Method for manufacturing an expandable neodymium-iron-boron sintered magnet

Abstract

The present invention discloses an expandable NdFeB sintered magnet, a manufacturing method thereof, and an application thereof, the magnet comprising a NdFeB sintered magnet and an expandable coating applied to the surface of the NdFeB sintered magnet. Instead of applying a potting resin adhesive to a conventional epoxy resin adhesive coated magnet, the present invention uses a NdFeB sintered magnet coated with an expandable coating, and the magnet coated with the expandable coating can be inserted into a magnetic steel groove and fixed in the magnetic steel groove by utilizing the irreversible expansion of the coating itself. At the same time, the use of the expandable coating of the present invention significantly shortens the assembly time of the motor and improves the assembly accuracy of the motor. In addition, the sintered magnet coated with the expandable coating of the present invention has excellent properties such as excellent resistance to high temperatures, anti-aging, corrosion resistance, and high bonding strength.

Description

Technical Field

[0001] This application claims the priority of a prior application filed with the China National Intellectual Property Administration on June 11, 2021, with the patent application number 202110657206.1 and the invention title "Expandable Neodymium Iron Boron Sintered Magnet, Its Manufacturing Method, and Application". The above prior application is incorporated herein by reference in its entirety.

[0002] The present invention relates to the technical field of surface protection of magnetic materials, specifically to an expandable neodymium iron boron sintered magnet and a manufacturing method Law thereof.

Background Art

[0003] Neodymium iron boron sintered magnets are called the modern "magnetic king" because they have excellent residual magnetism, coercive force, and magnetic energy product. Therefore, they are widely applied in industries such as energy, transportation, machinery, medical, IT, and household appliances. In particular, with the development and utilization of new energy, the demand and application of neodymium iron boron sintered magnets are expanding. The emergence of neodymium iron boron sintered magnets has greatly promoted the development of permanent magnet motors. Products that use conventional ferrite magnets or samarium cobalt magnets for motor stators or rotors are attempting to replace them with neodymium iron boron sintered permanent magnets to achieve weight reduction, improvement of power factor, and increase of output power of the motor. However, sintered neodymium iron boron consists of the main phase Nd2Fe 14 B, Nd-rich phase, and B-rich phase. Among them, the chemical activity of the metal Nd element is the strongest and it is very easily corroded in humid, high-temperature, and electrochemical environments, which significantly limits the further expansion of the application fields of neodymium iron boron sintered magnets.

[0004] Currently, in the electro-mechanical industry, electrophoretic epoxy resin coatings with a thickness of 15 - 30 μm are generally used for surface protection. Such coatings have a strong bonding force with the substrate and excellent advantages such as acid resistance and alkali resistance. However, in the motor assembly process, it is necessary to assemble using a potting resin adhesive, that is, insert the magnet into the magnetic steel groove, further inject the resin adhesive, and heat-cure it to fix the magnet in the magnetic steel groove. Therefore, in the motor assembly process, not only is it necessary to consider factors such as the fluidity, adhesiveness, thermal conductivity, oil resistance, and environmental protection of the resin adhesive, but at the same time, since the heat-curing of the resin adhesive takes a relatively long time, usually more than 24 h of heat-curing is required to meet the requirements of the adhesive force in motor assembly. Also, although the epoxy resin adhesive coating has excellent performance, the hardness of the coating is relatively small, the wear resistance is extremely poor, it is easily damaged during packaging and transportation and motor assembly, and since the epoxy resin adhesive coating is a cathodic protection coating, once the coating is damaged, the magnet will be severely corroded. Therefore, how to fix the magnet in the magnetic steel groove and increase the hardness and wear resistance of the coating are technical problems to be solved in this technical field.

Summary of the Invention

Means for Solving the Problems

[0005] To improve the above technical problems, the present invention provides an expandable neodymium iron boron sintered magnet including a neodymium iron boron sintered magnet and an expandable coating applied on the surface of the neodymium iron boron sintered magnet.

[0006] According to an embodiment of the present invention, the thickness of the expandable coating is 50 - 300 μm, preferably 80 - 150 μm, and illustratively 50 μm, 80 μm, 110 μm, 150 μm, 180 μm, 200 μm, 250 μm, 300 μm.

[0007] According to an embodiment of the present invention, the expandable coating is softened at 60 to 100 °C, and is exemplified by being softened at 60 °C, 70 °C, 80 °C, 90 °C, and 100 °C.

[0008] According to an embodiment of the present invention, in the absence of pressure, the expansion rate of the thickness of the expandable coating is 200 to 400%, preferably 300 to 400%, and is exemplified by 200%, 220%, 250%, 280%, 300%, 350%, and 400%.

[0009] The expandable coating of the present invention is heated at 165 to 210 °C, and the organic resin inside the expandable coating is crosslinked to achieve fixed molding.

