Method for manufacturing bonded magnet, and compound used therefor

Abstract

Provided is method for manufacturing a bonded magnet, comprising: a step of obtaining a compound by coating magnetic particles with a polyamic acid using a wet method; a step of obtaining a molded body by molding the compound; and a step of heating the molded body, imidizing the polyamic acid to obtain a polyimide, and of binding the magnetic particles to each other using the polyimide.

Description

Method for manufacturing bonded magnet and compound used therefor

[0001] The present disclosure relates to a method for manufacturing a bonded magnet and a compound used therefor.

[0002] A bonded magnet containing magnetic particles and a binder polymer is advantageous in terms of the high degree of freedom in shape and the high electrical resistance due to the presence of the binder polymer as compared with a sintered magnet. For example, Patent Document 1 discloses a method for manufacturing a bonded magnet using polyimide as a binder polymer.

[0003] Japanese Patent Application Laid-Open No. 2004-235458

[0004] An object of the present disclosure is to provide a novel method for manufacturing a bonded magnet using polyimide as a binder polymer and a compound used therefor.

[0005] This disclosure includes the following aspects: [1] A method for manufacturing a bonded magnet, comprising the steps of: obtaining a compound by coating magnetic particles with polyamic acid using a wet method; obtaining a molded body by molding the compound; and heating the molded body to imide the polyamic acid to obtain polyimide, and bonding the magnetic particles together with the polyimide. [2] The method for manufacturing a bonded magnet according to [1], wherein the wet method is a method of coating the magnetic particles with polyamic acid by dispersing magnetic particles in a polyamic acid solution containing polyamic acid and a solvent, and then volatilizing the solvent. [3] The method for manufacturing a bonded magnet according to [1], wherein the wet method is a method of coating magnetic particles with polyamic acid by dispersing magnetic particles in a polyamic acid solution containing polyamic acid and a solvent, and then adding a poor solvent to the polyamic acid to precipitate the polyamic acid on the magnetic particles. [4] A method for producing a bonded magnet according to [2] or [3], wherein the polyamic acid solution is reacted with a tetracarboxylic dianhydride and a diamine in a solvent using a wet method. [5] A method for producing a bonded magnet according to [4], wherein the diamine comprises a diamine having amino groups at both ends of the silicone. [6] A method for producing a bonded magnet according to [5], wherein the diamine further comprises an aromatic diamine. [7] A method for producing a bonded magnet according to any one of [1] to [6], wherein the weight-average molecular weight of the polyamic acid is 5,000 or more and 20,000 or less.

[0006] [8] A compound used in the manufacture of bonded magnets, comprising magnetic particles and a polyamic acid coating the magnetic particles. [9] The compound according to [8], wherein the polyamic acid is a reaction product of a tetracarboxylic dianhydride and a diamine.

[10] The compound according to [9], wherein the diamine comprises a diamine having amino groups at both ends of a silicone.

[11] The compound according to

[10] , further comprising an aromatic diamine.

[12] The compound according to any one of [8] to

[11] , wherein the weight-average molecular weight of the polyamic acid is 5,000 or more and 20,000 or less.

[0007] According to one aspect of this disclosure, a novel method for manufacturing bonded magnets using polyimide as a binder polymer, and a compound used therein can be provided. Furthermore, according to one aspect of this disclosure, coating magnetic particles with polyamic acid by a wet method is advantageous compared to coating by a dry method (e.g., vapor deposition polymerization) in that a sufficient amount of coating can be formed on the surface of the magnetic particles, improving the insulation between magnetic particles, and resulting in a higher strength of the resulting molded article.

[0008] Embodiments of the present disclosure are described below. Unless otherwise specified, the materials described herein may be used individually or in combination of two or more. The content of each component in the composition means the total amount of any multiple substances present in the composition if there are multiple substances corresponding to each component in the composition, unless otherwise specified.

[0009] One embodiment is a method for manufacturing bonded magnets, comprising: step S1 of coating magnetic particles with polyamic acid by a wet method to obtain a compound; step S2 of molding the compound to obtain a molded body; and step S3 of heating the molded body to imide the polyamic acid to obtain polyimide and to bond the magnetic particles together with the polyimide.

