Friction material comprising graphite, methods of making friction materials, and their uses

Abstract

To provide a friction material combining good thermal conductivity, and / or a sufficient friction coefficient and / or sufficient lubricity.SOLUTION: The friction material comprises graphite having a c / 2 of 0.3356 nm or less and a spring-back of 50% or more, such as 55% or more, or 60% or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] FIELD OF THE INVENTION The present invention relates to a friction material comprising an elastic graphitic material. The present invention further relates to a method for making such a friction material and to uses thereof. [Background technology]

[0002] Background of the Invention Friction materials are used in a variety of applications, such as for disc brakes, drum brakes, or clutches, and for end use in vehicles such as automobiles, heavy vehicles, wind turbines, and railways. Friction materials must meet various requirements depending on their intended use. Some of the desired properties include good heat dissipation achieved by high thermal conductivity, a well-defined and stable coefficient of friction, good lubricity, high compressibility, vibration damping properties, noise reduction, and low disc brake resistance. The use of asbestos in friction materials has been clearly phased out over the past several decades due to workplace safety, health, and environmental concerns. Additionally, the use of copper in friction materials, which combines good thermal conductivity with a good and stable coefficient of friction, has been phased out in light of environmental legislation that has been in place for many years. Graphite and graphitic carbon have been used in friction materials in the past. Elastic graphitic materials, in particular, provide the necessary springback properties for use in friction materials. For example, EP 3 088 764 A1 discloses the use of elastic graphitic materials in non-asbestos organic (NAO) brake pads. Elastic graphitic carbon particles are produced by expanding carbonaceous mesophase or coke, followed by graphitization at 1900-2700°C, resulting in a degree of graphitization of 80-95% as measured by X-ray analysis. This improves volume recovery upon removal of an applied compressive load. Furthermore, crack formation is reduced, thereby reducing chipping.

[0003] One drawback of using graphitic materials in friction materials stems from the conflicting requirements for springback and thermal conductivity. Graphite springback is generally correlated with crystallinity. Without being bound by theory, carbon springback tends to increase with decreasing crystallinity. For example, amorphous coke with a c / 2 value greater than about 0.34 nm and an Lc value less than about 50 nm will have a springback value of 50% or greater. Graphitized carbon with a c / 2 value less than about 0.3356 nm and an Lc value between about 100 and about 200 nm will have a lower springback. Typical flake-type natural graphite with a c / 2 value less than about 0.3358 nm and an Lc value greater than about 200 nm will have a very low springback value, for example, less than about 10%. High crystallinity provides high thermal conductivity for heat dissipation and good lubricity for stabilizing the friction coefficient, all of which are desirable properties of friction materials. On the other hand, high springback results in high compressibility of the friction material and reduced noise for good vibration damping, and low disc braking resistance. Therefore, the use of graphitic materials in friction materials provides a necessary balancing act between opposing properties of the material, requiring compromises in one or more properties of the friction material. Thus, the state of the art constitutes a problem. Summary of the Invention

[0004] BRIEF DESCRIPTION OF THE INVENTION The above-mentioned problems are solved by the present invention, as defined in the appended claims. More specifically, the present invention provides a material that combines good thermal conductivity and / or a sufficient coefficient of friction and / or sufficient lubricity. Furthermore, the material of the present invention provides a sufficiently high springback. In particular, the present invention is embodied in a friction material comprising graphite having a c / 2 of 0.3358 nm or less, e.g., 0.3357 nm or less, or 0.3356 nm or less, and a springback of 40% or more, e.g., 40.5% or more, or 41% or more. As known to those skilled in the art, graphitization of 95.3% or more corresponds to a c / 2 of 0.3358 nm or less. It has been found that graphite having these parameters can be obtained and provide good properties in friction materials. According to one embodiment, the graphite used in the friction material according to the present invention can have a springback of 45% or more, e.g., 50% or more, or 60% or more. It has been found that such materials are particularly advantageous for use in friction pad applications.

[0005] According to one embodiment of the present invention, the graphite included in the friction material has a degree of graphitization of 95.3% or more, such as 96% or more, such as 97% or more. According to one embodiment of the present invention, the graphite contained in the friction material has a density of 2.0 g / cm 3 The xylene density is equal to or greater than 1000 ppm. According to one embodiment of the present invention, the graphite contained in the friction material has a crystallinity (L c ) According to one embodiment of the present invention, the graphite contained in the friction material is 9m 2 / g or less BET surface area. According to one embodiment of the present invention, the graphite contained in the friction material is surface-modified graphite. For example, the graphite contained in the friction material may be surface-modified natural graphite or surface-modified synthetic graphite, or may be a mixture of surface-modified natural graphite and surface-modified synthetic graphite, such as surface-modified graphite by heat treatment and coated graphite by surface-coating treatment. According to one embodiment of the present invention, the surface modification of graphite includes surface modification by heat treatment. According to one embodiment of the present invention, the surface modification comprises an additional coating on the surface of the graphite particles, said surface coating can be carried out simultaneously with or separately from the heat treatment, for example subsequent to the heat treatment.

