Abstract: Bonded NdFeB magnets were prepared by using modified epoxy resin as binder, and their properties were studied. The results show that the performance of the modified epoxy resin plastic bonded rare earth magnets is higher than that of the traditional epoxy bonded magnet. The magnet prepared under the conditions of 130 ℃, 2min, 120min and 120 ℃, Magnetic best. Since the advent of bonded magnets since the mid-1980s, a variety of binder materials have been studied such as non-magnetic polymer compounds epoxy (thermoplastic), phenolic (thermosetting), nylon, polyphenylene sulfide, , Rubber and low melting point metal Bi, Sn, Pb, Zn, Al [1-3]. Liu Ying et al  studied the metallocene polymer ferromagnetic powder bonded permanent magnet composite material properties. Chen Debo et al  studied the amount of epoxy resin on the performance of the magnet, the results show that the binder content of 2.5% of the magnet has better performance. Li Jun et al  studied the effect of silane treatment on the performance of the magnet, indicating that magnetic properties of the magnetic powder after proper silane treatment are favored. Zhang Hong et al  studied the impact of five different epoxy resin on the performance of the magnet, that is solid at room temperature, high epoxy value and good compatibility with the surface of the magnetic powder resin is prepared plastic bonded neodymium magnets Ideal binder.
A single phenolic resin cured by chemical reaction of good heat resistance, but the nature of more brittle, so the bonding strength of pure phenolic resin is not high. In most cases, it is modified with a thermoplastic resin, a synthetic resin, or the like. Unmodified phenolic resin glue can only cement wood, rigid foam and other porous materials. Other polymer-modified phenolic resin-based adhesives, structural adhesives occupy an important position. In this paper, KY-2055 modified epoxy phenolic resin as a binder, prepared a good magnetic properties of plastic-bonded NdFeB magnets.
1 experimental method
1.1 Experimental Materials, Instruments and Equipment
The main raw material for the United States GM production of quenching MQP-16 magnetic powder, silane coupling agent KH-550, epoxy resin E-44 and KY-2055 and so on. Magnets were prepared using a 769YP-24B powder tablet press and the magnet performance was measured using a NIM-200C magnetic gauge.
1.2 Sample Preparation
After quenching NdFeB magnetic powder coupling agent KH-550 treatment, mixed with epoxy phenolic resin, and then placed in a drying oven dried at 60 ℃ 2h, pressed into a cylinder Φ10mm × 10mm. Finally, at different temperatures and time curing, measuring its magnetic properties.
2 Experimental results and analysis
2.1 Modified epoxy resin binder prepared magnet properties
The modified epoxy resin and the traditional epoxy resin are respectively mixed with the magnetic powder according to the mass fraction of 2%, and then pressed into a cylinder under the pressure of 600 ~ 1600MPa for 120s, and cured at 150 ° C for 3h. After cooling, The magnetic properties, the results shown in Figure 1.
As can be seen from FIG. 1, the performance of a magnet prepared using a modified epoxy resin as a binder is better than that of a magnet prepared using a conventional epoxy resin as a binder, especially when the pressure is low. This is because the modified epoxy resin bond strength than the traditional epoxy resin, when the pressure is small, the magnetic powder can be closely bonded together; with the pressure increase, the gap between the magnetic powder and the adhesive gradually Shrinking, the magnets made with both adhesives approach performance. ) 2.2 modified epoxy resin as a binder process parameters on the performance of the magnet
Using modified epoxy resin as a binder, at 150 ℃ curing time on the performance of the magnet experiment, the experimental results shown in Figure 2. As can be seen from Figure 2, the magnetic energy product increases from 90 min to 120 min and then begins to decrease. This is because the curing cross-linking reaction occurs between the magnet and the binder, and the chain binder forms a net-like structure after the reaction to make the binding between the magnetic powder and the binder more compact and the density of the magnet higher . At the same time, the binder and the magnetic powder penetrate each other and diffuse at a certain curing temperature, so that the density of the magnet is more uniform and the defects are reduced. Certainly, the infiltration and the diffusion take a certain amount of time to achieve the better effect. When the binder is fully cured, the binder loses its fluidity and forms an insoluble, infusible network. Curing time is too short, the reaction is not complete, after a certain period of time, the curing reaction basically completed, even if the longer curing time, the crosslinking degree will not be improved. In the meantime, the magnet is oxidized at a high temperature for a long time, and the magnet powder, which is not completely covered with the binder, is oxidized to deteriorate the performance of the magnet.