[0010] According to an embodiment of the present invention, the expandable coating exhibits a honeycomb shape after expansion. Preferably, the expandable coating basically has the form shown in FIG. 1 after expansion.

[0011] In the present invention, the expansion rate = the thickness of the coating after expansion in the free state / the thickness of the expandable coating before expansion.

[0012] According to an embodiment of the present invention, the expandable coating contains at least a water-soluble resin and a foaming agent.

[0013] Preferably, the water-soluble resin is at least one selected from water-soluble acrylic resins, water-based epoxy resins, and water-based urethane resins. Preferably, it is selected from water-soluble acrylic resins.

[0014] According to an embodiment of the present invention, in the expandable coating, the solid content of the water-soluble resin is 30 to 50%, and is exemplified by 30%, 35%, 40%, and 50%.

[0015] Preferably, the blowing agent is thermoplastic expandable microspheres. Preferably, the diameter of the thermoplastic expandable microspheres is 5 to 30 μm, preferably 5 to 20 μm, and illustratively 5 μm, 8 μm, 10 μm, 12 μm, 15 μm, 18 μm, 20 μm, 25 μm, 30 μm.

[0016] According to an embodiment of the present invention, after the expandable coating expands, the area of the expanded microspheres occupies 60 to 90% of the cross-sectional area of the expanded coating, and illustratively 60%, 70%, 80%, 90%.

[0017] According to an embodiment of the present invention, at 110 to 210 °C (illustratively 110 °C, 120 °C, 130 °C, 140 °C, 150 °C, 160 °C), the expandable microspheres inside the expandable coating expand step by step.

[0018] Preferably, the average diameter of the thermoplastic expandable microspheres is 10 to 15 μm, and illustratively 10 μm, 12 μm, 15 μm.

[0019] Preferably, the expansion temperature of the thermoplastic expandable microspheres is 110 to 210 °C, and illustratively 110 °C, 120 °C, 150 °C, 160 °C, 170 °C, 180 °C, 200 °C.

[0020] Preferably, the maximum heat-resistant temperature of the thermoplastic expandable microspheres is 145 to 235 °C, and illustratively 145 °C, 160 °C, 180 °C, 200 °C, 215 °C, 235 °C.

[0021] According to an embodiment of the present invention, the expandable coating is manufactured by applying an expandable paint containing at least a water-soluble resin and a blowing agent.

[0022] According to an embodiment of the present invention, the weight percentage of the water-soluble resin in the expandable paint is 45 to 65%, for example 50 to 60%.

[0023] According to an embodiment of the present invention, the weight percentage of the blowing agent in the above-expandable paint is 10 to 30%, for example, 15 to 25%.

[0024] According to an embodiment of the present invention, the above-expandable paint optionally further contains Hectorite . For example, the weight percentage of the above Hectorite is 0.1 to 0.5%, and for another example, 0.2 to 0.4%.

[0025] According to an embodiment of the present invention, the above-expandable paint optionally further contains dipropylene glycol butyl ether. For example, the weight percentage of the above dipropylene glycol butyl ether is 0.5 to 3%, and for another example, 0.8 to 2.5%.

[0026] According to an embodiment of the present invention, the above-expandable paint optionally further contains propylene glycol. For example, the weight percentage of the above propylene glycol is 1 to 3%, and for another example, 1.5 to 2.5%.

[0027] According to an embodiment of the present invention, the above-expandable paint optionally further contains an acrylic thickener. For example, the weight percentage of the above acrylic thickener is 0.2 to 0.8%, and for another example, 0.3 to 0.6%.

[0028] According to an embodiment of the present invention, the above-expandable paint optionally further contains a dispersant. For example, the weight percentage of the above dispersant is 0.1 to 0.5%, and for another example, 0.2 to 0.4%. Preferably, the above dispersant is ethylene glycol, sodium oleate, carboxylate, etc.

[0029] According to an embodiment of the present invention, the above-expandable coating optionally further contains a leveling agent. For example, the weight percentage of the above leveling agent is 0.1 to 0.5%, and for another example, 0.2 to 0.4%.

[0030] Preferably, the leveling agent is silicone oil, organic siloxane, or the like. According to an embodiment of the present invention, the expandable paint further contains water. According to an embodiment of the present invention, the sum of the weight percentages of each component in the expandable paint is 100%.

[0031] According to an embodiment of the present invention, the method for manufacturing the expandable paint includes mixing the above components to obtain the expandable paint.

[0032] Preferably, the method for manufacturing the expandable paint further includes dispersing each component after mixing in water to obtain a dispersion. Further, the method further includes stirring the dispersion to mix it uniformly. In the present invention, expandable microspheres are sorted by a sorting device to select expandable microspheres with a particle size range of 5 to 30 μm, and then mixed with a water-soluble resin and stirred at room temperature for 30 to 60 minutes.