[0006] According to a further embodiment of the invention, the surface modification of the graphite comprises a surface coating obtainable by a chemical vapor deposition (CVD) process, for example a carbon coating obtainable by a CVD process. According to yet another embodiment of the present invention, the surface modification of graphite includes surface modification by heat treatment and further surface coating obtained, for example by a CVD process, coating the graphite surface with amorphous carbon or with a coating of a carbon precursor on the graphite surface, followed by carbonization by heat treatment in an inert gas atmosphere. Also, after the CVD process, the material is typically hydrophobic. Further treatment of the material after the CVD process may improve its wettability with water and, if desired, help make the material more hydrophilic or more hydrophobic. Therefore, a further oxidation treatment may form part of the present invention. The degree of oxidation makes it possible to control the hydrophilicity of the graphite surface, and therefore its wettability by humidity. The same oxidation can be applied to graphitic materials having a surface coating of amorphous carbon.

[0007] According to one particular embodiment of the present invention, the friction material comprises 0.1% to 30% by weight of graphite having the above-defined characteristics, based on the total weight of the friction material. According to one particular embodiment of the present invention, the friction material has a copper content of 5% by weight or less, such as a copper content of 0.5% by weight or less. According to one embodiment of the present invention, the friction material is essentially copper-free. According to one particular embodiment of the present invention, the friction material has an in-plane thermal conductivity of 1.5 W / mK or greater as measured according to ASTM E1461 using a laser flash with a NETZSCH LFA447. According to one particular embodiment of the present invention, the friction material has a coefficient of friction of 0.5 or less. According to yet another embodiment of the present invention, the friction material can include one or more additional ingredients selected from the group consisting of a resin or cement binder, antimony trisulfide, copper, barium sulfate, metal powder, metal fiber fibers, mineral fiber, iron sulfide, coke, other natural, synthetic, expanded graphite, calcium carbonate, mica, talc, zirconia, and mixtures thereof, and additional ingredients typically used in friction materials known to those skilled in the art.

[0008] Also part of this invention is a method for producing a friction material, the method comprising the steps of: (a) providing graphite; (b) subjecting the graphite provided in step (a) to a heat treatment at 600°C or greater for 30 minutes or more; and (c) mixing the treated graphite obtained at the end of step (b) with additional ingredients and processing to form a friction material, for example, by compression molding or hot compression molding or curing by heat treatment, or any combination thereof. According to one embodiment, the graphite provided may be natural graphite, or synthetic graphite, or a mixture of natural and synthetic graphite. According to a further embodiment, the treatment in step (b) may further include a surface coating treatment, such as a CVD treatment or an amorphous carbon surface coating treatment. According to a further embodiment, the treatment step (b) of the method may include a first heat treatment that is part of the surface coating treatment, such as a CVD coating or pitch coating with subsequent carbonization, and a second heat treatment that is not part of the surface coating treatment and may be performed before or after the surface coating treatment. According to a further embodiment, the second heat treatment is a post-treatment (i.e., performed after the first treatment, such as a surface coating treatment). According to a further embodiment, the treatment step (b) may not include a surface coating treatment such as a CVD treatment.

[0009] Also part of the present invention is the use of the friction material according to the present invention in the manufacture of brake pads, for example in the manufacture of low copper brake pads or in the manufacture of copper-free brake pads. Also part of the present invention is a brake pad including a friction material according to the present invention, which may be for use in an electric vehicle. It should be understood that the following description and references to the drawings are directed to exemplary embodiments of the invention and are not intended to limit the scope of the claims. DETAILED DESCRIPTION OF THE INVENTION

[0010] Detailed Description of the Invention The present invention according to the appended claims provides a friction material containing graphite, wherein the graphite has a c / 2 value of 0.3358 nm or less, e.g., 0.3357 nm or less, or 0.3356 nm or less, and a springback of 40% or more, e.g., 40.5% or more, or 41% or more. The core of the present invention is that it is possible to achieve a balance between the essentially contradictory properties of graphitization and springback of graphite for effective use in friction materials. Those skilled in the art will recognize that the springback of graphite generally depends on its crystallinity. Graphite with low crystallinity generally has high springback properties, and vice versa. High springback results in low compressibility, which reduces the packed density of the graphite powder. A friction material comprising a graphitic material having the combination of physical parameters in accordance with the present invention has not previously been demonstrated.

[0011] graphitic material According to the present invention, the graphite contained in the friction material has a c / 2 value of 0.3358 nm or less, such as 0.3357 nm or less, or 0.3356 nm or less, and a springback of 40% or more, such as 40.5% or more, or 41% or more, such as 45% or more, such as 50% or more, such as 55% or more, such as 60% or more, such as 65% or more, such as 70% or more, for example 75% or more. According to the present invention, the graphite contained in the friction material has a c / 2 value of 0.3358 nm or less, such as 0.3357 nm or less, or 0.3356 nm or less. It will be clear to those skilled in the art that having a high degree of graphitization and a high springback is most advantageous. Thus, friction materials comprising graphite with a c / 2 value of 0.3358 nm or less and a springback of more than 40%, for example, 41% or more, and friction materials comprising graphite with a c / 2 value of 0.3358 nm or less and a springback of 50% or more, and friction materials comprising graphite with a c / 2 value of 0.3358 nm or less and a springback of 60% or more, and even friction materials comprising graphite with a degree of graphitization of 95.3% or more and a springback of 70% or more, and even friction materials comprising graphite with a c / 2 value of 0.3356 nm or less (a degree of graphitization of 98% or more) and a springback of 75% or more are considered part of the present invention.

