Friction materials

Abstract

To provide a friction material suitable for mass production through injection molding, which exhibits stable and good frictional properties at both a room temperature and higher temperatures beyond 120°C, while offering good wear resistance at higher temperatures beyond 120°C, and a servo motor equipped with a braking device including the friction material.SOLUTION: A friction material comprises an injection-molded article. The friction material comprises 40-55 vol.% of a phenolic resin, inorganic fibers with Mohs hardness of 5 or more, and an inorganic filler with the Mohs hardness of less than 5. The inorganic filler comprises tetrapod crystals.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a friction material, and more particularly to a friction material made of an injection-molded product containing a phenolic resin. [Background technology]

[0002] Conventionally, friction materials based on thermosetting resins such as phenolic resins have been used in brakes for automobiles and servo motors for large industrial robots. Because high safety is required for friction materials used in such applications, they are generally constructed to obtain a stable high friction coefficient by highly loading fillers or fibers with high Mohs hardness and using the minimum necessary amount of thermosetting resins such as phenolic resins as binder components. As a result, the resin compositions used as raw materials for friction materials have poor melt fluidity, and friction materials have generally been produced using compression molded products.

[0003] Meanwhile, in recent years, the industrial robotics field has seen growing demand for small, so-called collaborative robots that can operate in the same workspace as humans in various manufacturing processes without being enclosed by safety fences or other barriers, allowing them to work directly with humans. In such collaborative robots, servo motors are used to drive many of the joints, and when stopped, the servo motors' non-excitation brakes are used to stop or maintain the robot's position, allowing it to perform various tasks. Because these collaborative robots are smaller than large industrial robots, they do not require the high coefficient of friction required of friction materials provided in large industrial robots. However, they are still required to function as emergency brakes. To achieve emergency braking, thermosetting resins are required as binder components, as thermoplastic resins may melt if used as binder components. Furthermore, because non-excitation brakes are required for the servo motors provided in many of the joints of collaborative robots, they must also be affordable.

[0004] In response to market demand for friction materials applicable to the field of industrial robots, particularly collaborative robots, the present applicant has proposed a mass-producible friction material that has stable and favorable friction characteristics both at room temperature and in the range of temperatures above about 120°C (Patent Document 1). The friction material described in Patent Document 1 is an injection-molded product of a thermosetting resin composition that contains, for example, 30 to 50 volume % of inorganic fibers having a Mohs hardness of 5 or more, 5 volume % or more of at least three types of inorganic fillers having a Mohs hardness of less than 5, totaling 15 to 45 volume %, and 30 to 40 volume % of a phenolic resin. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent Publication No. 2021-107468 Summary of the Invention [Problem to be solved by the invention]

[0006] According to the invention described in Patent Document 1, injection molding is possible, making it possible to provide a friction material at low cost, and for example, even at 150°C, it is possible to achieve friction characteristics comparable to those at room temperature (25°C). However, as a result of the inventor's investigations, it was found that the friction material may wear out after a certain period of continued use. It was also found that this may result in the accumulation of friction powder inside the brake, which may affect the performance of the brake.

[0007] Therefore, an object of the present invention is to provide a friction material that can be mass-produced by injection molding, that has stable and good friction characteristics both at room temperature and in a range of temperatures above about 120° C., and that has good wear resistance in a range of temperatures above about 120° C. It is also an object of the present invention to provide a servomotor equipped with a braking device that uses the friction material. [Means for solving the problem]

[0008] The present inventors have conducted extensive research to solve the above-mentioned problems. As a result, they have found that the above-mentioned problems can be solved by configuring a friction material so that it contains a phenolic resin in a volume ratio within a predetermined range, and contains inorganic fibers having a Mohs hardness of 5 or more and an inorganic filler containing tetrapod-type crystals having a Mohs hardness of less than 5. The gist of the present invention is as follows.

[0009] The first aspect of the present invention relates to a friction material made of an injection-molded product, which contains 40 to 55 volume % of phenolic resin, inorganic fibers having a Mohs hardness of 5 or more, and an inorganic filler having a Mohs hardness of less than 5, and the inorganic filler contains tetrapod-type crystals.

