Method for manufacturing a magnetic encoder

Abstract

Method for manufacturing a magnetic encoder with a magnetic body comprising a magnetic rubber molded part, comprising a mixing step of mixing and then kneading a nitrile rubber (A), a magnetic ferrite powder (B) and a vulcanizing agent (C) to provide a magnetic rubber composition, and a forming step of shaping and vulcanizing the magnetic rubber composition in a mold to which a magnetic field is applied, so that the magnetic rubber molded item is provided, wherein the content of the magnetic ferrite powder (B) is 700 to 1500 parts by mass based on 100 parts by mass of the nitrile rubber (A), where the density of the magnetic ferrite powder (B) is 3.5 g / cm³ 3 or more wherein an average particle diameter of the magnetic ferrite powder (B) is 0.5 to 2 µm, and wherein the magnetic ferrite powder (B) has a particle size distribution with a plurality of peaks.

Description

TECHNICAL AREA

[0001] The present invention relates to a method for manufacturing a magnetic encoder comprising a magnetic body comprising a magnetic rubber molded part produced by vulcanizing a magnetic rubber composition comprising a nitrile rubber and a magnetic ferrite powder; and a magnetic encoder with a magnetic body comprising a magnetic rubber molded part. STATE OF THE ART

[0002] A magnetic rubber molded article, produced by vulcanizing a magnetic rubber compound containing rubber and magnetic powder, has been used in various applications. In particular, a suitable application of a magnetic rubber molded article is a magnetic encoder, produced by magnetizing such an article. Depending on the performance required, different rubbers are used, and considering the balance of oil resistance, heat resistance, and cost, nitrile rubber is expediently employed. As for the magnetic powder, depending on the required performance, magnetic ferrite powder, rare-earth magnetic powder, and the like are used, and considering cost and durability, magnetic ferrite powder is expediently employed (see, e.g., [reference]).patent documents Nos. 1 to 3).

[0003] To improve the accuracy of various sensors using a magnetic encoder and to reduce their size, it is essential to enhance the magnetic properties of a magnetic rubber molded part. To this end, a large quantity of magnetic powder is routinely added to improve the magnetic properties; however, an excessive amount leads to a deterioration in formability, thus limiting the amount that can be increased. Therefore, there was a need to provide a magnetic rubber molded part that could improve the magnetic properties while maintaining formability.

[0004] Patent document No. 4 describes an anisotropic bonded magnet made from a ferrite powder in which a particle size distribution has a plurality of peaks and a compact density (CD) of 3.5 g / cm³. 3 or more, and a green die has a coercive force (p-iHc) of 2100 Oe or more. Specifically, the document describes how a composition containing ferrite powder and Nylon 6 is injection molded in a magnetic field to produce an anisotropic bonded magnet. The document states that this enables the production of a highly magnetic bonded magnet with excellent fillability and orientation while maintaining coercive force.

[0005] The polyamide (Nylon 6) used in patent document No. 4 exhibits low viscosity at high temperatures above its melting point and can be injection molded at high speeds even when containing a large amount of magnetic ferrite powder. Furthermore, a magnetic rubber molded part is produced by kneading a high-viscosity magnetic rubber composition at a low temperature for an extended period and subsequently vulcanizing it. However, it has been found that when a high-viscosity magnetic rubber composition contains a large amount of magnetic ferrite powder for an extended period, the coercive force of a magnetic rubber molded part obtained after vulcanization is considerably reduced due to the intense shearing force exerted on the contained magnetic ferrite powder for a prolonged period.This is a problem specific to a magnetic rubber composition that is not significant for injection molding a polyamide-containing bonded magnet, as described in patent document No. 4. Therefore, there is a strong need to provide a method for manufacturing a magnetic rubber molded article with high coercivity. Patent document No. 5 describes a magnetic encoder and a rolling bearing. Patent document No. 6 relates to a ferrite powder for connecting magnets and a connecting magnet that uses it. Patent document No. 7 discloses a manufacturing process for rubber magnets. Patent document No. 8 describes a sealing element with excellent wear resistance and a sealing structure that uses it. DOCUMENTS OF THE STATE OF TECHNOLOGY Patent documents Patent Document No. 1: JP S60 - 14 405 A Patent Document No. 2: JP 2003 - 183 443 A Patent Document No. 3: JP 2006 - 225 601 A Patent Document No. 4: JP 2010 - 263 201 A Patent Document No. 5: JP 2009 - 097 995 A Patent Document No. 6: JP 2014 - 078 757 A Patent Document No. 7: JP S63 - 284 804 A Patent Document No. 8: US 2013 / 0 277 916 A1 SUMMARY OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION

[0006] To solve the aforementioned problems, it is an object of the present invention to provide a method for producing a magnetic encoder with a magnetic body having a high coercivity and residual magnetic flux density by vulcanizing a magnetic rubber composition with advantageous formability. MEANS TO SOLVE THE PROBLEMS

[0007] The aforementioned problems are solved by providing a method for manufacturing a magnetic encoder with a magnetic body comprising a magnetic rubber molded part, comprising a mixing step of mixing and then kneading a nitrile rubber (A), a magnetic ferrite powder (B) and a vulcanizing agent (C) to provide a magnetic rubber composition, and a forming step of shaping and vulcanizing the magnetic rubber composition in a mold to which a magnetic field is applied, so that the magnetic rubber molded item is provided, wherein the content of the magnetic ferrite powder (B) is 700 to 1500 parts by mass based on 100 parts by mass of the nitrile rubber (A), where the density of the magnetic ferrite powder (B) is 3.5 g / cm³ 3 or more wherein an average particle diameter of the magnetic ferrite powder (B) is 0.5 to 2 µm, and wherein the magnetic ferrite powder (B) has a particle size distribution with a plurality of peaks.

[0008] It is preferred that the magnetic ferrite powder (B) is an anisotropic magnetic powder.

[0009] It is preferred that the minimum torque ML of the magnetic rubber composition is 29.4 to 78.5 N·cm (3 to 8 kgf·cm), determined in a vulcanization curve at 180 °C. It is also preferred that, in the mixing step, the nitrile rubber (A), the magnetic ferrite powder (B), and the vulcanizing agent (C) are mixed and then kneaded for 10 to 60 min at 60 to 130 °C to provide the magnetic rubber composition. Furthermore, it is also preferred that the vulcanization is carried out at 140 to 250 °C for 1 to 30 min in the mold to which a magnetic field is applied.

[0010] It is preferred that the generated encoder comprises a carrier element that can be attached to a rotor and an annular magnetic rubber molded part attached to the carrier element, wherein the magnetic rubber molded part is magnetized alternately as the N pole and the S pole in the circumferential direction. EFFECTS OF THE INVENTION

[0011] According to the manufacturing process of the present invention, a magnetic encoder with a magnetic body exhibiting high coercivity and residual magnetic flux density can be produced by vulcanizing a magnetic rubber composition with advantageous formability. A high-performance magnetic encoder manufactured using the method of the present invention can contribute to improved accuracy and a reduction in size of various sensors in which the encoder is used. MODES FOR EXECUTING THE INVENTION

[0012] A magnetic encoder according to the present invention comprises a magnetic body that includes a magnetic rubber molded part. The magnetic rubber molded part is produced by vulcanizing a magnetic rubber composition containing a nitrile rubber (A) and a magnetic ferrite powder (B). The proportion of magnetic ferrite powder (B) is 700 to 1500 parts by mass based on 100 parts by mass of the nitrile rubber (A), and the density of the magnetic ferrite powder (B) is 3.5 g / cm³. 3or more. The average particle diameter of the magnetic ferrite powder (B) is 0.5 to 2 µm, and the magnetic ferrite powder (B) exhibits a particle size distribution with a plurality of peaks. As described above, the magnetic rubber composition used in the present invention is characterized in that it contains a magnetic powder with a high pressing density in a high concentration.