[0012] As described in the introduction above, a high degree of graphitization results in good thermal conductivity of the graphite and therefore good heat dissipation in friction materials containing this graphite, and good springback results in good compressibility of the material, which in turn results in improved vibration damping and noise reduction. According to one embodiment of the present invention, the graphite contained in the friction material has a degree of graphitization of 95.3% or greater and a springback of more than 40%, such as 41% or greater, for example, a degree of graphitization of 96% or greater, or 97% or greater, or even a degree of graphitization of 98% or greater. According to one embodiment of the present invention, the friction material has a crystallinity L of 50 nm or more. c For example, the friction material includes graphite having a crystallinity L of 100 nm or more. c , or a crystallinity L of 150 nm or more c , or a crystallinity L of 200 nm or more c , or crystallinity L of 250 nm or more c As used herein, this includes graphite having a crystallinity L c indicates the average crystallite size of graphite.

[0013] According to one embodiment of the present invention, the friction material has a surface roughness of 2.0 g / cm 3 Xylene density above 2.1g / cm 3 Xylene density above 2.2g / cm 3 Xylene density above 2.23g / cm 3 Xylene density above 2.24 g / cm 3 Xylene density above 2.25g / cm 3 Xylene density greater than or equal to 2.26 g / cm 3 For example, the graphite contained in the friction material according to one embodiment of the present invention has a xylene density of 2.26 g / cm or greater. 3 The xylene density will not be higher than Higher xylene density generally indicates higher crystallinity of the graphite, and therefore improved thermal diffusivity of the material, without directly providing information on the numerical value of the crystallite size of the c / 2 distance, providing improved properties for use in friction materials. According to one embodiment of the present invention, the friction material is 2 For example, the graphite contained in the friction material according to the present invention has a BET surface area of ​​8 m 2 / g or less BET surface area, 8.0 m 2 / g or less BET surface area, or 7.0 m 2 / g or less BET surface area, or 6.0 m 2 / g or less BET surface area, or 5.0 m 2 / g or less BET surface area, or 4.5m 2 / g or less BET surface area, or 4.0 m 2 / g or less. One advantage of a lower BET surface area is generally lower resin consumption.

[0014] Preparation of graphite materials with desired properties by surface treatment According to some embodiments of the present invention, the graphitic material for use in the friction material may be natural graphite, synthetic graphite, or a mixture thereof. For example, the graphitic material may be surface-treated natural graphite or surface-treated synthetic graphite. According to one embodiment, the graphite may be chosen from expanded graphite and / or non-expanded graphite. According to a further embodiment, the graphite is selected from non-expanded graphites. According to an embodiment of the present invention, the surface treatment of the graphitic material may be a heat treatment, for example, at a temperature of 600°C or higher, such as 800°C or higher, or 1000°C or higher, or 1200°C or higher, such as 1400°C. According to some embodiments of the present invention, the surface treatment of the graphitic material may be a surface coating treatment such as a chemical vapor deposition (CVD) treatment or coating the graphite particles with a carbon precursor followed by carbonization in an inert gas atmosphere.

[0015] Typical surface coating processes are based on the coating of carbon precursors, such as coal tar or petroleum pitch (typically referred to as pitch coating), or organic polymers, such as phenolic resins or polystyrene, polyvinyl alcohol, furan resins, or furfuryl alcohol (known to produce high carbon yields upon carbonization), onto the graphite surface in a dry or wet mixing process, followed by carbonization at high temperatures in an inert gas atmosphere (Wan et al., Journal of Applied Electrochemistry, 2009, 39, 1081; Yoon et al., Journal of Power Sources, 2001, 94, 68). Another known process described in the art involves the coating of pyrolytic carbon on the graphite surface, typically referred to as CVD coating, achieved by treating graphite particles in hydrocarbon gases or vapors at high temperatures (chemical vapor deposition). The surface coating described forms an amorphous carbon coating on the surface of the graphite particles. Examples of these surface modifications include heat treatments and surface coating treatments, which can be simultaneous, for example in the case of CVD coating, or independent, for example in the case of pitch coating of a carbon precursor with subsequent carbonization.