[0010] In an embodiment of the present invention, the inorganic fibers may be contained in an amount of 20 to 40% by volume, the inorganic filler in an amount of 5 to 40% by volume, and the tetrapod-type crystals may be contained in an amount of 5% by volume or more based on the total volume of the inorganic filler.

[0011] In an embodiment of the present invention, the tetrapod-shaped crystal may be a crystal of zinc oxide whiskers.

[0012] In an embodiment of the present invention, the inorganic fibers may be glass fibers and / or rock wool.

[0013] In an embodiment of the present invention, the inorganic filler may contain at least one selected from layered silicates, inorganic whiskers, and sulfates in addition to the tetrapod-type crystals.

[0014] In an embodiment of the present invention, the rubber component may be contained in an amount of 5 to 15% by volume.

[0015] The above-described embodiments of the present invention can be combined in any manner.

[0016] A second aspect of the present invention relates to a servomotor equipped with a braking device using the above-mentioned friction material. [Effects of the Invention]

[0017] According to the present invention, it is possible to provide a friction material that can be mass-produced by injection molding, that has stable and good friction characteristics both at room temperature and in a range of temperatures above about 120° C., and that has good wear resistance in a range of temperatures above about 120° C. It is also possible to provide a servomotor equipped with a braking device that uses the friction material. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 2 is an explanatory diagram schematically illustrating a wear progress curve used when calculating a specific wear rate. [Figure 2] FIG. 1 is a graph showing wear progress curves at 150° C. for Example 3 and Comparative Example 4. DETAILED DESCRIPTION OF THE INVENTION

[0019] The friction material according to an embodiment of the present invention is made of an injection-molded product and contains 40 to 55 volume % of a phenolic resin, inorganic fibers having a Mohs hardness of 5 or more, and an inorganic filler having a Mohs hardness of less than 5. The inorganic filler further contains tetrapod-type crystals.

[0020] In this way, the friction material contains a phenolic resin in a predetermined volume ratio, as well as specific inorganic fibers and a specific inorganic filler, and in particular, contains tetrapod-shaped crystals as the inorganic filler. This makes it easier for the tetrapod-shaped crystals to adhere to the phenolic resin component, making the friction material less susceptible to wear even when the phenolic resin content is 40 to 55 volume %, and improving wear resistance compared to conventional materials, especially in high temperature ranges above about 120°C.

[0021] The phenolic resin may be any known phenolic resin that can be used as a friction material, such as novolac phenolic resin, resole phenolic resin, or aryl alkylene phenolic resin. Examples of novolac phenolic resins include cresol novolac resin and bisphenol A novolac resin. Examples of resole phenolic resins include methylol resole resin and dimethylene ether resole resin. Examples of aryl alkylene phenolic resins include phenol-aralkyl resin. These may be used alone or in combination of two or more. Of these, novolac phenolic resins and aryl alkylene phenolic resins are preferred.

[0022] The phenolic resin used in injection molding may contain a curing agent if necessary. For example, in the case of novolac-type phenolic resins or aryl alkylene-type phenolic resins, hexamethylenetetramine is usually suitable. Depending on the conditions, the curing agent may remain in the injection-molded product, but it is treated as a component constituting the phenolic resin.

[0023] In addition, from the viewpoint of heat resistance, it is preferable that the phenol resin has a glass transition temperature (Tg) of 120° C. or higher after curing.

[0024] The phenolic resin content in the injection-molded product, i.e., the friction material, is 40 to 55% by volume. This is a higher blend ratio than the approximately 20% by volume used in general compression molding. This ensures fluidity during injection molding and also prevents inorganic fibers with a Mohs hardness of 5 or higher from attacking the injection molding machine.

[0025] Furthermore, friction materials obtained by conventional compression molding have a low resin content and are brittle, resulting in low mechanical properties such as flexural modulus and flexural strength. Therefore, to ensure strength, they are typically fixed to a metal plate or the like. However, by using the aforementioned phenolic resin content and a specified inorganic fiber and inorganic filler, it is possible to improve mechanical properties such as flexural strength and flexural modulus compared to compression molding. Therefore, unlike compression-molded friction materials, the friction material can function as a friction material by itself without the use of a reinforcing member such as a metal plate.