[0013] It is known that the residual magnetic flux density of a magnetized magnetic rubber molded object can be increased by adding a large quantity of magnetic powder. Consequently, for various types of magnetic powder, a magnetic rubber composition containing a large quantity of the magnetic powder based on a nitrile rubber was formed and vulcanized in a mold to which a magnetic field was applied, resulting in a magnetic rubber molded object whose magnetic properties were determined.The results showed that using a magnetic powder with a higher compression density allows for the production of a magnetic rubber composition with advantageous moldability, even when containing a high concentration of magnetic powder. It also enables the production of a magnetic rubber molded part with high coercivity, despite prolonged kneading of a highly viscous magnetic rubber composition. Furthermore, molding and vulcanizing the composition in a tool to which a magnetic field is applied increases the residual magnetic flux density of the magnetic rubber molded part. A magnetic rubber molded part with high residual magnetic flux density and high coercivity is required for a high-performance magnetic encoder.

[0014] A magnetic rubber composition of the present invention comprises a nitrile rubber (A). There are no specific restrictions regarding the nitrile rubber (A) used in the present invention, and a copolymer of acrylonitrile and 1,3-butadiene may be used. Hydrogenation of double bonds remaining in 1,3-butadiene units after polymerization is optional. Unhydrogenated rubbers (NBR) and hydrogenated rubbers (HNBR) may be used, depending on the application. As long as the effects of the present invention are not impaired, the rubber may contain a structural unit derived from another copolymerizable monomer. Such a structural unit may contain functional groups, such as carboxyl groups and carboxylic anhydride groups.

[0015] The acrylonitrile content in the nitrile rubber (A) is preferably 15 to 50 wt%. The 1,3-butadiene content is preferably 50 to 85 wt%, including hydrogenated units. The nitrile rubber (A) can be unhydrogenated (NBR) or hydrogenated (HNBR). The Mooney viscosity (ML) 1+10 The viscosity (at 100 °C) of the nitrile rubber (A) is preferably 20 to 100. To maintain malleability despite the large amount of magnetic powder present, a lower Mooney viscosity is preferred, and it is therefore more preferably 70 or less, more preferably 55 or less. Although a nitrile rubber that is liquid at room temperature (25 °C) can be added, it is preferred, for handling reasons, to use only a nitrile rubber that is solid at room temperature.

[0016] The magnetic rubber composition of the present invention contains a magnetic ferrite powder (B). There are no specific restrictions regarding the magnetic ferrite powder (B), and magnetic strontium ferrite powder and magnetic barium ferrite powder can be used advantageously. The magnetic ferrite powder (B) of the present invention must have a density of 3.5 g / cm³. 3 or more, preferably 3.55 g / cm² 3 or more. In this way, a magnetic rubber molded article with advantageous formability and magnetic properties can be provided. The compression density is generally 4 g / cm³. 3 or less. The density (g / cm³) is... 3) of the magnetic ferrite powder (B) the density of a sample obtained by placing 10 g of a magnetic ferrite powder into a cylindrical mold with an inner diameter of 2.54 cm and pressing it at a pressure of 1 tonne / cm² 2is obtained. With regard to such a compression density, the particle size distribution exhibits a plurality of peaks. The particle size distribution of the magnetic ferrite powder (B) can be determined using a dry laser diffraction particle size distribution measuring device. The average particle diameter of the magnetic ferrite powder (B) is 0.5 to 2 µm. Furthermore, the magnetic ferrite powder (B) is preferably an anisotropic magnetic powder. An anisotropic magnetic powder can be vulcanized in a mold to which a magnetic field is applied, so that a magnetic rubber molded article with advantageous magnetic properties is obtained. In general, such an anisotropic magnetic powder suitable for such a vulcanization process is commercially available as a magnetic powder "for magnetic field orientation".A magnetic powder for magnetic field orientation has a small aspect ratio (diameter / thickness ratio in a plate-like body) so that it can easily rotate in a rubber compound within a magnetic field. Furthermore, for an orientation process involving mechanical deformation without the application of a magnetic field, a magnetic powder with a large aspect ratio, commercially available as a magnetic powder "for mechanical orientation," is commonly used.

[0017] In the magnetic rubber composition of the present invention, the content of magnetic ferrite powder (B) is 700 to 1500 parts by mass of 100 parts by mass of nitrile rubber (A). If the content of magnetic ferrite powder (B) is less than 700 parts by mass, problems of deterioration of formability and lower coercivity compared to a conventional magnetic ferrite powder are not caused, and therefore the application of the present invention is less significant. The content of magnetic ferrite powder (B) is preferably 850 parts by mass or more, more preferably 1000 parts by mass or more.