[0033] According to an embodiment of the present invention, the expandable coating is prepared by applying an expandable paint having a composition of weight percentages such as 45 to 65% of a water-soluble resin, 10 to 30% of a foaming agent, Hectorite 0.1 to 0.5%, 0.5 to 3% of dipropylene glycol butyl ether, 1 to 3% of propylene glycol, 0.2 to 0.8% of an acrylic thickener, 0.1 to 0.5% of a dispersant, and 0.1 to 0.5% of a leveling agent.

[0034] The inventors of the present invention have surprisingly found that in the thermal expansion process, it is necessary to control the expansion rate within the range of 300 to 400%. When the expansion rate is less than 200%, the gap between the magnet and the magnetic steel groove cannot be densely filled. On the other hand, when the expansion rate is greater than 400%, the cross-linking inside the coating is destroyed and the pressing force becomes insufficient. Moreover, the expansion rate is related to conditions such as the content of microspheres, particle size, expansion temperature, expansion time, and coating thickness in the coating. The present invention optimizes the usage amount of raw materials for each composition and the expansion conditions in the expandable coating, thereby controlling the expansion ratio within an appropriate range and manufacturing a neodymium iron boron-based sintered magnet having a better adhesive pressing force.

[0035] According to an embodiment of the present invention, the neodymium iron boron-based sintered magnet consists of a main phase Nd2Fe 14 B, an Nd-rich phase, and a B-rich phase.

[0036] The present invention further provides a manufacturing method of the expandable sintered magnet, which includes applying an expandable paint of each composition on the surface of the neodymium iron boron-based sintered magnet and manufacturing the expandable sintered magnet through a pre-curing treatment.

[0037] According to an embodiment of the present invention, before applying the expandable paint on the neodymium iron boron-based sintered magnet, it further includes a surface pretreatment step.

[0038] According to an embodiment of the present invention, the surface pretreatment includes a process of chemical, ultrasonic degreasing, pickling, and water washing on the surface of the neodymium iron boron-based sintered magnet. Preferably, the degreasing solution used in the degreasing process is a composite solution of a base and a surfactant.

[0039] Preferably, the base is sodium hydroxide or sodium carbonate, and its concentration is 10 to 20 g / L, illustratively 10 g / L, 15 g / L, 20 g / L.

[0040] Preferably, the surfactant is sodium dodecyl sulfonate or sodium dodecyl sulfate, and its concentration is 2 to 6 g / L, illustratively 2 g / L, 3 g / L, 4 g / L, 5 g / L, 6 g / L.

[0041] Preferably, the temperature of the above oil removal liquid is 30 to 70 °C, illustratively 30 °C, 40 °C, 50 °C, 60 °C, 70 °C. Further, the time of the above oil removal is 1 to 20 min, illustratively 1 min, 5 min, 10 min, 15 min, 20 min.

[0042] Preferably, the acid used for the above pickling may be nitric acid or an aqueous citric acid solution.

[0043] Preferably, the concentration of the acid solution used for the above pickling is 5 to 30 wt%, illustratively 5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%, 30 wt%. Further, the time of the above pickling is 5 to 30 s, illustratively 5 s, 10 s, 15 s, 20 s, 25 s, 30 s.

[0044] Preferably, the above water washing includes ultrasonic water washing using pure water. Further, the time of the above water washing is 20 to 120 s, illustratively 20 s, 30 s, 50 s, 80 s, 100 s, 120 s.

[0045] According to an embodiment of the present invention, the above coating method includes, but is not limited to, spraying, printing, dipping, smearing, etc. Preferably, it is spraying.

[0046] According to an embodiment of the present invention, the thickness of the above coating is 50 to 300 μm, the preferred thickness is 80 to 150 μm, illustratively 50 μm, 80 μm, 110 μm, 150 μm, 180 μm, 200 μm, 250 μm, 300 μm.

[0047] The inventors of the present invention surprisingly found that if the coating thickness of the expandable coating is too thin, the expansion force is insufficient in the assembly process, and thus a predetermined pressing force cannot be achieved. However, if the coating thickness of the expandable coating is too thick, peeling and cracking are likely to occur on the surface of the coating. Moreover, the coating thickness of the expandable coating is related to the preliminary dimensions of the magnetic steel groove, and the larger the preliminary dimensions, the thicker the required coating thickness. The inventors surprisingly found that after the coating expands, the expansion compression rate of the coating in the magnetic steel groove is most effective between 35% and 65%. At this time, the adhesive force between the magnet and the magnetic steel groove is the highest, and the pressing force of the magnet in the magnetic steel groove is the largest.