[0016] During the CVD process, a carbon source in the gas phase, usually a hydrocarbon, is decomposed at high temperatures, and carbon particles are deposited as so-called pyrolytic carbon on the graphite surface. The hardening effect of pyrolytic carbon, especially isotropic carbon, has been demonstrated in the literature (see, for example, Handbook of Carbon, Graphite, Diamond and Fullerenes, Properties, Processing and Applications, Hugh O. Pierson, published in 1993 by Noyes Publications, ISBN: O-8155-1339-9, Printed in the United States, Published in the United States of America by Noyes Publications Mill Road, Park Ridge, New Jersey 07656). Due to its random structure, the deposited isotropic pyrolytic carbon lacks orientation and is, as a result, very hard. According to certain embodiments, the CVD process can be carried out using, for example, a rotary kiln, a fluidized bed furnace, or a fixed-bed furnace, as known from prior art applications WO 2016 / 008951 or EP 0 977 292. According to the methods disclosed in these publications, hydrocarbon gases such as propane, methane, or toluene and benzene vapor are decomposed at temperatures between 600 and 1200 °C. The resulting final material is coated with a continuous layer of amorphous carbon, 10 nm to 100 nm thick, of which 0.5 to 30 wt. % can be hydrophobic or hydrophilic. Any other known CVD process for depositing pyrolytic carbon can also be used, such as thermal CVD, plasma-enhanced CVD, hot-filament CVD, low-pressure CVD, or liquid injection CVD.

[0017] More detailed methods for producing surface-treated graphitic materials for use in friction materials are described further below. According to the present invention, the carbon-coated graphite material prepared in this manner was analyzed, and an increase in springback was indeed observed. In addition to the effect of pyrolytic carbon acting as a hardener as described above, heat treatment in an inert atmosphere also has the effect of increasing springback. It is speculated that even at temperatures above 500°C with significant residence times, small amounts of non-graphitic carbon within the graphite particles undergo structural changes that lead to increased springback. For these reasons, it is part of the present invention to provide a friction material comprising a graphitic material derived from surface-treated synthetic or natural graphite, wherein the surface treatment can be a heat treatment under an inert atmosphere, or a surface coating treatment such as a CVD treatment, or both, performed simultaneously or subsequently.

[0018] friction material According to one embodiment of the present invention, there is provided a friction material comprising the above-described graphite material, wherein the graphite material is contained in the friction material in an amount of 0.1 mass % to 30 mass % with respect to the total amount of the friction material. For example, the graphitic material may be present in the friction material in an amount of 0.1% by weight or greater, such as 0.1% by weight or greater, or 0.5% by weight or greater, or 1% by weight or greater, or 5% by weight or greater, or 10% by weight or greater, or 15% by weight or greater, or 20% by weight or greater, or 25% by weight or greater, such as about 30% by weight. For example, the graphitic material may be present in the friction material in an amount of 30% by weight or less, e.g., 25% by weight or less, or 20% by weight or less, or 15% by weight or less, or 10% by weight or less, or 5% by weight or less, or 1% by weight or less, or 0.5% by weight or less, or about 0.1% by weight. For example, the graphite material according to the present invention may be present in the friction material in an amount of 0.5% by mass to 30% by mass, such as 1% by mass to 25% by mass, or such as 2% by mass to 10% by mass, based on the total amount of the friction material.

[0019] The friction material may further include other materials suitable for use in friction materials known to those skilled in the art, such as resin or cement binders, antimony trisulfide, copper, barium sulfate, metal powders, metal fibers, mineral fibers, iron sulfide, coke, other natural, synthetic, or expanded graphite, calcium carbonate, mica, talc, and zirconia. For example, the friction material may further include expanded graphite (e.g., TIMREX, C-THERM) for high thermal conductivity friction materials. According to some embodiments, the friction material contains less than 5% by weight of copper, e.g., less than 1% by weight of copper, or less than 0.5% by weight of copper, or less than 0.1% by weight of copper. According to some embodiments, the friction material is copper-free. As used herein, a friction material is considered copper-free if it contains less than 0.05% by weight of copper or no detectable copper. The friction material may further include expanded graphite (e.g., TIMREX C-THERM) for high thermal conductivity friction materials, for example. The friction material according to the present invention may have an in-plane thermal conductivity of 1.5 W / mK or greater, such as 5 W / mK or greater, as measured according to ASTM E1461 using a laser flash with a NETZSCH LFA 447. Friction materials according to the present invention can have a coefficient of friction of 0.5 or less, such as between 0.2 and 0.5, or between 0.3 and 0.5.

[0020] Friction material manufacturing method According to one embodiment of the present invention, the friction material may be formed by providing a graphitic material having a springback of 40% or more, or 41% or more, and a c / 2 value of 0.3358 nm or less, e.g., 0.3357 nm or less, or 0.3356 nm or less. Such graphite may be obtained using a method such as those described above, which includes the steps of heat treatment and / or surface coating treatment, e.g., CVD treatment, of synthetic or natural particulate graphite. According to the present invention, a friction material can be formed by providing graphite, heat treating the provided graphite at 600°C or greater for 30 minutes or greater, and mixing and processing the resulting treated graphite with additional components to form the friction material. Such additional components can be selected from the group consisting of resin or cement binders, antimony trisulfide, copper, barium sulfate, metal powders, metal fibers, mineral fibers, iron sulfide, coke, other natural, synthetic, expanded graphite, calcium carbonate, mica, talc, and zirconia, and / or other components typically used in friction materials and mixtures thereof. After mixing with these additional ingredients, the mixture may be subjected to compression molding, for example, cold compression molding, or hot compression molding, or curing by heat treatment, or a combination thereof.