[0010] The magnetic particles used in step S1 may be composed of permanent magnets. The permanent magnets may be samarium-cobalt (Sm-Co) alloy magnets, neodymium-iron-boron (Nd-Fe-B) alloy magnets, samarium-iron-nitrogen (Sm-Fe-N) alloy magnets, iron-cobalt (Fe-Co) alloy magnets, or Al-Ni-Co alloy magnets (alnico magnets). When the magnetic particles are composed of permanent magnets, the particle size of the magnetic particles may be, for example, 20 μm or more or 40 μm or more, 300 μm or less or 250 μm or less.

[0011] The magnetic particles may be composed of a soft magnetic material. The soft magnetic material may contain at least one metal selected from the group consisting of pure iron and iron-containing alloys. The iron-containing alloy may be at least one selected from the group consisting of, for example, Fe-Cr alloys (stainless steel), Fe-Ni-Cr alloys (stainless steel), Fe-Si alloys, Fe-Si-Al alloys (Sendust), Fe-Ni alloys (Permalloy), Fe-Cu-Ni alloys (Permalloy), Fe-Co alloys (Permendur), Fe-Cr-Si alloys (electromagnetic stainless steel), and Fe-Ni-Mn-C alloys (Invar). The soft magnetic material may be amorphous. The soft magnetic material may be an Fe amorphous alloy. The magnetic particles composed of the soft magnetic material may be composed of amorphous iron or carbonyl iron. When the magnetic particles are composed of a soft magnetic material, the particle size of the magnetic particles may be 60 μm or larger and 150 μm or smaller.

[0012] Magnetic particles may contain multiple types of metallic elements. In addition to the above elements, magnetic particles may further contain at least one element selected from the group consisting of base metal elements, noble metal elements, transition metal elements, and rare earth elements. Magnetic particles may further contain at least one element selected from the group consisting of copper (Cu), titanium (Ti), manganese (Mn), cobalt (Co), nickel (Ni), zinc (Zn), aluminum (Al), tin (Sn), chromium (Cr), barium (Ba), strontium (Sr), lead (Pb), silver (Ag), oxygen (O), beryllium (Be), phosphorus (P), boron (B), and silicon (Si).

[0013] The wet process used in step S1 may be any method using a polyamic acid solution. The polyamic acid solution contains polyamic acid and a solvent that dissolves the polyamic acid. Examples of solvents include N-methyl-2-pyrrolidone (NMP) and γ-butyrolactone.

[0014] In one embodiment, the wet method may be a method of coating magnetic particles with polyamic acid by dispersing magnetic particles in a polyamic acid solution and then volatilizing the solvent (hereinafter also referred to as the "solution coating method"). The solution coating method is advantageous in that it allows for easy coating of magnetic particles with polyamic acid when the boiling point of the solvent (boiling point at atmospheric pressure; the same applies hereinafter) is low, for example, when the boiling point is 210°C or lower.

[0015] In the solution coating method, the solvent can be evaporated, for example, by heating under reduced pressure. The degree of reduced pressure (vacuum) may be 0.01 to 100 Pa. The heating temperature and heating time should be set appropriately according to the type of solvent (boiling point) so that the solvent evaporates.

[0016] In another embodiment, the wet method may be a method of coating magnetic particles with polyamic acid by dispersing magnetic particles in a polyamic acid solution and then adding a poor solvent relative to the polyamic acid to precipitate the polyamic acid on the magnetic particles (hereinafter also referred to as the "reprecipitation method"). The reprecipitation method is advantageous when the boiling point of the solvent is high and it is difficult to volatilize the solvent, for example, when the boiling point is 190°C or higher.

[0017] In the reprecipitation method, an excess amount of poor solvent is added relative to the volume of the polyamic acid solution. The amount of poor solvent added may be 500 parts by mass or more per 100 parts by mass of the polyamic acid solution.

[0018] Examples of poor solvents include aliphatic alcohols and water. Examples of aliphatic alcohols include methanol and ethanol.

[0019] In the reprecipitation method, after adding a poor solvent, the solvent contained in the polyamic acid solution is washed away with the poor solvent, the magnetic particles coated with polyamic acid may be filtered, and the filtered magnetic particles coated with polyamic acid may be dried (to volatilize the poor solvent).