[0048] Expansion compression rate m = (H1 - H0) / (H2 - H0) Among them, H0 is the coating thickness of the expandable coating, H1 is the thickness of the expandable coating after expansion in the magnetic steel groove, and H2 is the thickness of the expandable coating after expansion in the natural state.

[0049] According to an embodiment of the present invention, the above pre-curing treatment may be to naturally dry the coating under normal temperature conditions (15 - 35°C), or may be to dry it under baking conditions at medium and low temperatures (35 - 90°C) to form a complete coating.

[0050] The present invention further provides the application of the above expandable neodymium iron boron-based sintered magnet in a motor rotor.

[0051] The present invention further provides a motor rotor workpiece including the above expandable neodymium iron boron-based sintered magnet.

[0052] The present invention further provides an assembly method of the above motor rotor workpiece, including assembling the above expandable neodymium iron boron-based sintered magnet into a magnetic steel groove and manufacturing the above motor rotor workpiece through a heat expansion treatment.

[0053] The inventors of the present invention surprisingly found that the final use performance of the expansion coating is related to the expansion temperature, heating rate, and heat preservation time in the expansion assembly process, and the expansion temperature directly affects the final structure of the expansion coating.

[0054] According to an embodiment of the present invention, the above heat expansion treatment adopts a two-stage heating method. The expansion temperature in the first expansion stage is 110-160°C, illustratively 110°C, 120°C, 130°C, 140°C, 150°C, 160°C. Further, the heating rate in the first expansion stage is 5-15°C / min, illustratively 5°C / min, 10°C / min, 15°C / min.

[0055] When the expansion temperature in the first expansion stage is lower than 110°C, a large number of microspheres cannot expand and the required expansion ratio cannot be achieved. However, when the expansion temperature in the first expansion stage is higher than 160°C, the microspheres expand rapidly. However, since the particle sizes of the microspheres in the coating are different, the microspheres with smaller particle sizes expand rapidly and rupture, and the coating collapses prematurely, so the corresponding supporting force cannot be obtained. And when the heating rate in the first expansion stage is controlled at 5-15°C / min, the expansion coating can be expanded uniformly, and the expanded microspheres can vaporize the liquefied hydrocarbon in the body sufficiently during the heating process, and the coating can be expanded gently and uniformly.

[0056] According to an embodiment of the present invention, the expansion temperature in the second expansion stage of the above heat expansion treatment is 180-210°C, illustratively 180°C, 190°C, 200°C, 210°C. Further, the heating rate in the second expansion stage is 30-60°C / min, illustratively 30°C / min, 40°C / min, 50°C / min, 60°C / min. After the coating expands uniformly, the heating is accelerated to rapidly cure the thermoplastic resin in the coating.

[0057] The thermoplastic expandable microspheres of the present invention are composed of a thermoplastic resin outer shell and a propellant encapsulated therein, and more than 70% of the propellant consists of isooctane. In addition to isooctane, the propellant may be selected from other appropriate liquids within a boiling point range such as butane, pentane, and heptane. In the heating expansion process, after the liquid propellant in the expandable microspheres reaches its boiling point, the propellant evaporates to increase the internal pressure of the microspheres, causing a volume change of the propellant and deforming and expanding the outer shell softened by heat. At the same time, the outer shell is softened to limit the expansion of the microspheres. Let the temperature at the start of expansion be Tstar and the temperature when the maximum degree of expansion is reached be T max When the temperature exceeds T max , the propellant is released through the thermoplastic resin outer shell to such an extent that the microspheres begin to rupture.

[0058] In the first expansion stage, the high-boiling solvent in the coating volatilizes, dissolving the outer shell of the expandable microspheres with a particle size <10 μm and a wall thickness <3 μm, thinning the outer shell wall, and causing rupture when the maximum expansion temperature has not been reached. More than 60% of the microspheres in the expansion coating rupture. After rupture, the propellant in the microspheres is released, forming a void structure without accelerating the expansion of the microspheres. The thermoplastic resin on the outer wall of the microspheres crosslinks with the water-soluble resin in the coating to form a network structure, increasing the adhesive force in the magnetic steel groove of the magnet.

[0059] In the second expansion stage, the expandable microspheres with a particle size >10 μm in the coating continue to expand. In this case, the high-boiling solvent in the coating has already volatilized and does not accelerate the rupture of the outer shell of the expandable microspheres. In this case, the maximum expansion temperature in the second expansion stage is lower than the maximum expansion temperature of the microspheres with a particle size >10 μm, and the release rate of the propellant becomes slower.

[0060] Since the expandable microspheres of the present invention contain more than 70% isooctane as the release agent, by measuring the release amount of isooctane during the expansion process, the expansion rate of the coating can be controlled, more than 60% of the microspheres in the coating can be ruptured, and further crosslinked with the water-soluble resin to form a network structure.