[0021] The graphite prepared by the method according to the invention may be selected from natural graphite, synthetic graphite and mixtures thereof, for example, the graphite may be crushed natural graphite or crushed synthetic graphite, or a combination of crushed natural graphite and synthetic graphite. The heat treatment can be carried out at a temperature of at least 700°C, or at least 800°C, or at least 850°C. The heat treatment can be carried out for a time of at least 30 minutes, preferably at least 60 minutes, or at least 120 minutes. According to one embodiment, the heat treatment can be carried out at a temperature between 600°C and 850°C inclusive, for a time of at least 120 minutes. According to one embodiment, the heat treatment is performed at about 1000°C or higher for at least 30 minutes. According to one embodiment, the heat treatment is performed at about 1000°C or higher for at least 60 minutes. According to one embodiment, the heat treatment is performed at about 1200°C or higher for at least 30 minutes. According to one embodiment, the heat treatment is performed at about 1200°C or higher for at least 60 minutes. According to one embodiment, the heat treatment is performed at about 1300°C or higher for at least 30 minutes. According to one embodiment, the heat treatment is performed at about 1300°C or higher for at least 60 minutes. According to one embodiment, the heat treatment is performed at about 1500°C or higher for at least 30 minutes. According to one embodiment, the heat treatment is performed at about 1500°C or higher for at least 60 minutes.

[0022] According to one embodiment of the present invention, step (b) of the method for producing a friction material may include a surface coating, such as a CVD process, and the heat treatment can be performed simultaneously with the coating process, or the surface coating process can be performed before the heat treatment, for example, by coating the graphite surface with a carbon precursor and then carbonizing under an inert gas, in which case the carbonization is a heat treatment. According to some embodiments of the present invention, such a CVD process uses an amorphous hydrocarbon gas, such as methane, ethane, propane, butane, benzene, or toluene, in the presence of a carrier gas, such as nitrogen or argon. According to another embodiment, step (b) comprises a separate CVD treatment (including a heat treatment and a surface coating treatment) and a separate heat treatment (not part of the CVD treatment), as described above. In this embodiment, the heat treatment is independent of the CVD treatment, and both treatments may be performed subsequent to each other, with or without other intermediate steps such as cooling, quenching, or chemical treatment. According to one embodiment of the present invention, the prepared graphite comprises natural graphite, and step (b) comprises a heat treatment and a surface coating treatment.

[0023] According to a further embodiment, step (b) of the above method may include a first heat treatment that is part of the surface coating process (e.g., CVD coating or pitch coating followed by carbonization) and a second heat treatment (not part of the surface coating process), where the second heat treatment can be performed before or after the surface coating process. According to a further embodiment, such second heat treatment is a post-treatment. According to a further embodiment, step (b) of the above method may not include a surface coating treatment. According to one embodiment of the present invention, the provided graphite comprises synthetic graphite, and method step (b) may or may not include a surface coating treatment such as a CVD treatment. Additionally, various other materials are provided in accordance with the present invention to form the friction material, such as resin or cement binders, antimony trisulfide, copper, barium sulfate, metal powders, metal fibers, mineral fibers, iron sulfide, coke, other natural, synthetic, or expanded graphite, calcium carbonate, mica, talc, and zirconia.

[0024] Also part of this invention is the use of the graphitic materials disclosed herein in the formation of friction materials, and the use of such friction materials in the formation of brake pads for disc brakes, drum brakes, or clutches, and for applications in automobiles, heavy vehicles, railroads, etc., including electric vehicles. In electric vehicles, there is no noise from the engine, so it is even more advantageous to use brake pads that produce reduced noise. The friction material according to the present invention can also be used, for example, in carbon brushes and bipolar plates for fuel cells, or for disc brakes, drum brakes, or clutches, and for applications in vehicles such as automobiles, heavy vehicles, wind turbines, railways, etc. According to one embodiment of the present invention, the prepared graphite can be used for self-lubricating polymer compounds, for example, in carbon brushes and bipolar plates for fuel cells. Also part of the present invention is a method for improving brake pad performance, comprising using a friction material according to the present invention, the friction material including graphite having a c / 2 value of 0.3358 nm or less, e.g., 0.3357 nm or less, or 0.3356 nm or less, and a springback of 40% or more, e.g., 41% or more, where the brake pad performance can be evaluated in terms of noise reduction, durability, vibration damping, braking power, coefficient of friction stabilization, etc. Also part of this invention is a brake pad containing a friction material according to the invention.

[0025] Graphite springback Springback is a source of information about the elasticity of compressed graphite powder. A predetermined amount of powder is injected into a 20 mm diameter die. After inserting a punch and sealing the die, air is evacuated from the die. A compression force of 1.5 metric tons is applied, resulting in a springback of 0.477 t / cm. 2 A pressure of 1000 psi is created and the height of the powder is recorded. This height is recorded again after the pressure is released. Springback is the percentage difference in height relative to the height under pressure.

[0026] Layer spacing c / 2 and degree of graphitization The interlayer spacing c / 2 was measured by X-ray diffraction.