[0026] The inorganic fibers have a Mohs hardness of 5 or more. From the viewpoint of ensuring good injection moldability, the Mohs hardness is more preferably 5 or more but less than 7. Examples of inorganic fibers having a Mohs hardness of 5 or more include glass fiber, rock wool, alumina fiber, basalt fiber, and zirconia fiber. These may be used alone or in combination of two or more. Of these, glass fiber and / or rock wool are preferred as inorganic fibers having a Mohs hardness of 5 or more, and glass fiber and rock wool are more preferred. Mohs hardness can be determined by whether or not the standard mineral is scratched when rubbed with 10 standard minerals of different hardness. A commercially available Mohs hardness tester can be used to measure Mohs hardness.

[0027] The content of inorganic fibers with a Mohs hardness of 5 or more in the injection-molded product, i.e., the friction material, is preferably 20 to 40% by volume, more preferably 25 to 40% by volume. This allows for good friction characteristics to be imparted. When multiple types of inorganic fibers are contained, this refers to the total amount.

[0028] The inorganic filler has a Mohs hardness of less than 5 and contains tetrapod-type crystals.

[0029] Examples of inorganic fillers that can form tetrapod-shaped crystals include zinc oxide whisker crystals, with zinc oxide whisker crystals being particularly preferred. Tetrapod-shaped zinc oxide whisker crystals can be produced, for example, by the method described in JP-A-1-252600. Commercially available zinc oxide whiskers may also be used. Zinc oxide whiskers can include thin, short, needle-like crystals, amorphous crystals, etc., but the use of tetrapod-shaped crystals makes it possible to impart good wear resistance to the friction material at temperatures above 120°C.

[0030] The tetrapod-type crystals may, for example, be composed of a central base and needle-like crystal portions extending from the base in four axial directions. The size of the base may be, for example, 0.7 to 14 μm, and the length from the base to the tip of the needle-like crystal may be, for example, 3 to 200 μm. Commercially available zinc oxide tetrapod-type crystals have needle-like crystal portions with lengths of approximately 10 μm or 20 μm.

[0031] The inorganic filler may be other inorganic fillers in addition to the tetrapod-shaped crystals. Examples of such other inorganic fillers include layered silicates, inorganic whiskers, sulfates, carbonates, non-layered silicates (excluding inorganic whiskers), minerals other than silicates (excluding inorganic whiskers), and calcium fluoride (excluding minerals). When using these, they may be used alone or in combination of two or more. From the viewpoints of injection moldability, suppression of deterioration in friction characteristics at high temperatures, and strength of the friction material, it is preferable to use three or more inorganic fillers in addition to the tetrapod-shaped crystals. The other inorganic filler other than the tetrapod-shaped crystals is preferably one or more selected from layered silicates, inorganic whiskers, and sulfates, and more preferably one or more selected from layered silicates and sulfates.

[0032] Examples of layered silicates include clay minerals such as kaolin, serpentine, talc, biotite, phlogopite, muscovite, and vermiculite.

[0033] Examples of inorganic whiskers include potassium titanate whiskers, zinc oxide whiskers (excluding tetrapod-type crystals), magnesium sulfate whiskers, wollastonite, and sepiolite.

[0034] Examples of sulfates include barium sulfate and calcium sulfate hydrate.

[0035] Examples of carbonates include calcium carbonate, magnesium carbonate, lithium carbonate, barium carbonate, potassium carbonate, and sodium carbonate.

[0036] Examples of non-layered silicates (excluding inorganic whiskers) include zeolites (boiling stones) and calcium silicates (excluding minerals).

[0037] Examples of minerals other than silicates (excluding inorganic whiskers) include dolomite, fluorite, calcite, and dolomite.