[0061] The present invention can control the rupture ratio of expanded microspheres by detecting the collected gas and determining the gas emission amount of isooctane during the expansion process. The detection method is as follows: Take a magnet (specification: 35.5 mm × 16.5 mm × 5.5 mm) coated with an expansion coating, place it in a sealed sample bottle, heat it at the same heating temperature and heating rate as the expansion process conditions during the expansion assembly process, after the heating is completed, let it cool naturally to 60°C, purge the gas in the sample bottle into the gas chromatographic column using nitrogen gas for detection, record the isooctane content w1 at that time, Continuously raise the temperature of the magnet in the sample bottle to a temperature higher than the maximum expansion temperature T of the expanded microspheres max Heat it at a higher temperature (240°C) for 3 h to completely rupture the microspheres in the expansion coating and completely release the isooctane in the microspheres, and record the isooctane content w2 at that time.

[0062] Calculate the rupture ratio q = w1 / (w1 + w2) of the microspheres from the emission amount of isooctane.

[0063] As a result of a large number of experiments, the inventors surprisingly found that when the rupture ratio q of the microspheres is within the range of 60% to 85%, all the expansion coatings show excellent supporting effects, and in the use environment, all the expansion magnets have relatively high pressing forces and the highest coating adhesion.

[0064] The expansion coating first undergoes a softening process. The expanded microspheres in the coating increase in volume due to heat, the thickness of the entire expansion coating increases, generating a pressing force on the inner wall of the motor assembly. At the same time, the thermoplastic resin in the expansion coating generates a certain adhesive force after softening, fixing the sintered magnet to the motor assembly to complete the assembly process.

Effect of the Invention

[0065] (1) Instead of applying a potting resin adhesive to a conventional epoxy resin-bonded coated magnet, the present invention uses a neodymium iron boron sintered magnet coated with an expandable coating. By inserting the magnet coated with the expandable coating into a magnetic steel groove and utilizing the irreversible expansion of the coating itself, the magnet can be fixed to the magnetic steel groove. At the same time, by using the expandable coating of the present invention, the assembly time of the motor can be significantly shortened, and the assembly accuracy of the motor can be improved. Moreover, the sintered magnet coated with the expandable coating of the present invention has excellent properties such as excellent high-temperature resistance, anti-aging, corrosion resistance, and high bonding strength.

[0066] (2) Instead of using a conventional potting resin, the present invention utilizes a specially formulated environmentally friendly water-based expandable paint, which can reduce VOC emissions and environmental problems caused by solvent-based binders.

[0067] (3) By controlling the content of expandable microspheres in the expandable coating to 10-30%, after expansion by heat, the water-soluble resin cross-links with the expandable microspheres to form a honeycomb-like porous structure, among which the porosity reaches 60-90%, and the adhesive pressing force in the assembled product at room temperature is greater than 1000 N / cm 2 greater.

[0068] (4) The expandable coating manufactured by the present invention reaches a hardness of Mitsubishi pencil H or higher under the condition of room temperature (25 °C), has high wear resistance, and the coating surface is not easily damaged during the transportation process. Moreover, even if the expandable coating of the present invention is immersed in 10% hydrochloric acid and 10% sodium hydroxide solution for more than 24 h, the coating has no defects such as softening, dissolution, and peeling, indicating relatively high acid and base resistance. At the same time, even if the coating is immersed in engine oil at 170 °C for more than 500 h, the coating still has no problems such as softening, swelling, and peeling, indicating relatively high heat and oil resistance.

Brief Description of the Drawings

[0069]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0070] Hereinafter, in accordance with specific examples, the technical solution of the present invention will be described in more detail. It should be understood that the following examples are merely illustrative and explanatory of the present invention and should not be construed as limiting the scope of the claims of the present invention. Any technology realized based on the above content of the present invention is included within the scope of the claims of the present invention.

[0071] Unless otherwise specified, all raw materials and reagents used in the following examples are commercially available products or can be manufactured by known methods. 〔Example 1〕 In this example, a neodymium iron boron sintered magnet (not magnetized) with dimensions of 35.5 mm × 16.5 mm × 5.5 mm was used, and the dimensions of the magnetic steel groove equipped with the assembled motor rotor were 36 mm × 17 mm × 6 mm.

[0072] Surface pretreatment of the magnet: Using a composite oil removal solution with a sodium hydroxide concentration of 15 g / L and a sodium dodecyl sulfonate concentration of 3 g / L, perform oil removal on the neodymium iron boron sintered magnet at 60 °C for 2 minutes, then pickle with a 25 wt% aqueous citric acid solution for 15 seconds, and finally wash the neodymium iron boron sintered magnet with deionized water by ultrasonic wave for more than 2 minutes.