[0020] The polyamic acid solution used in the wet process (solution coating method and reprecipitation method) may be a commercially available solution (varnish), or a solution obtained by reacting tetracarboxylic dianhydride and diamine in a solvent. The commercially available polyamic acid solution may be a recycled product.

[0021] From the viewpoint of controlling the physical properties of the polyamic acid, the polyamic acid solution is preferably a solution obtained by reacting a tetracarboxylic dianhydride and a diamine in a solvent. That is, the wet method (solution coating method and reprecipitation method) may further include obtaining a polyamic acid solution by reacting a tetracarboxylic dianhydride and a diamine in a solvent. Examples of solvents are the same as the examples of solvents described as being included in the polyamic acid solution.

[0022] The tetracarbone dianhydride may be an aromatic tetracarbone dianhydride. Examples of aromatic tetracarbone dianhydrides include pyromellitic acid dianhydride, benzophenone tetracarboxylic acid dianhydride, and biphenyl tetracarboxylic acid dianhydride.

[0023] In one embodiment, the diamine includes an aromatic diamine. Examples of aromatic diamines include 4,4-diaminodiphenyl ether, p-phenyldiamine, and 2,2-bis(4-aminophenoxyphenyl)propane.

[0024] In one embodiment, the diamine includes a diamine having an amino group at the end or side chain of a silicone (polysiloxane chain). That is, the two amino groups in the diamine may be bonded to each of the Si atoms located at both ends of the silicone (polysiloxane chain), or they may be bonded to Si atoms other than those located at the ends.

[0025] The diamine preferably includes a diamine having amino groups at both ends of the silicone. In this case, the gas permeability of the polyamic acid (polyimide) increases, making it easier to discharge (degas) the water generated in the dehydration condensation reaction in step S3, where the polyamic acid is imidized to obtain polyimide, from the system.

[0026] The functional group equivalent (amino group equivalent) of a diamine having amino groups at both ends of the silicone may be 200 g / mol or more, 300 g / mol or more, or 400 g / mol or more, and may be 2000 g / mol or less, 1800 g / mol or less, or 1600 g / mol or less. Such diamines are available, for example, as "KF-8010", "X-22-161A", and "X-22-161B" of Shin-Etsu Silicone (registered trademark) manufactured by Shin-Etsu Industries Ltd.

[0027] In one embodiment, the diamine preferably includes a diamine having amino groups at both ends of the silicone and an aromatic diamine. In this case, the gas permeability of the polyamic acid (polyimide) is further increased, and in step S3, the water generated during the dehydration condensation reaction to imidize the polyamic acid to obtain polyimide is further discharged (degassed) from the system. In this case, the mass ratio of the diamine having amino groups at both ends of the silicone to the aromatic diamine may be 5 / 95 or more, 10 / 90 or more, or 15 / 85 or more, and may be 35 / 65 or less, 30 / 70 or less, or 25 / 75 or less.

[0028] The weight-average molecular weight of the resulting polyamic acid can be adjusted by controlling the molar ratio of tetracarboxylic dianhydride and diamine during their reaction. The weight-average molecular weight of the polyamic acid may be 3000 or more, 4000 or more, or 5000 or more, and may be 25000 or less, 22000 or less, or 20000 or less. The weight-average molecular weight of the polyamic acid refers to the weight-average molecular weight in terms of polystyrene, as measured by GPC.

[0029] From the viewpoint of easily obtaining polyamic acids having the weight-average molecular weight described above, the amount of diamine used in the reaction between tetracarboxylic dianhydride and diamine may be in excess of the amount of tetracarboxylic dianhydride. Specifically, per mole of tetracarboxylic dianhydride, the amount of diamine may be 1.01 moles or more, 1.05 moles or more, or 1.10 moles or more, and may be 1.30 moles or less, 1.20 moles or less, or 1.15 moles or less.

[0030] The content of magnetic particles in the compound obtained in step S1 may be 95 parts by mass or more, or 96 parts by mass or more, and 99.5 parts by mass or less, or 99 parts by mass or less, based on 100 parts by mass of the total mass of magnetic particles and polyamic acid. The content of polyamic acid 2 in the compound may be 0.5 parts by mass or more, or 1 part by mass or more, and 5 parts by mass or less, or 4 parts by mass or less, based on 100 parts by mass of the total mass of magnetic particles and polyamic acid.

