristics, and it is preferable to add a large amount of raw materials with relatively low thermal conductivity.

[0029] Therefore, in the present invention, instead of adding a metal element or alloy with a relatively high thermal conductivity to the friction material composition, zirconium oxide, which has a relatively low thermal conductivity, is added as an inorganic friction modifier in an amount of 10 to 35% by weight based on the total amount of the friction material composition, thereby suppressing the outflow of polyethylene particles from the friction material matrix.

[0030] In the present invention, the average particle size is the value of the 50% primary particle size measured by a laser diffraction particle size distribution method.

[0031] As such ultra-high molecular weight polyethylene particles, roughly spherical particles in the Mipelon (registered trademark) series manufactured by Mitsui Chemicals, Inc. can be used. For example, XM-330, XM-220, XM-221U, and PM-200 all have an average particle size in the range of 10 μm to 70 μm.

[0032] The friction material of the present invention comprises a friction material composition containing, in addition to the ultra-high molecular weight polyethylene particles, a binder, a fibrous base material, a lubricant, an inorganic friction modifier, an organic friction modifier, a pH adjuster, a filler, and the like, which are commonly used in friction materials.

[0033] Examples of binders include those commonly used in friction materials, such as straight phenolic resin, cashew oil-modified phenolic resin, acrylic rubber-modified phenolic resin, silicone rubber-modified phenolic resin, nitrile rubber (NBR)-modified phenolic resin, phenol-aralkyl resin (aralkyl-modified phenolic resin), and silicone rubber-dispersed phenolic resin. These can be used alone or in combination of two or more. The content of the binder is preferably 4 to 9% by weight, and more preferably 5 to 8% by weight, of the total amount of the friction material composition.

[0034] Examples of the fibrous base material include organic fibers commonly used in friction materials, such as aramid fibers, cellulose fibers, poly-paraphenylene benzobisoxazole fibers, and acrylic fibers, which can be used alone or in combination of two or more. The content of the fibrous base material is preferably 1 to 7% by weight, and more preferably 2 to 4% by weight, of the total amount of the friction material composition.

[0035] Examples of lubricants include carbonaceous lubricants such as natural graphite, artificial graphite, elastic graphitized carbon, petroleum coke, activated carbon, and oxidized polyacrylonitrile fiber pulverized powder, and metal sulfide lubricants such as zinc sulfide, molybdenum disulfide, antimony trisulfide, iron sulfide, and tin sulfide, which are commonly used in friction materials. These can be used alone or in combination of two or more. The content of the lubricant is preferably 3 to 20% by weight, and more preferably 5 to 15% by weight, of the total amount of the friction material composition.

[0036] Examples of inorganic friction modifiers include particulate inorganic friction modifiers such as phlogopite, muscovite, iron oxide, glass beads, silicon oxide, magnesium oxide, zirconium oxide, zirconium silicate, γ-alumina, α-alumina, silicon carbide, calcium silicate particles, and non-whisker-like titanates (potassium hexatitanate, potassium octatitanate, magnesium potassium titanate, lithium potassium titanate, etc.), and fibrous inorganic friction modifiers such as wollastonite, sepiolite, basalt fiber, glass fiber, biosoluble artificial mineral fiber, and rock wool. These may be used alone or in combination of two or more. The content of the inorganic friction modifier is preferably 40% to 70% by weight, and more preferably 45% to 65% by weight, of the total amount of the friction material composition.

[0037] In addition to the ultra-high molecular weight polyethylene particles, examples of organic friction modifiers include cashew dust, ground tire tread rubber powder, and organic friction modifiers commonly used in friction materials, such as vulcanized or unvulcanized rubber powders of nitrile rubber, acrylic rubber, silicone rubber, and butyl rubber. These may be used alone or in combination of two or more. The content of the organic friction modifier, combined with the ultra-high molecular weight polyethylene particles, is preferably 2 to 12% by weight, more preferably 5 to 10% by weight, of the total amount of the friction material composition.

[0038] As the pH adjuster, a pH adjuster that is normally used in friction materials, such as calcium hydroxide, can be used. The amount of the pH adjuster is preferably 1 to 7% by weight, more preferably 2 to 4% by weight, based on the total amount of the friction material composition.

[0039] The remainder of the friction material composition may be a filler such as barium sulfate or calcium carbonate.

[0040] It has been suggested that the copper component contained in the friction material may flow into rivers, lakes, and oceans as wear particles, thereby polluting water bodies. Therefore, it is preferable not to add a copper component to the friction material composition.

[0041] The friction material used in the disc brake of the present invention is manufactured through a mixing process in which a predetermined amount of friction material composition is mixed uniformly using a mixer, a heat-pressure molding process in which the resulting friction material raw material mixture is placed on a back plate that has been separately cleaned, surface-treated, and coated with an adhesive, and the resulting mixture is placed in a thermoforming mold and heated and pressurized to form a molded product, a heat treatment process in which the resulting molded product is heated to complete the curing reaction of the binder, an electrostatic powder coating process in which a powder paint is applied, a paint baking process in which the paint is baked, and a polishing process in which a friction surface is formed using a rotary grinding wheel. Note that the heat-pressure molding process may be followed in this order by the painting process, the heat treatment process which also serves as paint baking, and the polishing process.