[0073] Manufacture of expandable paint: 55% water-soluble acrylic resin, 30% water, 10% foaming agent, Hectorite 0.2%, 1.5% dipropylene glycol butyl ether, 2% propylene glycol, 0.5% acrylic thickener, 0.4% ethylene glycol, 0.4% polydimethylsiloxane (calculated by weight percentage), and an expandable paint was manufactured with such a composition. As the foaming agent, thermoplastic expandable microspheres with an average diameter of 13 μm were selected.

[0074] Coating treatment: The above expandable paint was applied to the surface of the magnet by compressed air spraying at a spraying speed of 120 mm / s, a coating thickness of 110 μm, a distance between the nozzle and the workpiece of 15 cm, an angle between the spray gun and the workpiece of 25°, and an argon gas pressure of the spray gun of 0.6 MPa.

[0075] Pre-curing treatment: The surface of the magnet coated with the expandable paint was heated to 50 °C for pre-curing treatment to obtain a coating with a thickness of 110 μm.

[0076] Coating expansion: In the absence of pressure, the magnet coated with the expansion coating was placed in a high-temperature drying box, and the magnet was heated to 120°C in two stages, first at 120°C and kept at 4°C for 5 min, and then rapidly heated to 170°C and kept at 4°C for 3 min. The heating rate of the first stage was 5°C / min, and the heating rate of the second stage was 50°C / min. With the increase in temperature, the gas pressure inside the shell of the expansion microsphere increased, the thermoplastic shell softened, and the expansion volume of the microsphere increased significantly. At this time, the resin inside the expansion coating softened, and the thickness of the expansion coating increased with the increase in the volume of the expansion microsphere. As shown in Figure 1, the microspheres expanded uniformly, and the microsphere walls and the resin cross-linked with each other to form a stable support structure. The expansion coating was composed of water-soluble acrylic resin and expansion microspheres. The thickness of the expansion coating expanded from 110 μm after the pre-curing treatment to 394 μm, and the expansion rate reached 358%. After expansion, the expanded coating had a honeycomb shape, and the cross-sectional area of ​​the expanded microspheres accounted for 82% of the cross-sectional area of ​​the expanded coating. (In the present invention, the ratio of the cross-sectional area of ​​the expanded microspheres to the cross-sectional area of ​​the expanded coating in one cross section was calculated by first taking a scanning electron microscope photograph of the cross section of the expanded coating after expansion, then recognizing the gaps in the cross section from the image, calculating the sum of the gap areas, and using the gap area as the ratio of the cross-sectional area of ​​the expanded microspheres to the cross-sectional area of ​​the expanded coating.)

[0077] The magnet coated with the expansion coating was assembled into the magnetic steel groove of the motor rotor, placed in a high-temperature drying oven, and two-stage heating was used. First, it was heated to 120 °C and kept warm for 5 min. Then, it was rapidly heated to 170 °C and kept warm for 3 min. Among them, the heating rate of the first-stage heating was 5 °C / min, and the heating rate of the second-stage heating was 50 °C / min. (The magnet was assembled into the magnetic steel groove, and the expansion coating expanded due to heat, generating pressure on the inner wall of the magnetic steel groove, filling the gap between the magnetic steel groove and the magnet. At the same time, the resin and the expanded microspheres inside the coating cross-linked to form a honeycomb-like coating structure. Due to being restricted by the inner wall of the magnetic steel groove, the coating could not expand to the maximum extent during the expansion process, so that the honeycomb-like structure was compressed and wrinkled after expansion.) After the heating was completed, the motor rotor was cooled under natural conditions. Due to the increase in the volume of the expanded microspheres caused by the change in the ambient temperature and the irreversible change, and the increase in the coating volume, the gap between the magnet and the inner wall of the magnetic steel groove was filled, and the magnet was closely fixed inside the magnetic steel. The magnet product obtained at that time was named Sample 1.

[0078] In this state, the adhesive pressing force of the magnet at room temperature and the adhesive pressing force at high temperature were measured. The adhesive pressing force at room temperature (25 °C) was 1200 N / cm 2 and the adhesive pressing force at high temperature (170 °C) was 530 N / cm 2 . 〔Examples 2 - 5〕 The surface pretreatment method, the expandable paint, and the coating treatment process were the same as those in Example 1. The magnet with an expansion coating thickness of 110 μm was inserted into the magnetic steel groove, and different first expansion temperatures, expansion temperatures in the second expansion stage, heating rates, and expansion times were used to obtain the optimal assembly process conditions, and the results are shown in Table 1 below.

[0079]

Table 1

[0080] As can be seen from the results in Table 1, the pressing force at room temperature and the pressing force at high temperature are correlated with the expansion temperature and the heating rate, and the optimal assembly process conditions can be obtained through optimization.