[0031] In step S1, the magnetic particles may be coated with polyamic acid alone, or with a mixture of polyamic acid and other components. That is, the compound obtained in step S1 may consist only of magnetic particles and polyamic acid, or it may further contain other components. Examples of other components include polymers other than polyimide, curing agents, curing accelerators (curing catalysts), coupling agents (e.g., silane coupling agents), flame retardants, waxes (lubricants), and organic solvents.

[0032] The shape of the compound obtained in step S1 is not particularly limited. The compound may be, for example, a powder (compound powder).

[0033] In step S2, the compound obtained in step S1 is filled into a mold and compression molding is performed to obtain a molded body. The molding pressure may be 500 MPa or more or 700 MPa or more, and may be 2500 MPa or less or 2000 MPa or less. The compound may be molded so that the density of the molded body is 75% to 90% of the true density of the magnetic particles.

[0034] In step S3, the molded body obtained in step S2 is heated. The heating temperature may be 180°C or higher, 200°C or higher, or 250°C or higher, and may be 350°C or lower, 300°C or lower, or 280°C or lower. The heating time may be 0.5 hours or higher, 1.0 hour or higher, or 2.0 hours or higher, and may be 4.0 hours or lower, 3.0 hours or lower, or 2.5 hours or lower.

[0035] The heating described above causes a dehydration condensation reaction of the polyamic acid to proceed, resulting in the imidization of the polyamic acid to produce polyimide, which then binds the magnetic particles together.

[0036] The bonded magnet obtained by the above manufacturing method contains magnetic particles and polyimide that binds the magnetic particles together. Because this bonded magnet contains polyimide as a binder polymer, it has excellent heat resistance. The polyimide may be a dehydrated condensate of polyamic acid. The polyamic acid may be a reaction product of tetracarboxylic dianhydride and diamine. The details of the polyamic acid, tetracarboxylic dianhydride and diamine are the same as those described in the manufacturing method of the bonded magnet.

Claims

1. A method for manufacturing a bonded magnet, comprising:

1. A step of coating magnetic particles with polyamic acid by a wet method to obtain a compound; 2. A step of molding the compound to obtain a molded body; and 3. A step of heating the molded body to imide the polyamic acid to obtain a polyimide, and bonding the magnetic particles together with the polyimide.

2. The method for manufacturing a bonded magnet according to claim 1, wherein the wet method is a method of coating the magnetic particles with the polyamic acid by dispersing the magnetic particles in a polyamic acid solution containing the polyamic acid and a solvent, and then volatilizing the solvent.

3. The method for manufacturing a bonded magnet according to claim 1, wherein the wet method involves dispersing the magnetic particles in a polyamic acid solution containing the polyamic acid and a solvent, and then adding a poor solvent to the polyamic acid to precipitate the polyamic acid on the magnetic particles, thereby coating the magnetic particles with the polyamic acid.

4. The method for producing a bonded magnet according to claim 2 or 3, wherein the polyamic acid solution is obtained by reacting a tetracarboxylic dianhydride and a diamine in the solvent in the wet method.

5. The method for producing a bonded magnet according to claim 4, wherein the diamine includes a diamine having amino groups at both ends of the silicone.

6. The method for producing a bonded magnet according to claim 5, wherein the diamine further comprises an aromatic diamine.

7. The method for producing a bonded magnet according to claim 1, wherein the weight-average molecular weight of the polyamic acid is 5,000 or more and 20,000 or less.

8. A compound used in the manufacture of bonded magnets, comprising: magnetic particles; and a polyamic acid coating the magnetic particles.

9. The compound according to claim 8, wherein the polyamic acid is a reaction product of a tetracarboxylic dianhydride and a diamine.

10. The compound according to claim 9, wherein the diamine comprises a diamine having amino groups at both ends of the silicone.

11. The compound according to claim 10, wherein the diamine further comprises an aromatic diamine.

12. The compound according to claim 8, wherein the weight-average molecular weight of the polyamic acid is 5,000 or more and 20,000 or less.