1. Bonded magnet is a kind of composite permanent magnet material, which is made by mixing the powder of permanent magnet material with binder and other additives in a certain proportion, and then by compression, extrusion or injection moulding. There are two main types of binder materials used: polymers (such as rubber, thermoplastic or thermosetting resins) and low melting point metals (such as Zn, Sn, Cu, Al and their alloys). Resin bonded magnets are widely used in electrical appliances, instrumentation, micro-motors and other fields, although their magnetic properties are lower than sintered magnets, but they have the advantages of high dimensional accuracy, easy secondary processing, good toughness, easy mass production, low price and light weight. When preparing bonded magnets, the combination of magnetic powder and resin binder is not ideal. In order to improve the affinity between fillers and resins, a more effective method is to use a coupling agent with amphoteric structure, which uses different groups in its molecule to bind closely with magnetic powder and resin respectively, so as to improve the performance of magnetic powder/resin composite system. In this paper, two coupling agents, titanate and silane, were used to treat the magnetic powder/epoxy resin composite system. The effects of the type, dosage and treatment process of coupling agents on the magnetic properties and mechanical properties of bonded magnets were studied.

2 The bonded magnet samples were prepared by using titanate (monoalkoxy titanate coupling agent) and silane (KH-560) as coupling agents according to the process shown in Fig. 1. The magnetic powder is BMXF-2 type magnetic powder produced by Beijing General Institute of Mining and Metallurgical Research. The use of E-44 epoxy resin as binder is 3% (mass fraction) and the pressure is 800MPa. The magnetic properties were measured by CL6-1 magnetometer and the compressive strength of the magnet was tested on the universal material testing machine.

3. Experimental results and discussion

3.1 In order to improve the coupling effect of coupling agent, appropriate solvent should be used for wet mixing. In this experiment, 1% (mass fraction) titanate coupling agent was used to couple magnetic powder, 3% E-20 (mass fraction) epoxy resin was used as binder, forming under 800 MPa pressure, and then its magnetic properties were measured. Figure 2 shows the effect of different solvents on the maximum magnetic energy product of magnets. From Figure 2, it can be seen that the bonded magnet prepared by dry mixing method has the lowest maximum magnetic energy product, and the effect of wet mixing with organic solvents increases in turn according to acetone, ethanol and xylene. Titanate coupling agent is waxy powder at room temperature. If the magnetic powder is treated by dry mixing method, the coupling agent is not easy to disperse due to the small amount of coupling agent used, and can not be evenly wrapped on the surface of magnetic powder, but tends to concentrate in local clusters, which can not achieve the desired effect of coupling agent. Different magnetic properties are obtained by using different solvents, which is related to the solubility of coupling agents in solvents. The solubility of titanate in three solvents was xylene > ethanol > acetone in turn. The bigger the solubility is, the better the dispersion of coupling agent is. When the surface of magnetic powder is treated, the coupling agent molecule can be fully contacted with magnetic powder, which is beneficial to the surface modification of magnetic powder, promotes the combination of magnetic powder and binder, improves the coating effect of binder on the surface of magnetic powder, and consequently improves the performance of magnet.

3.2 The effect of the type of coupling agent on the magnetic properties of magnets is that appropriate amount of magnetic powder is treated with silane and titanate of 1%, 2%, 3% (mass fraction) respectively (xylene is used as coupling agent solvent). After drying, 3% (mass fraction) of epoxy resin E-44 and polyamide are added to mix well and dried and granulated in drying oven. The treated magnetic powders were pressed into the mould at 800 MPa pressure and then cured in the drying box at 100 C. Its density, magnetic properties and compressive strength were measured. The test results are shown in Table 1, where 1 #, 2 #, 3 # use 1%, 2%, 3% silane coupling agent, 4 #, 5 #, 6 # use 1%, 2%, 3% titanate coupling agent respectively. When the amount of silane and titanate is the same, the magnet density using titanate as coupling agent is higher, and the remanence, coercivity, intrinsic coercivity and maximum magnetic energy product are higher. The same amount of silane and titanate is used to prepare magnets with different properties, which is mainly due to the different molecular structures of silane and titanate. The molecular structure of titanate coupling agent is shown in Fig. 3, and that of silane coupling agent. Hydroxyl groups exist on the surface of magnetic powder due to the adsorption of trace moisture and gas in air, as shown in Fig. 5. The isopropoxy group in the molecular structure of titanate coupling agent is a hydrolytic group which can be coupled with magnetic powder. The isopropoxy group of titanate reacts with hydroxyl groups on the surface of magnetic powder to form an organic active monomolecular adsorption layer on the surface of magnetic powder. The three alkoxy groups in the molecular structure of silane coupling agent can not only hydrolyze with hydroxyl groups on the surface of magnetic powder, but also condensate with alkoxy groups in the molecule of silane coupling agent itself. As a result, a multi-molecular layer is formed on the surface of magnetic powder.

Comparison of treatment effect of titanate coupling agent and silane coupling agent on magnetic powder
The monolayer structure on the surface of magnetic powder can obtain good dispersion, wettability and coupling efficiency, which can change the external surface energy of magnetic powder, improve the fluidity of magnetic powder and improve its dispersion ability in resin. However, the multi-layer structure on the surface of magnetic powder is not conducive to the dispersion of magnetic powder, and the fluidity of magnetic powder is poor. Therefore, the bonded magnets prepared with titanate coupling agent have higher density and magnetic properties than those prepared with silane coupling agent. Titanate-treated magnetic powder surface-oriented binder I