[0042] If necessary, before the heat-pressure molding step, a granulation step of granulating the friction material raw material mixture, a kneading step of kneading the friction material raw material mixture, and a pre-molding step of putting the friction material raw material mixture, the granulated product obtained in the granulation step, or the kneaded product obtained in the kneading step into a pre-molding mold to mold a pre-molded product are performed, and a scorching step is performed after the heat-pressure molding step.

[0043] The present invention will be described in detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples. <Method for Manufacturing Friction Materials of Examples 1 to 14, Comparative Examples 1 to 3, and Reference Example> Friction material compositions having the compositions shown in Tables 1 and 2 were mixed for 5 minutes in a Lödige mixer and then premolded in a molding die at 30 MPa for 10 seconds. These premolded products were placed on a steel back plate that had been previously cleaned, surface-treated, and coated with an adhesive. They were then molded in a thermoforming die at a molding temperature of 150°C and a molding pressure of 40 MPa for 10 minutes, after which they were polished to form a friction surface. A paint was applied to the areas excluding the friction surface, and the friction materials were then heat-treated at 200°C for 5 hours to complete the curing reaction of the thermosetting resin and simultaneously bake the paint applied in the painting process.

[0044]

[0045]

[0046] The obtained friction materials were evaluated for noise, squealing, and wear resistance.

[0047] <Abnormal noise> Using an actual vehicle, a running test equivalent to JASO C406 was performed, and the vehicle was left outdoors overnight, after which a comparative test of the sound pressure level of abnormal creep noise was conducted. The evaluation of abnormal creep noise was performed by gradually releasing the brake pedal while the vehicle was stopped with a pad surface pressure of 1.0 MPa, maintaining the brake pedal depression force when the vehicle started to move, and measuring the abnormal creep noise generated at this time with a sound level meter. This procedure was repeated 10 times. The evaluation criteria were as follows: Excellent: No abnormal noise at all Good: Abnormal noise was generated but the sound pressure level was 57 dB or less Fair: Abnormal noise was generated but the sound pressure level was higher than 57 dB and lower than 73 dB Poor: Abnormal noise was generated and the sound pressure level was 73 dB or higher

[0048] <Squeal> After 200 runs of running-in at an initial speed V of 60 km / h, deceleration α of 0.3 G, and a braking start temperature of 120°C, the tire was left outdoors overnight and then braked 20 times at an initial speed V of 10 km / h and deceleration α of 0.1 G, and the rate of occurrence of squeal of 70 dB or more was calculated using the following formula: Squeal occurrence rate (%) = Number of times squeal of 70 dB or more occurred / Number of times braking x 100 The evaluation criteria were as follows: Excellent: Less than 20% Good: 20% or more but less than 30% Passable: 30% or more but less than 50% Poor: 50% or more

[0049] <Wear Resistance> In accordance with JASO C427 "Automobiles - Brake linings and disc brake pads - Dynamometer wear test method," the wear amount (mm) of the friction material was measured under the conditions of an initial braking speed of 50 km / h, a braking deceleration of 0.3 G, appropriate number of braking cycles, and a brake temperature before braking of 200°C, and was converted into the wear amount per 1000 braking cycles and evaluated. The evaluation criteria are as follows: Excellent: Less than 0.15 mm Good: 0.15 mm or more but less than 0.20 mm Passable: 0.20 mm or more but less than 0.50 mm Poor: 0.50 mm or more

[0050] The evaluation results are shown in Tables 3 and 4.

[0051]

[0052]

[0053] As can be seen from each table, compositions satisfying the composition of the present invention have been evaluated as having good results, such as suppressing the generation of abnormal noise and squealing and having sufficient wear resistance.

[0054] According to the present invention, in a friction material used in a disc brake pad, which is obtained by molding a friction material composition of an NAO material containing a binder, a fiber base material, an organic friction modifier, an inorganic friction modifier, and a lubricant, it is possible to provide a friction material in which the generation of abnormal noises and squeals when braking after being left outdoors or under low temperature conditions in winter, etc., is suppressed without using a fluororesin such as polytetrafluoroethylene, and which is of extremely high practical value.

Claims

1. A friction material used in disc brake pads, which is obtained by molding a friction material composition of an NAO material containing a binder, a fiber base material, an organic friction modifier, an inorganic friction modifier, and a lubricant, wherein the friction material composition contains 0.1 to 3% by weight of ultra-high molecular weight polyethylene particles as an organic friction modifier based on the total weight of the friction material composition.

2. The friction material according to claim 1, wherein the ultra-high molecular weight polyethylene particles are ultra-high molecular weight polyethylene particles having a weight average molecular weight of 1,000,000 to 4,000,000.

3. The friction material according to claim 1 or 2, characterized in that the ultra-high molecular weight polyethylene particles have an average particle size of 10 μm to 70 μm.

4. A friction material according to any one of claims 1 to 3, characterized in that the friction material composition does not contain any metal element or alloy, and contains zirconium oxide as an inorganic friction modifier in an amount of 10 to 35% by weight based on the total amount of the friction material composition.

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