[0038] The content of inorganic fillers with a Mohs hardness of less than 5 is preferably 5 to 40% by volume in the injection-molded product, i.e., the friction material. Furthermore, the content of tetrapod-type crystals in the entire inorganic filler with a Mohs hardness of less than 5 can be 100% by volume or less. When tetrapod-type crystals and other inorganic fillers are contained, the tetrapod-type crystals are preferably contained in an amount of 5% by volume or more, more preferably 10 to 50% by volume, and even more preferably 10 to 39% by volume, based on the entire inorganic filler. When one or more other inorganic fillers are contained, each of the other inorganic fillers is preferably contained in an amount of 5% by volume or more, based on the entire inorganic filler, more preferably 50 to 90% by volume, and even more preferably 61 to 90% by volume.

[0039] The inorganic fibers and inorganic fillers described above may be surface-treated with, for example, aminosilane, if necessary, to improve the bonding strength with the phenolic resin.

[0040] In addition to the above-mentioned components, the friction material may contain a rubber component to further improve the adhesion between the inorganic filler and the phenolic resin and to further increase the contact area between the friction material and the mating material. Examples of such rubber components include natural rubber (NR), nitrile rubber (NBR), butadiene rubber (BR), styrene-butadiene rubber (SBR), chloroprene rubber (CR), polyisoprene rubber (IR), acrylic rubber A (CM), high-styrene rubber, ethylene-propylene-diene copolymer (EPDM), silicone rubber, fluororubber, and epichlorohydrin rubber. These may be used alone or in combination of two or more. Among these, at least one selected from nitrile rubber, butadiene rubber, and styrene-butadiene rubber is preferred, with nitrile rubber being more preferred. As the nitrile rubber, powdered nitrile rubber is particularly preferred.

[0041] The form of the rubber component is not particularly limited as long as it can be dispersed in the phenolic resin, and examples thereof include powder and other forms. The rubber component particles may have an average particle diameter of 1 mm or less. The average particle diameter can be measured, for example, by laser diffraction / scattering (JIS Z 8825).

[0042] When a rubber component is used, the content of the rubber component in the friction material is preferably 5 to 15% by volume from the viewpoint of further improving the adhesion between the inorganic filler and the phenolic resin.

[0043] The friction material may contain other components in addition to the above-mentioned components. Such components include those commonly used in the relevant technical field, such as flame retardants, release agents, plasticizers, colorants, weathering agents, antioxidants, etc. When added, the total amount of such components is preferably 5% by volume or less of the friction material.

[0044] The above-described friction material can have mechanical properties such as a bending strength of 100 MPa or more at 25°C as an injection-molded product. Therefore, the injection-molded product can function as a friction material by itself without being fixed to a metal plate. This allows for weight reduction and also makes it possible to omit the process of joining to a metal plate. Furthermore, the above-described friction material can have a lower blend ratio of inorganic fibers and inorganic fillers than compression-molded products, meaning that it can be made smaller than conventional compression-molded products. This also allows for weight reduction compared to conventional compression-molded products.

[0045] The shape of the friction material can be determined appropriately depending on the application, etc. Examples include a plate shape, a ring shape, etc.

[0046] The friction material can be obtained by a typical injection molding method of a thermosetting resin composition. For example, it can be produced as follows. First, the aforementioned phenolic resin (including a curing agent, if necessary), predetermined inorganic fibers, predetermined inorganic fillers, and other optional components are mixed in a blending ratio that will result in the desired content in the injection-molded product, i.e., the friction material. This blending ratio is maintained in the cured friction material, as described below. The content in the cured friction material can be measured and calculated from images obtained using, for example, a measuring device that combines an SEM (scanning electron microscope), an EDX (energy dispersive X-ray spectrometer), and an FIB (focused ion beam) device. Alternatively, the inorganic fibers and inorganic fillers can be measured and calculated (including density conversion) using a simultaneous differential thermal and thermogravimetric analyzer (TG-DTA) based on the weight ratio remaining after heating under predetermined conditions. After mixing, the components are kneaded using a roll or a kneading extruder according to a standard method to obtain a kneaded product of the desired shape as a thermosetting resin composition. The resulting thermosetting resin composition may be in any shape commonly used for injection molding, such as pellets or flakes.