[0018] The magnetic rubber composition of the present invention may contain a rubber other than a nitrile rubber, provided that it does not impair the effects of the present invention. However, the content of the other rubber is generally 10% by mass or less, preferably 5% by mass or less, based on the total amount of the rubber component, and more preferably, a rubber other than a nitrile rubber is essentially absent. Furthermore, the magnetic rubber composition of the present invention may contain a magnetic powder other than a magnetic ferrite powder, such as a magnetic rare-earth powder, provided that it does not impair the effects of the present invention.However, its content is generally 10% by mass or less, preferably 5% by mass or less, based on the total amount of magnetic powder, and more preferably a magnetic powder that is different from a magnetic ferrite powder, or is essentially absent.

[0019] The magnetic rubber composition of the present invention contains a vulcanizing agent (C). The vulcanizing agent (C) can be selected from those commonly used for vulcanizing nitrile rubber (A), such as sulfur, a peroxide, and a polyamine compound. The vulcanizing agent (C) content is generally 0.1 to 10 parts by mass based on 100 parts by mass of nitrile rubber (A).

[0020] The magnetic rubber composition of the present invention may contain a component different from the nitrile rubber (A), the magnetic ferrite powder (B), and the vulcanizing agent (C), as long as it does not impair the effects of the present invention. The composition may contain various additives, such as a vulcanization accelerator, a vulcanization aid, an acid acceptor, a colorant, a filler, and a plasticizer, which are commonly used in magnetic rubber compositions.

[0021] The method for producing a magnetic encoder of the present invention comprises a mixing step of mixing and then kneading a nitrile rubber (A), a magnetic ferrite powder (B) and a vulcanizing agent (C) to provide a magnetic rubber composition and a forming step of forming and vulcanizing the magnetic rubber composition in a forming tool to which a magnetic field is applied, so that the magnetic rubber molded article is obtained.

[0022] In the mixing step described above, the individual components are blended to produce a magnetic rubber composition. There are no specific restrictions regarding the mixing method, and the kneading can be carried out using an open roller, a kneader, a Banbury mixer, an internal mixer, an extruder, or the like. Kneading is preferably performed in an open roller or a kneader. The temperature of the magnetic rubber composition during kneading is preferably 60 to 130 °C. The kneading time is preferably 10 to 60 minutes.

[0023] As described above, the production of a rubber molded part typically involves kneading a highly viscous composition at a relatively low temperature for a relatively long period, followed by vulcanization. However, it has been found that when a highly viscous magnetic rubber composition containing a large amount of magnetic ferrite powder is kneaded for a long time, a strong shear force is exerted on the magnetic ferrite powder for an extended period, leading to a deterioration in the coercivity of the manufactured magnetic rubber molded part. It was found that the use of a magnetic ferrite powder (B) with a density of 3.5 g / cm³ 3or can further reduce the problem. This is a problem specific to a magnetic rubber composition and does not occur when manufacturing a polyamide-containing bonded magnet by injection molding.

[0024] The minimum torque ML of the magnetic rubber composition thus obtained is preferably 29.4 to 78.5 N·cm (3 to 8 kgf·cm), determined in a vulcanization curve at 180 °C. If the ML is less than 29.4 N·cm (3 kgf·cm), air may remain trapped in a molded part. If the ML is more than 78.5 N·cm (8 kgf·cm), the moldability may be insufficient, leading to inadequate filling.

[0025] The subsequent step is a forming step involving the molding and vulcanization of the magnetic rubber composition in a mold to which a magnetic field is applied, thus producing the magnetic rubber object. In this forming step, the aforementioned magnetic rubber composition is generally formed into a desired shape and then vulcanized by heating. Examples of methods for forming a magnetic rubber composition include extrusion and compression molding. Compression molding is particularly suitable. The vulcanization temperature is preferably 140 to 250 °C. The vulcanization time is preferably 1 to 30 minutes.Depending on the shape or dimensions of the magnetic rubber molded part, the interior may not be sufficiently vulcanized while the surface is vulcanized, and consequently, the compound may require further heating for secondary vulcanization. The heating method for vulcanization can be one commonly used for vulcanizing rubber, such as press heating, steam heating, furnace heating, or hot air heating, and press heating is suitable.