[0002] The layer spacing was calculated by determining the angular position of the peak maximum in the reflection profile and applying the Bragg equation (Klug and Alexander, X-ray Diffraction Procedures, John Wiley & Sons Inc., New York, London (1967)). To avoid problems due to the low absorption coefficient of carbon, instrument alignment, and non-planarity of the sample, an internal standard, silicon powder, was added to the sample, and the graphite peak position was recalculated based on the position of the silicon peak. The graphite sample was mixed with the silicon standard powder by adding a mixture of polyglycol and ethanol. The resulting slurry was then spread onto a glass plate with a blade spaced 150 mm apart and dried. Layer spacing (d 002 ) and degree of graphitization (g) are directly related by the following formula:

[0027]

number

[0028] Graphite crystallite size L c Crystallite size L c is determined by analyzing the (002) and (004) diffraction profiles. In the present invention, the method proposed by Iwashita (N. Iwashita, C. Rae Park, H. Fujimoto, M. Shiraishi and M. Inagaki, Carbon 42, 701-714 (2004)) is used. The algorithm proposed by Iwashita was specifically developed for carbon materials. The width of the line profile at half maximum of the sample and reference is measured. A correction function allows the width of the pure diffraction profile to be determined. The crystallite size is then calculated by applying Scherrer's formula (P. Scherrer, Göttinger-Nachrichten 2 (1918) p. 98).

[0029] xylene density The analysis is based on the liquid exclusion principle as specified in DIN 51901. Approximately 2.5 g (accuracy 0.1 mg) of powder is weighed into a 25 mL pycnometer. Xylene is added under vacuum (15 Torr (20 mbar)). After a residence time of several hours at normal pressure, the pycnometer is adjusted and weighed. The density represents the ratio of mass to volume. The mass is given by the mass of the sample, and the volume is calculated from the difference in mass of the pycnometer filled with xylene with and without the sample powder.

[0030] BET specific surface area The method is based on the registration of the adsorption isotherm of liquid nitrogen at 77 K in the range of p / p0 = 0.04-0.26. The monolayer capacity can be determined according to the procedure proposed by Brunauer, Emmett, and Teller (Adsorption of Gases in Multimolecular Layers, J. Am. Chem. Soc., 1938, 60, 309-319). Based on the cross-sectional area of ​​the nitrogen molecule, the monolayer capacity, and the mass of the sample, the specific surface area can be calculated. It should be noted that the present invention may include any combination of the features and / or limitations referenced herein, except for combinations of features that are mutually exclusive. The foregoing description is directed to and is intended to describe specific embodiments of the present invention. However, it will be apparent to those skilled in the art that many modifications and variations of the embodiments described herein are possible. All such modifications and variations are intended to be within the scope of the present invention, as defined by the appended claims. [Example]

[0031] Example 1: Hydrophobic CVD coated synthetic graphite using a rotary furnace D 10 = 5 μm and D 90Ground synthetic graphite "GRAPHITE SGA" with a particle size distribution of 73 μm was used as a starting material to improve its springback and other properties for use as a graphite-containing friction material. This starting material was continuously fed into a rotary kiln reactor heated to 1050 °C using a single screw for 2 hours, producing approximately 2000 g of material. Chemical vapor deposition (CVD) was carried out using a mixture of hydrocarbons and inert gases (amorphous carbon precursor: CH (3 L / min) and carrier gas: N (1 L / min)) fed into the reactor to maintain the pressure within the reactor at 0-8 mbar above atmospheric pressure. The tube was tilted at 4°, the rotation speed was set at 6 rpm, and the residence time in the kiln was approximately 30 minutes. To eliminate the amount of polycyclic aromatic hydrocarbons (PAHs), further processing was performed in a muffle furnace or in a rotary kiln at 700 °C in an inert atmosphere (N). Ground synthetic graphite "GRAPHITE SGA" was again used as the starting material to improve its springback and other properties for use as a graphite-containing friction material. This starting material was charged into a fluidized-bed reactor and heated to approximately 900°C under inert gas. Chemical vapor deposition (CVD) was carried out using a mixture of organic solvent and inert gas (nitrogen flow at 4 L / min) while maintaining atmospheric pressure in the reactor. The total processing time (including heating and cooling) was 7 hours. The discharged material, SG HSB B, was then subjected to controlled sieving using a 150 μm sieve. The properties of the untreated starting material "Graphite SGA" and the resulting high springback materials "Graphite SG HSB A" and "Graphite SG HSB B" are shown in Table 1:

[0032] [Table 1]