[0047] Next, using an injection molding machine, the phenolic resin in the resulting thermosetting resin composition is molten and filled into a mold having a cavity of the desired shape for the friction material, and injection molding is performed. The injection molding conditions can be appropriately set depending on the compounding ratio, etc. Thereafter, the phenolic resin is cured under temperature conditions that satisfy the curing conditions of the phenolic resin, thereby obtaining an injection-molded friction material. The phenolic resin is cured after injection molding, i.e., the crosslinking reaction of the phenolic resin is promoted, thereby obtaining a friction material with the desired characteristics. These conditions can be adjusted by appropriately changing the temperature, time, etc., depending on the type of phenolic resin, etc.

[0048] The above-described friction material has stable and good friction characteristics both at room temperature and in the range of temperatures above about 120°C, as well as good wear resistance, and can be mass-produced by injection molding, making it suitable for use in braking devices. The friction material provided in the braking device may be in a form fixed to a metal substrate, but it is preferable to use an injection-molded product molded into a desired shape as is. Braking devices to which the above-described friction material can be applied are not particularly limited, and examples include non-excitation operating brakes.

[0049] The braking device having the above-described friction material is suitable for use in, for example, a servomotor. A servomotor having such a braking device can be applied to various devices requiring drive control, and is suitable, for example, for collaborative robots that require the use of many servomotors. Because the collaborative robot having the above-described servomotor uses a servomotor equipped with a braking device having the above-described friction material, it can be lightweight despite using many servomotors. Furthermore, because the above-described servomotor is provided in the moving part of the collaborative robot, it can have stable and excellent braking characteristics even in high-temperature environments caused by driving the moving part of the collaborative robot. [Example]

[0050] Hereinafter, embodiments of the friction material according to the present invention will be described based on examples.

[0051] Examples 1 to 7 The components were mixed in the blending ratios (volume %) shown in Table 1, and then kneaded at 110°C using a roll mixer to obtain a sheet-like kneaded product. The sheet-like kneaded product was pulverized to produce flakes of a thermosetting resin composition. Using an injection molding machine, the obtained thermosetting resin composition was filled into a mold capable of molding test pieces (4 x 10 x 80 mm) conforming to ISO 178, and injection-molded. The test pieces were then heat-treated at 180°C for 3 hours to obtain injection-molded test pieces. The molding conditions were a cylinder temperature of 90°C, a mold temperature of 190°C, and a curing time of 80 seconds.

[0052] (Comparative Examples 1 to 5) Test pieces were prepared by injection molding in the same manner as in Example 1, except that the compounding ratios shown in Table 1 were used.

[0053] (Comparative Example 6) A sheet-shaped kneaded product was prepared using a roll mixer in the same manner as in Example 1, except that the compounding ratios were as shown in Table 1. A flake-shaped thermosetting resin composition was then prepared, and an attempt was made to prepare a test piece by injection molding. However, it was not possible to obtain a test piece usable for evaluation.

[0054] The components used in the examples and comparative examples shown in Table 1 are as follows. Phenolic resin: Gun-ei Chemical Co., Ltd., Milex XL-325M, phenol-aralkyl resin, Glass fiber: Central Glass Fiber Co., Ltd., chopped strand ECS03-615, Mohs hardness 6.5 Rock wool: RB220 manufactured by Lapinas, surface treated with aminosilane, Mohs hardness 6, Kaolin: Takehara Chemical Industry Co., Ltd., Satenton W, calcined kaolin, Mohs hardness 1, Barium sulfate: Sakai Chemical Industry Co., Ltd., BA, elutriation grade, Mohs hardness 3, Tetrapod-type crystal: Panatetra, manufactured by Amtec Co., Ltd., zinc oxide whisker crystal (single crystal), needle-shaped crystal length 20 μm, Mohs hardness 4, Potassium titanate: Otsuka Chemical Co., Ltd., Tismo D, Mohs hardness 4, potassium titanate whisker Rubber component: Nipol 1411C, powdered nitrile rubber, manufactured by Nippon Zeon Co., Ltd.