Figure 3 shows the relationship between the curing temperature and the performance of the magnet. As can be seen from Fig. 3, before 130 ℃, remanence, coercivity and energy product increase with the increase of curing temperature. After 130 ℃, the remanence, coercivity and energy product decrease with increasing temperature. This is because at about 130 ℃ modified epoxy resin and the occurrence of cross-linking reaction of magnetic powder, cross-linked by the linear structure into a network structure, the magnets are closely pulled together to make the gap between the magnetic particles smaller and closer contact , Magnet performance improved. However, as the temperature continues to rise, the phenomenon of magnet oxidation gradually becomes obvious. The magnetic powder in the periphery of the magnet is oxidized first, and the magnetic powder in the magnet is also gradually oxidized. Oxidation inside the magnet is in the process of preparation, the air inside the magnet is inevitably enclosed by air, and when the temperature rises, the temperature of the gas in the space increases, the pressure increases, the reactivity is enhanced, and the air and the magnetic powder The oxidation reaction takes place. Remaining magnetism gradually decreases with the increase of curing temperature and curing time, because the magnet slowly demagnetizes at 150 ℃ and oxidation of magnetic powder aggravates.
The effect of molding temperature and dwell time on the performance of the magnets is shown in Figures 4 and 5. It can be seen from Fig. 4 that the magnetic properties Br, (BH) max and Hci decrease with the increase of the molding temperature in the temperature range of 90 ~ 100 ℃; the magnetic properties gradually increase with the increase of the molding temperature in the range of 130 ~ 140 ℃ Molded temperature increases Magnetic properties decline.
Modified epoxy resin is a polymer, which forms a three-dimensional network with the curing agent in the curing process. Thermosetting resins are similar to thermoplastic resins before crosslinking and belong to linear polymers, exhibiting good ductility below the cure temperature. Molding these molecules come with reactive groups and curing agent cross-linked together to form a tight rigid body structure. The cross-linking process is divided into three stages : ① Stage A: It has good solubility and fusibility in the range of 90 ~ 100 ℃. As the temperature rises, the binder gradually changes from solid to liquid, The molecules of the binder change from the frozen state to the moving state, the magnetic powder and the magnetic powder adhere to each other, the friction between the magnetic powder increases, and the magnetic powder can not flow easily. At the same time, the small molecular gas produced in the binder increases with temperature, the volume of pores in the bonded magnet increases, and the defects increase, resulting in the decrease of the density of the bonded magnet and seriously affecting the magnetic properties. 100 ~ 130 ℃ binder viscosity becomes smaller, further enhance the mobility of the magnetic powder, magnetic powder can uniformly fill the space around the cavity, the gap between the magnetic powder decreases, after pressing the density of the magnet uniform throughout. When the temperature is raised to 130 ° C, the mixed fluidity of the binder and the magnetic powder reaches the maximum, and the magnetic powders can be closely bonded together to form a dense bonded body. ② B stage: above 130 ℃, the intermolecular began to produce cross-link, the resin soluble, fusibility decline, but still plastic. ③ C stage: the temperature reaches the curing temperature at this stage, the reaction rate is rapidly increased at this temperature, in a very short period of time cross-linked to the molecular structure of the binder, the resin insoluble insolubles deep cross-linked, magnetic powder Time to free flow, curing reaction is completed, resulting in incomplete cavity filling.
It can be seen from Fig. 5 that the Br, (BH) max and Hci show an upward trend with the prolongation of the holding time, and the magnetic properties are the best when the holding time is 120s. Then the holding time, Br, BH, decline. The reason is that the flaky magnetic powder relatively slides during the packing process, the gap in the mold is fully filled, the pressure is more fully transmitted in the magnet, the magnetic force is uniformly applied to various parts of the magnet, the magnetic powder deforms and intermeshes with each other, and the performance of the magnet is increased. Continue to extend the dwell time, the gap between the powder gradually reduced, the maximum deformation of the magnetic powder under pressure and began to break, damaged the crystal lattice structure, resulting in decreased performance of the magnet.
Compared with the traditional epoxy bonded NdFeB magnets, modified epoxy resin bonded NdFeB magnets with higher magnetic properties; modified epoxy resin as a binder, at a molding temperature of 130 ℃, dwell time 2min, Curing time 120min, the curing temperature of 120 ℃ prepared under the conditions of the magnet, the best magnetic properties.
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