[0026] In the manufacturing process of the present invention, vulcanization is carried out in a mold to which a magnetic field is applied. Consequently, the residual magnetic flux density of the magnetic rubber molded part can be increased. During compression molding, it is advantageous to apply a magnetic field in a direction perpendicular to the surface of the molded part.

[0027] A magnetic encoder produced by the method of the present invention comprises a magnetic body that includes the resulting magnetic rubber molded object. Although the magnetic body may have a set of S and N poles, it is frequently a magnetic body with a plurality of poles in which the magnetic poles are arranged alternately; however, the type of magnetization is not limited to this. There are no specific restrictions regarding the shape of the magnetic body, but it is preferably ring-shaped, such as a disk or cylinder for detecting rotational motion. In such a case, the S and N poles are arranged alternately in the circumferential direction so that an angle can be detected. Consequently, this is the most important aspect in practice.In an application for detecting linear motion, a magnetic body in the form of a flat strip can be used. If the S and N poles are positioned close together and the dimension of each pole is small, a higher coercive force is required, and therefore the use of a magnetic encoder produced by the method of the present invention is particularly advantageous.

[0028] A magnetic encoder produced by the method of the present invention optionally comprises a support element that carries or supports the magnetic body. The support element is preferably a metal element, in particular a metal plate. There are no specific restrictions regarding a method for joining a magnetic rubber molded part and a support element, and these can be joined directly by vulcanizing a magnetic rubber molded part. For a more secure bond between a magnetic rubber composition of the present invention and a support element, the magnetic rubber molded part and the support element are preferably joined by means of a heat-curing adhesive.The magnetic rubber composition can be molded and vulcanized, and then the heat-curing adhesive can be cured, so that the magnetic rubber molded part is bonded to the substrate by means of the heat-curing adhesive. Alternatively, the magnetic rubber composition can be molded and vulcanized while the heat-curing adhesive is curing, so that the magnetic rubber molded part is bonded to the substrate by means of the heat-curing adhesive. The heat-curing adhesive can be any adhesive that can be cured by allowing a crosslinking reaction to occur upon heating. Examples of adhesives that can be used include a phenolic resin, an epoxy resin, a urethane resin, a rubber adhesive produced by dissolving unvulcanized rubber in a solvent, and a silane adhesion promoter.

[0029] A suitable embodiment of a magnetic encoder produced by the method of the present invention comprises a carrier element that can be attached to a rotor and an annular magnetic rubber element mounted on the carrier element, wherein the magnetic rubber element is magnetized alternately as the N-pole and S-pole in the circumferential direction. This is suitable as a magnetic encoder for detecting a rotary motion. When a small angle needs to be measured accurately, the use of a magnetic encoder having a magnetic rubber element with a high coercivity is particularly advantageous.

[0030] There are no specific limitations regarding the application of a magnetic encoder produced by the method of the present invention. A magnetic encoder with a ring-shaped or disc-shaped magnetic body having a plurality of poles, in which the magnetic poles are arranged alternately in the circumferential direction, is used in a sensor for detecting rotary motion. For example, it can be used in an axle speed sensor, a crank angle sensor, a motor rotation angle sensor, and the like. A magnetic encoder with a magnetic body having a plurality of poles, in which the magnetic poles are arranged alternately in a linear direction, is used in a sensor that detects linear motion. It is used, for example, in a linear guide device, an electric window regulator, an electrically adjustable seat, a brake pedal pressure sensor, an office device, and the like.Of these, the most suitable application of a magnetic encoder produced by the method of the present invention is its use as a magnetic encoder for a sensor rotor in an anti-lock braking system for a motor vehicle, which has excellent flexibility and excellent magnetic properties as well as a high residual magnetic flux density and a high coercivity force. EXAMPLES Example 1 [Production of an unvulcanized rubber layer]