[0081] Meanwhile, by measuring the release amount of isooctane to estimate the rupture rate of the microspheres, it was found that the rupture rate of the microspheres was between 60% and 85%, and the ruptured microspheres crosslinked with the resin coating, and the obtained coating had a stable structure and could stably support the gap between the magnet and the magnetic steel groove.

[0082] Figure 1 shows the coating state after expansion of Sample 1. As can be seen from the figure, the microspheres expanded uniformly, and the microsphere wall and the resin crosslinked with each other to form a stable support structure.

[0083] Figure 2 shows the coating state after expansion of Sample 5. As can be seen from the figure, there were many ruptures of the microspheres and large gaps due to fusion between the microspheres, so the adhesive force of the coating at high temperature was relatively low. [Example 6] In this example, a neodymium iron boron sintered magnet (not magnetized) with specifications of 35.5 mm × 16.5 mm × 5.5 mm was used, and the dimensions of the magnetic steel groove equipped with the assembled motor rotor were 36 mm × 17 mm × 6 mm.

[0084] Using the same surface pretreatment as in Example 1, a composite oil removal solution with a sodium hydroxide concentration of 15 g / L and a sodium dodecyl sulfonate concentration of 3 g / L was used to perform oil removal on the neodymium iron boron sintered magnet at 60°C for 2 minutes, then pickling was performed with a 25 wt% aqueous citric acid solution for 15 s, and finally the neodymium iron boron sintered magnet was washed with deionized water by ultrasonic wave for more than 2 minutes.

[0085] An expandable powder coating manufactured by AKZO-Nobel, consisting of 50% epoxy resin powder, 20% hardener, 10% elastomer resin, and 20% thermoplastic expandable microspheres, was selected. The above expandable coating was applied to the surface of the magnet by compressed air spraying at a spraying speed of 60 mm / s, a coating thickness of 110 μm, a distance between the nozzle and the workpiece of 10 cm, an angle between the spray gun and the workpiece of 25°, and an argon gas pressure of the spray gun of 0.6 MPa.

[0086] The magnet with the expandable coating was assembled into the magnetic steel groove of the motor rotor and placed in a high-temperature drying oven, and heated for 20 min by applying a temperature of 190°C. Since the volume of the thermoplastic expandable microspheres increases due to temperature change, the entire coating expands. At the same time, the epoxy resin cures and the coating does not shrink stably. The cross-sectional structure of the expandable coating was observed with an electron microscope. As shown in Figure 3, the thickness of the expandable coating expanded from 110 μm to 180 μm, and the expansion rate reached 163%. Moreover, it was clearly visible after expansion. The edges of the thermoplastic expandable microspheres were clearly visible. When the expandable microspheres were heated, the volume of the microspheres increased only due to the vaporization of the liquid alkane inside the microspheres. By detecting the release amount of isooctane in the expandable microspheres, the presence or absence of rupture of the microspheres in the coating was inferred. As can be seen from the detection, when heated at 190°C for 20 min, the release amount w1 of isooctane was 1.52 mg / L. When the temperature was raised to 240°C and held for 3 h to completely rupture the expandable microspheres in the coating, the release amount w2 of isooctane was 5.34 mg / L. By calculation of the formula, in the actual assembly process, the rupture rate q = w1 / (w1 + w2) = 1.52 / (1.52 + 5.34) = 22% of the expandable microspheres could be estimated.

[0087] As can be seen from Figure 3, the cross-sectional area of the expandable microspheres accounted for 35% of the expandable coating. In this state, the adhesion pressing force of the magnet at room temperature and at high temperature was detected. The adhesion pressing force at room temperature (25°C) was 920 N / cm 2, the adhesion pressing force at high temperature (170 °C) was 310 N / cm 2 It was. 〔Examples 7 - 14〕 In this example, a neodymium iron boron sintered magnet (not magnetized) with a standard size of 35.5 mm × 16.5 mm × 5.5 mm was used, and the dimensions of the magnetic steel groove equipped with the assembled motor rotor were 36 mm × 17 mm × 6 mm.

[0088] Using the same surface pretreatment process and expandable paint as in Example 1, the expandable paint was applied to the surface of the magnet by compressed air spraying so that the spray thicknesses were 80 μm, 90 μm, 100 μm, 110 μm, 120 μm, 130 μm, 140 μm, and 150 μm respectively.

[0089] The magnet coated with the expansion coating was assembled into the magnetic steel groove of the motor rotor, placed in a high - temperature drying oven, and two - stage heating was used. First, it was heated to 120 °C and kept warm for 5 min, then rapidly heated to 170 °C and kept warm for 3 min. Among them, the heating rate of the first - stage heating was 5 °C / min, and the heating rate of the second - stage heating was 50 °C / min. The pressing force of the magnet at normal temperature and high temperature in the operating state was measured, and the results are shown in Table 2 below.