[0033] Example 2: Hydrophilic CVD coated natural graphite with additional heat treatment using a rotary furnace As a starting material for hydrophilic graphite friction materials based on treated natural graphite with high springback, D 10 = 6 μm and D 90 Flaky natural graphite "GRAPHITE NGB" with a particle size distribution of 42 μm was used. This starting material was continuously fed into a rotary kiln reactor externally heated to 1050 °C for 2 hours to produce approximately 700 g of material. Chemical vapor deposition (CVD) was carried out using a mixture of hydrocarbons and inert gases (amorphous carbon precursor: CH (3 L / min) and carrier gas: N (1 L / min)) fed into the reactor to maintain the pressure within the reactor at 0-8 mbar above atmospheric pressure. The tube was tilted at 4°, the rotation speed was set at 6 rpm, and the residence time within the kiln was approximately 30 minutes. To improve the wettability of the resulting CVD-modified "GRAPHITE NGB," an additional process step was added to Example 1. The resulting CVD-modified "GRAPHITE NGB" was fed into a rotary furnace heated to 650°C and filled with an oxygen-containing atmosphere (2 L / min synthetic air flow), with an inclination of 6° and a rotation speed of 6 rom. 350 g of material was fed over approximately 30 minutes. The properties of the untreated starting material "GRAPHITE NGB" and the resulting high springback material "GRAPHITE NG HSB B" are shown in Table 2:

[0034] [Table 2]

[0035] Example 3: CVD-processed synthetic graphite using a fluidized bed As a starting material for fluidized bed batch processes, D 10 = 7 μm and D 90Potato-shaped synthetic graphite "GRAPHITE PSG" with a particle size distribution of 36 μm was used. This starting material (8500 g) was charged into a fluidized-bed reactor and then heated to 920 °C under a nitrogen atmosphere. CVD treatment was carried out for 260 minutes using a mixture of hydrocarbon and inert gas (amorphous carbon precursor: toluene C7H8 and carrier gas: N2). The reactor and treated graphite were then cooled under a nitrogen atmosphere. When the material reached ambient temperature, it was discharged from the fluidized bed. The properties of the untreated starting material "Graphite PSG" and the resulting high springback material "Graphite PSG HSB C" are shown in Table 3:

[0036] [Table 3]

[0037] Example 4: Heat-treated synthetic graphite using a box furnace Approximately 450 g of crushed synthetic graphite "GRAPHITE PSG" (see Example 3) was placed in a crucible and placed in a high-temperature gas-tight box furnace. The starting material, which had a springback of 12%, was heated to 1500°C at a ramp rate of 10°C / min. This was done with a constant nitrogen flow rate of 10 L / min. Once 1500°C was reached, this temperature was held for a residence time of 60 minutes. The sample was then cooled in a nitrogen atmosphere (still flowing at 10 L / min), discharged, and analyzed once it had reached ambient temperature. The properties of the untreated starting material "Graphite PSG" and the resulting heat-treated high springback material "Graphite PSG HSB HT" are shown in Table 4:

[0038] [Table 4]

[0039] Example 5: Preparation of brake pads Brake pads having the following ingredients according to Table 5 were prepared.

[0040] [Table 5]

[0041] These components were dry mixed, cold pressed in a performer at 140 bar, cured by compression molding at 160°C for 9 minutes, post-cured in an oven (120°C for 2 hours, then 160°C for 5 hours), and then ground into brake pads. Four brake pads were prepared according to the above procedure: BP1 contains "Graphite NGB" as the graphite type, BP2 contains "Graphite SGA" as the graphite type, BP3 contains "Graphite SG HSB A" as the graphite type, and BP4 contains "Graphite SG HSB B" as the graphite type. The properties of these graphite types can be found in Tables 1 and 2 above. The density and porosity of brake pads BP1 to BP4 were measured. Density was measured using standard SAE 9380. Porosity was measured using standard JIS D4418:1996, using either water or oil. The physical properties of these brake pads can be seen in Table 6 below.

[0042] [Table 6]

[0043] The inventive brake pads BP3 and BP4 were found to have a lower density and higher porosity than the comparative brake pads BP1 and BP2. The higher porosity of the brake pads is believed to reduce noise generation during use. Brake pads BP1 to BP4 were then tested for their performance by testing the coefficient of friction and mass loss according to standard procedures. The brake pads were mounted and subjected to bedding by applying the brakes 100 times at 30 bar at a speed of 80 km / h on the brake pads. After bedding, the brake disc was replaced with a test disc having a surface roughness of Ra = 3 to 4 μm. The following test cycle was applied to each test brake pad: Cycle 1: 25 braking applications at 20 bar, followed by 25 braking applications at 30 bar, followed by 25 braking applications at 40 bar at a speed of 60 km / h on the brake pads; Cycle 2: 25 braking applications at 20 bar, followed by 25 braking applications at 30 bar, followed by 25 braking applications at 40 bar at a speed of 80 km / h on the brake pads; Cycle 3: 25 braking applications at 20 bar, followed by 25 braking applications at 30 bar, followed by 25 braking applications at 40 bar at a speed of 100 km / h on the brake pads. Each of these test brake pads was therefore subjected to 225 braking applications, 75 at 60 km / h, 75 at 80 km / h, and 75 at 100 km / h. The coefficient of friction was measured for each pressure / speed combination. At the end of cycle 3, the mass loss of the brake pad was measured by determining the difference in mass of the brake pad before test cycle 1 and after test cycle 3. The coefficient of friction measurements are summarized in Table 7.