[0031] The starting materials listed below were kneaded using an open roller with a diameter of 20.3 cm (8 in) for 35 min while the composition was kept at 60 to 100 °C, producing unvulcanized rubber layers with thicknesses of 1 mm, 1.5 mm and 2 mm. • Nitrile rubber (unhydrogenated: NBR): 100 parts by mass, acrylonitrile content 34%, Mooney viscosity (ML 1+10 , 100 °C) 45 • Magnetic strontium ferrite powder A (for magnetic field orientation): 1100 parts by mass Average particle diameter: 1.2 µm (the particle size distribution exhibits a plurality of peaks.) Density: 3.6 g / cm³ 3 Residual magnetic flux density of a pressed body: 196 mT Coercivity of a compact: 236 kA / m • Plasticizer TOTM [Tris(2-ethylhexyl) trimellitate]: 3 parts by mass • Zinc oxide: 4 parts by mass • Stearic acid: 3 parts by mass • Anti-aging agent: [4,4'-Bis(α,α-dimethylbenzyl)diphenylamine]: 5 parts by mass • Solid paraffin: 2 parts by mass • Sulfur: 0.4 parts by mass • Vulcanization accelerator MBTS (2,2'-Dibenzothiazolyl disulfide): 2 parts by mass • Vulcanization accelerator TETD (tetraethylthiuram disulfide): 1.5 parts by mass [Vulcanization properties]

[0032] The unvulcanized rubber sample was tested for its vulcanization properties using a Curelastometer 7 from A&D Company, Limited, according to JIS K6300-2. A vulcanization curve was generated at a measurement temperature of 180 °C for 5 minutes, and the minimum torque ML (N · cm (kgf · cm)), the maximum torque MH (N · cm (kgf · cm)), the time to 10% torque of MH t10 (min), and the time to 90% torque of MH t90 (min) were determined from the resulting graph, where the vertical axis represents torque and the horizontal axis represents time. [Mechanical properties]

[0033] A tensile test was performed according to JIS K6251. The resulting unvulcanized rubber layer was pressure-vulcanized at 170 °C for 10 minutes, yielding a vulcanized rubber layer 1 mm thick. The tensile strength (MPa) and elongation (%) were determined at 23 °C and 50% relative humidity, at a tensile speed of 500 mm / min, using a No. 3 dumbbell-type test specimen prepared by cutting the resulting vulcanized rubber layer. The tensile strength was 4.0 MPa and the elongation was 30%. [Hardness]

[0034] The hardness was determined according to JIS K6253. A test specimen, produced by laminating three vulcanized rubber layers 2 mm thick, as used for the tensile test, was measured for hardness at a temperature of 23 °C and a relative humidity of 50% using a Type A hardness tester to determine a peak value. The resulting Type A hardness was 90. [Magnetic properties]

[0035] From the obtained unvulcanized rubber layer, a disc-shaped test specimen with a diameter of 18 mm and a thickness of 6 mm was produced and then press-vulcanized at 170 °C for 10 min in a magnetic field in the direction of the specimen thickness, thus producing a vulcanized rubber test specimen. The resulting molded specimen was measured for residual magnetic flux density and coercivity using a "BH curve tracer" DC magnetization test device from METRON Inc. The results showed a residual magnetic flux density of 300 mT and a coercivity of 270 kA / m. [Adhesive capacity on a support element]

[0036] A ring-shaped carrier element made of SUS430 (oil-sling ring) with a sheet thickness of 0.6 mm and an L-shaped cross-section was used. The carrier element had the following dimensions: an inner diameter of the inner cylinder of 55 mm, an outer diameter of the outer ring of 67 mm, and an axial length of the inner cylinder of 4.0 mm. Separately, an unvulcanized rubber layer, obtained with a thickness of 1.5 mm, was cut into a toroidal sheet with an inner diameter of 56 mm and an outer diameter of 67 mm. This sheet was then placed on the carrier element, which had been pre-coated with an adhesive made from a phenolic resin. It was then pressure-vulcanized at 180 °C for 3 minutes, resulting in a magnetic body with an inner diameter of 56 mm, an outer diameter of 67 mm, and a thickness of 1.0 mm. The magnetic body was firmly bonded to the carrier element, and the adhesion was good.The results mentioned above are summarized in Table 1. Reference example 2