[0090] Expansion ratio = H2 / H0 Compression ratio = (H1 - H0) / (H2 - H0) Among them, H0 is the coating thickness, H1 is the thickness of the expandable coating after expansion in the magnetic steel groove, and H2 is the thickness of the expandable coating after expansion in the natural state.

[0091]

Table 2

[0092] Figure 4 shows the coating state of Sample 10 in the magnetic steel groove. As can be seen from Figure 4, since the coating is restricted by the inner wall of the magnetic steel groove during the expansion process, the expansion coating is compressed and becomes wrinkled.

[0093] When the expansion coating is strongly compressed, relatively large stresses are generated at the contact surface of the expansion coating and the inner wall of the magnetic steel groove, the stress per unit area of the coating increases, the internal defects of the coating increase exponentially, and the cohesive strength of the adhesive layer decreases. Therefore, by controlling the compression ratio to 35% or more, magnets with relatively good pressing forces at normal temperature and high temperature can be manufactured.

[0094] The larger the compression ratio, the more economically advantageous it is. As a result of a large number of experiments, the inventors surprisingly found that when the compression ratio is greater than 65%, the coating becomes sensitive to shrinkage stress and thermal stress caused by temperature changes, thereby causing a loss of the cohesive strength of the magnet, and thus reducing the pressing force of the magnet at normal temperature and high temperature.

[0095] The embodiments of the present invention have been described above. However, the present invention is not limited to the above embodiments. Modifications, equivalent replacements, improvements, etc. made without departing from the spirit and principles of the present invention are all included within the scope of the claims of the present invention.

Claims

1. A method for manufacturing an expandable neodymium iron boron sintered magnet, wherein the manufacturing method includes applying an expandable paint on the surface of the neodymium iron boron sintered magnet and manufacturing the expandable neodymium iron boron sintered magnet through a preliminary curing process, the expandable paint includes, by weight percentage, 45-65% of a water-soluble resin, 10-30% of a foaming agent, 0.1-0.5% of hectorite, 0.5-3% of dipropylene glycol butyl ether, 1-3% of propylene glycol, 0.2-0.8% of an acrylic thickener, 0.1-0.5% of a dispersant, and 0.1-0.5% of a leveling agent, and the solid content of the water-soluble resin is 30-50%, the foaming agent is a thermoplastic expandable microsphere, the thermoplastic expandable microsphere consists of a thermoplastic resin outer shell and a propellant encapsulated therein, more than 70% of the propellant is isooctane, butane, pentane, or heptane, and the diameter of the thermoplastic expandable microsphere is 5-30 μm, the expandable neodymium iron boron sintered magnet includes a neodymium iron boron sintered magnet and an expandable coating applied on the surface of the neodymium iron boron sintered magnet, and the expandable coating is manufactured by applying the expandable paint, the thickness of the expandable coating is 50-300 μm, when the heating expansion treatment is performed at a temperature of 110-210 °C without pressure, the expansion rate of the thickness of the expandable coating is 200-400%, the heating expansion treatment adopts a two-stage temperature rising method, the expansion temperature in the first expansion stage is 110-160 °C, the temperature rising rate in the first expansion stage is 5-15 °C / min, the expansion temperature in the second expansion stage of the heating expansion treatment is 180-210 °C, and the temperature rising rate in the second expansion stage is 30-60 °C / min, after the expandable coating expands, the area of the expandable microspheres occupies 60-90% of the cross-sectional area of the expanded coating, characterized in that it is a method for manufacturing an expandable neodymium iron boron sintered magnet.

2. The water-soluble resin is at least one selected from water-soluble acrylic resin, water-based epoxy resin, and water-based urethane resin, characterized in that it is the manufacturing method according to Claim 1.

3. The average diameter of the thermoplastic expandable microspheres is 10-15 μm, characterized in that it is the manufacturing method according to Claim 1.

4. The weight percentage of the water-soluble resin in the expandable paint is 50 to 60%, and the weight percentage of the foaming agent in the expandable paint is 15 to 25%. The manufacturing method according to claim 1, characterized in that.

5. The dispersant is ethylene glycol, sodium oleate, or carboxylate, and the leveling agent is silicone oil or organosiloxane. The manufacturing method according to claim 1, characterized in that.

6. The neodymium iron boron-based sintered magnet consists of a main phase Nd 2 Fe 14 B, an Nd-rich phase, and a B-rich phase, The manufacturing method according to claim 1, characterized in that.

7. Before applying the expandable paint to the neodymium iron boron sintered magnet, it further includes a surface pretreatment step, and the surface pretreatment includes a process of chemical, ultrasonic oil removal, pickling, and water washing on the surface of the neodymium iron boron sintered magnet. The manufacturing method according to claim 1, characterized in that.

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