[0055] (evaluation) <Friction characteristics> Cylindrical samples measuring φ5 mm × 10 mm were machined from the test pieces obtained as injection-molded articles by complete filling in the examples and comparative examples. The static friction coefficient of each sample was measured using a pin-on-disk friction and wear tester (Auto Pin Disk APD-101, manufactured by Starlite Industrial Co., Ltd.) as described below, and the specific wear rate was calculated. The test conditions were as follows: Surface pressure (load on sample): 0.1 MPa, Speed ​​(setting value): 0.05 m / s (static friction coefficient), 1.0 m / s (specific wear rate), Atmosphere: Air, room temperature and 150℃ Lubrication: No lubrication, Counterpart material: SPHC steel with surface roughness Ra of 0.8 to 1.6 μm.

[0056] <<Static friction coefficient>> The cylindrical sample was placed with its bottom surface in contact with the mating material, and the pin-on-disk friction and wear tester was started. The peak value of the friction coefficient at the start was used as the static friction coefficient. The speed (set value) at this time was 0.05 m / s. This test was carried out at room temperature (25°C) and at 150°C. The test at 150°C was carried out while heating the mating material to 150°C. The measurement results are shown in Table 1.

[0057] <<Specific wear rate>> Immediately after measuring the static friction coefficients at room temperature (25°C) and 150°C, the speed was changed to 1.0 m / s and the change in wear volume over time was measured. The specific wear volume was calculated based on the wear progress curve (see, for example, the schematic diagram in Figure 1) showing the change in wear volume (height) over time, using the following formula (1). The calculation results are shown in Table 1. The wear progress curves at 150°C for Example 3 and Comparative Example 4 are shown in Figure 2. In this experimental system, T' was defined as 20 hours after the start of the test, and T was defined as 10 hours after the start of the test. K = (H'-H) / {P·V·(T'-T)} (1) K: specific wear amount [mm 3 / N·m] H': Wear amount (height) at test time T' [mm] H: Wear amount (height) at test time T [mm] P: Surface pressure [N / mm 2 ] V: Velocity [m / s] T': Test time in steady state [s] T: Test time in steady state [s] Steady state: The state after the start of the test when the wear rate has stabilized. (See the "steady state" section in the schematic diagram in Figure 1. This section is linear.)

[0058] [Table 1]

[0059] As shown in Table 1, the Examples enable the formation of friction materials by injection molding, and they exhibit stable and favorable friction characteristics both at room temperature and at a high temperature of 150°C. It can also be seen that at a high temperature of 150°C, the Examples have a significantly lower wear rate than the Comparative Examples. For example, comparing Example 3 with Comparative Example 4, it can be seen that by including tetrapod-type crystals as a predetermined inorganic filler, the wear rate of Example 3 is reduced to about one-third of that of Comparative Example 4, which simply translates to a lifespan that is approximately three times longer. Furthermore, comparing Example 3 with Examples 4, 5, and 7, it can be seen that the inclusion of a rubber component further reduces the wear rate at 150°C.

Claims

1. A friction material made of an injection-molded product, 40 to 55% by volume of phenolic resin, Inorganic fibers with a Mohs hardness of 5 or more, Contains an inorganic filler with a Mohs hardness of less than 5, The friction material, wherein the inorganic filler contains tetrapod-type crystals, and the tetrapod-type crystals are crystals of zinc oxide whiskers.

2. The inorganic fiber is contained in an amount of 20 to 40% by volume, and the inorganic filler is contained in an amount of 5 to 40% by volume.

2. The friction material according to claim 1, wherein the tetrapod-type crystals are contained in an amount of 5% by volume or more based on the total volume of the inorganic filler.

3. 3. The friction material according to claim 1, wherein the inorganic fibers are glass fibers and / or rock wool.

4. 3. The friction material according to claim 1, wherein the inorganic filler contains at least one selected from the group consisting of layered silicates, inorganic whiskers, and sulfates, in addition to the tetrapod-type crystals.

5. 3. The friction material according to claim 1, wherein the rubber component is contained in an amount of 5 to 15% by volume.

6. A servomotor equipped with a braking device using the friction material according to claim 1 or 2.

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