[0044] [Table 7]

[0045] All results shown are average values ​​obtained after two test runs using equivalent brake pads. The mass losses measured for the inventive brake pads BP3 and BP4 were 58 mg and 55 mg, respectively, while the mass losses measured for the comparative brake pads BP1 and BP2 were 50 mg and 66 mg, respectively. It can be seen that the inventive brake pads BP3 and BP4 achieve higher coefficients of friction than the comparative brake pads BP1 and BP2, except for the low-stress test at 20 bar / 60 km / h. In all cases, the coefficient of friction for BP4 (varying between 0.33 and 0.44) is more stable than those for BP1 (varying between 0.30 and 0.43) and BP2 (varying between 0.30 and 0.46).

Claims

1. A friction material comprising graphite having a c / 2 of 0.3358 nm or less, e.g., 0.3357 nm or less, or 0.3356 nm or less, and a springback of 40% or more, e.g., 41% or more.

2. The friction material of claim 1 , wherein the graphite has a degree of graphitization of 95.3% or more, such as 96% or more, such as 97% or more.

3. The friction material of claim 1 or 2, wherein the graphite has a springback of 45% or more, or 50% or more, or 60% or more.

4. The graphite has a density of 2.0 g / cm 3 Xylene density above 2.1g / cm 3 Xylene density above 2.2g / cm 3 Xylene density above 2.23g / cm 3 The friction material according to any one of claims 1 to 3, having a xylene density equal to or greater than 1000 kJ / g.

5. The graphite has a crystallinity (L c The friction material of any one of claims 1 to 4, comprising:

6. The graphite is 9m 2 / g or less BET surface area, e.g., 8.0 m 2 / g or less BET surface area, or 7.0 m 2 / g or less BET surface area, or 6.0 m 2 / g or less BET surface area, or 5.0 m 2 / g or less BET surface area, or 4.5m 2 / g or less BET surface area, or 4.0 m 2 The friction material of any one of claims 1 to 5, having a BET surface area of ​​less than or equal to 1 / g.

7. The friction material according to any one of claims 1 to 6, wherein the graphite is a surface-modified graphite, such as a surface-modified natural graphite or a surface-modified synthetic graphite, or a mixture thereof, such as a surface-modified graphite by a heat treatment and a coated graphite that may be subjected to a surface coating treatment.

8. 8. The friction material of claim 7, wherein the surface modification of the graphite includes a surface modification by a heat treatment and / or a surface coating treatment, such as a surface coating obtained by a chemical vapor deposition (CVD) process, and the surface modification may include a further treatment to increase wettability, such as an oxidation treatment, and the surface coating may be performed simultaneously with or separately from the heat treatment, such as subsequent to the heat treatment.

9. The friction material according to any one of claims 1 to 8, comprising 0.1% by mass to 30% by mass of the graphite according to any one of claims 1 to 8, based on the total mass of the friction material.

10. Friction material according to any one of claims 1 to 9, having a copper content of 5% by mass or less, for example a copper content of 0.5% by mass or less.

11. 11. The friction material of any one of claims 1 to 10, having an in-plane thermal conductivity of 1.5 W / mK or greater when measured according to ASTM E1461 using a laser flash with a NETZSCH LFA447.

12. The friction material of any one of claims 1 to 11, having a coefficient of friction of 0.5 or less.

13. 13. The friction material of any one of claims 1 to 12, further comprising one or more selected from the group consisting of resin or cement binders, antimony trisulfide, copper, barium sulfate, metal powders, metal fibers, mineral fibers, iron sulfide, coke, other natural, synthetic, expanded graphite, calcium carbonate, mica, talc, and zirconia.

14. A method for producing the friction material according to any one of claims 1 to 13, (a) providing graphite; (b) subjecting the graphite prepared in step (a) to a heat treatment at 600°C or higher for 30 minutes or more; (c) mixing the treated graphite obtained at the end of step (b) with additional ingredients and processing it, such as by compression molding, or hot compression molding, or hardening by heat treatment, or a combination thereof, to form a friction material. A method comprising:

15. 15. The method of claim 14, wherein the graphite provided in step (a) is selected from natural graphite, synthetic graphite, or a mixture thereof.

16. 16. The method of claim 14 or 15, wherein the heat treatment in step (b) includes a surface coating treatment, such as a CVD treatment or an amorphous carbon coating treatment.

17. The heat treatment A first heat treatment that is part of the surface coating process, such as CVD coating or pitch coating followed by carbonization, and A second heat treatment that is not part of the surface coating process but may be carried out before or after the first heat treatment, e.g., as a post-treatment.

17. The method of claim 16, comprising:

18. The method of claim 17 , wherein the second heat treatment does not include a surface coating treatment, such as a CVD treatment.

19. Use of a friction material according to any one of claims 1 to 13 in the manufacture of brake pads, for example in the manufacture of low-copper brake pads or in the manufacture of copper-free brake pads.

20. Use of the friction material according to any one of claims 1 to 13 in carbon brushes, bipolar plates for fuel cells, disc brakes, drum brakes or clutches for applications in automobiles, heavy vehicles, wind turbines or railways.

21. A brake pad comprising the friction material of any one of claims 1 to 13, which may be for use in an electric vehicle.

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