[0037] An unvulcanized rubber layer was prepared as described in Example 1, except that magnetic strontium ferrite powder A was replaced by magnetic strontium ferrite powder B. The properties of magnetic strontium ferrite powder B are as follows. Using the resulting unvulcanized rubber layer, the vulcanization properties, magnetic properties, and adhesion to a substrate were measured as described in Example 1. The results are summarized in Table 1. Average particle diameter: 1.14 µm (the particle size distribution shows a peak.) Density: 3.5 g / cm³ 3 Residual magnetic flux density of a pressed body: 185 mT Coercivity of a compact: 273 kA / m Example 3

[0038] An unvulcanized rubber layer was prepared as described in Example 1, except that the nitrile rubber (NBR) was replaced by hydrogenated nitrile rubber (HNBR), and the amount of stearic acid added was 2 parts by mass, and the amount of sulfur added was 0.5 parts by mass. The properties of the hydrogenated nitrile rubber used here are as described below. Using the resulting unvulcanized rubber layer, the vulcanization properties, magnetic properties, and adhesion to a support element were measured as described in Example 1. The results are summarized in Table 1. Acrylonitrile content: 36% Mooney viscosity (ML) 1+10 , 100 °C): 57 Lod value: 28 g / 100 g Comparative example 1

[0039] An unvulcanized rubber layer was produced as described in Example 1, except that the magnetic strontium ferrite powder A was replaced by a magnetic strontium ferrite powder C (for magnetic field orientation). The properties of the magnetic strontium ferrite powder C are as described below. Using the resulting unvulcanized rubber layer, the vulcanization properties, magnetic properties, and adhesion to a substrate were measured as described in Example 1. The results are summarized in Table 1. Average particle diameter: 1.4 µm (the particle size distribution shows a peak.) Density: 3.4 g / cm³ 3 Residual magnetic flux density of a pressed body: 185 mT Coercivity of a compact: 207 kA / m Comparative example 2

[0040] An unvulcanized rubber layer was produced as described in Example 1, except that the magnetic strontium ferrite powder A was replaced by a magnetic strontium ferrite powder D (for mechanical orientation), and vulcanization was carried out without applying a magnetic field. The properties of the magnetic strontium ferrite powder D are as described below. Using the resulting unvulcanized rubber layer, the vulcanization properties, magnetic properties, and adhesion to a substrate were measured as described in Example 1. The results are summarized in Table 1. Average particle diameter: 1.1 µm (the particle size distribution shows a peak.) Density: 3.2 g / cm³ 3 Residual magnetic flux density of a pressed body: 193 mT Coercivity of a compact: 235 kA / m [Table 1] Example 1 Reference example 2 Example 3 Comparative example 1 Comparative example 2 composition NBR 100 100 - 100 100 HNBR - - 100 - - Ferrite A (3.6 g / cm 3 ) 1100 - 1100 - - Ferrit B (3,5g / cm 3 ) - 1100 - - - Ferrit C (3,4g / cm 3 ) - - - 1100 - Ferrite D (3.2 g / cm 3 ) - - - - 1100 Plasticizer TOTM 3 3 3 3 3 zinc oxide 4 4 4 4 4 Stearic acid 3 3 2 3 3 Anti-aging agents 5 5 5 5 5 Solid paraffin 2 2 2 2 2 sulfur 0,4 0,4 0,5 0,4 0,4 MBTS vulcanization accelerator 2 2 2 2 2 Vulcanization accelerator TETD 1,5 1,5 2 1,5 1,5 Vulcanization curve T10 [min] 1,13 1,25 1,40 0,88 0,90 T90 [min] 2,52 3,01 2,91 2,40 3,29 ML [N · cm] ([kgf · cm]) 51,8 (5,28) 78,6 (8,02) 63,8 (6,51) 89,5 (9,13) 102,0 (10,40) MH [N · cm] ([kgf · cm]) 371,6(37,89) 589,4(60,10) 218,9(22,32) 466,1 (47,53) 796,3 (81,20) Residual magnetic flux density [mT] 300 290 298 292 260 Coercivity [kA / m] 270 273 260 229 280 Adhesive strength on a support element Good Good Good Good Good

[0041] As can be seen in Table 1, in examples 1 to 3, a magnetic powder with a density of 3.5 g / cm³ was used. 3 or more, when vulcanized in a magnetic field, a magnetic body with a high residual magnetic flux density and high coercivity was obtained. Furthermore, in Examples 1 to 3, the ML value in the vulcanization curve was small, and the fluidity during molding was advantageous. In particular, it is evident that in Example 1, where the particle size distribution of the magnetic powder exhibited a plurality of peaks, the ML value is particularly low, and the fluidity is significantly improved. In contrast, in the magnetic body of Comparative Example 1, where a magnetic powder with a compaction density of less than 3.5 g / cm³ was used, the ML value was significantly lower. 3In the case of vulcanization in a magnetic field, the coercive force was reduced by the shear force during kneading. In the magnetic body of comparison example 2, which had a mechanical orientation without the application of a magnetic field during press vulcanization, the magnetic powder was insufficiently oriented and consequently the residual magnetic flux density was reduced.

Claims

[1] Method for manufacturing a magnetic encoder with a magnetic body comprising a magnetic rubber molded article, comprising a mixing step of mixing and then kneading a nitrile rubber (A), a magnetic ferrite powder (B) and a vulcanizing agent (C) to provide a magnetic rubber composition, and a forming step of shaping and vulcanizing the magnetic rubber composition in a mold to which a magnetic field is applied, so that the magnetic rubber molded item is provided, wherein the content of the magnetic ferrite powder (B) is 700 to 1500 parts by mass based on 100 parts by mass of the nitrile rubber (A), where the density of the magnetic ferrite powder (B) is 3.5 g / cm³ 3 or more wherein an average particle diameter of the magnetic ferrite powder (B) is 0.5 to 2 µm, and wherein the magnetic ferrite powder (B) has a particle size distribution with a plurality of peaks. [2] Method for producing a magnetic encoder according to claim 1, wherein the magnetic ferrite powder (B) is an anisotropic magnetic powder. [3] Method for manufacturing a magnetic encoder according to claim 1 or 2, wherein the minimum torque ML of the magnetic rubber composition is 29.4 to 78.5 N · cm (3 to 8 kgf · cm) determined in a vulcanization curve at 180 °C. [4] Method for producing a magnetic encoder according to any one of claims 1 to 3, wherein in the mixing step the nitrile rubber (A), the magnetic ferrite powder (B) and the vulcanizing agent (C) are mixed and then kneaded for 10 to 60 min at 60 to 130 °C to provide the magnetic rubber composition. [5] Method for producing a magnetic encoder according to any one of claims 1 to 4, wherein the vulcanization is carried out at 140 to 250 °C for 1 to 30 min in the molding tool to which a magnetic field is applied. [6] Method for manufacturing a magnetic encoder according to any one of claims 1 to 5, wherein the magnetic encoder comprises a carrier element which can be attached to a rotor and an annular magnetic rubber molded part which is attached to the carrier element, wherein the magnetic rubber molded part is magnetized alternately as the N pole and the S pole in the circumferential direction. [7] Magnetic encoder with a magnetic body comprising a magnetic rubber molded part, wherein the magnetic rubber molded article is produced by vulcanizing a magnetic rubber composition containing a nitrile rubber (A) and a magnetic ferrite powder (B), a content of the magnetic ferrite powder (B) is 700 to 1500 parts by mass based on 100 parts by mass of the nitrile rubber (A), a density of the magnetic ferrite powder (B) 3.5 g / cm³ 3or more, an average particle diameter of the magnetic ferrite powder (B) is 0.5 to 2 µm, and The magnetic ferrite powder (B) exhibits a particle size distribution with a plurality of peaks. [8] Magnetic encoder according to claim 7, wherein a minimum torque ML of the magnetic rubber composition is 29.4 to 78.5 N · cm (3 to 8 kgf · cm), determined in a vulcanization curve at 180 °C. [9] Magnetic encoder according to claim 7 or 8, wherein the magnetic ferrite powder (B) is an anisotropic magnetic powder. [10] Magnetic encoder according to one of claims 7 to 9, comprising a carrier element that can be attached to a rotor and an annular magnetic rubber molded part that is attached to the carrier element, wherein the magnetic rubber molded part is magnetized alternately as the N pole and the S pole in the circumferential direction.