[0001] This invention relates to a plastic bonded magnet containing an anisotropic neodymium-iron-boron
system magnetic powder. Such magnets are used in various devices including stepping
motors, spindle motors, torque motors, automotive motors, various actuators, speakers,
and other magnetic-field generating devices.
[0002] Extensive efforts have been made to improve the maximum energy product (hereinafter
abbreviated to (BH)
max of resin-bonded rare-earth magnets. There are three essential requirements for a
higher (BH)
max plastic bonded magnet, namely (1) higher magnetization 4 I and higher coercive force
iH
c, (2) higher mold density, and (3) higher magnetic orientation of the magnetic powder.
While various ways have been found to attain requirements (1) and (2), including those
methods disclosed by in Japanese Laid-Open Patents Sho 60-207302 and Sho 60-220907,
virtually no satisfactory way has to date been found to improve the magnetic powder
orientation.
[0003] We have now devised a new neodymium-iron-boron system plastic bonded magnet having
an improved (BH)
max achieved by attaining a higher orientation of the magnetic powder thereof. In particular
the invention provides a plastic bonded magnet which comprises an anisotropic neodymium-iron-boron
system magnetic powder and a resin binder characterised in that from 10 to 20% weight
of the magnetic powder has a grain size in the range 10 to 49 microns (µm).
[0004] The magnetic powder grain distribution used in the magnets of the invention can be
obtained by kneading and dispersing the powder (together with a binder) in a high-speed
shearing machine.
[0005] By using the above-described magnetic powder constitution, a higher (BH)
max is obtained due to a substantially improved magnetic orientation in the compressed
molded body of the plastic bonded magnet. Furthermore, an extended pot-life of molding
compound and higher (BH)
max value can be obtained by employing an epoxy resin as the binder and curing it with
a latent hardening agent which is an amine adduct of the epoxy resin.
[0006] In order that the invention may be more fully understood, reference will be made
to the accompanying drawings, wherein:
Figure 1 is an enlarged cross-section of one embodiment of plastic bonded magnet of
the invention;
Figure 2 shows the relationship between the grain-size of magnetic powder of the invention
and the magnetic characteristics;
Figure 3 is a schematic representation of magnetic powder employed in Example 1;
Figure 4 is a schematic representation showing the magnetic powder grain configuration
after kneading and dispersion of the magnetic powder shown in Figure 1; and
Figure 5 is a schematic diagram of the magnetic powder grain configuration after kneading
and dispersion of magnetic powder used in Example 3.
[0007] Referring now to Figure 1, there is shown a magnet of the invention wherein 1 is
anisotropic neodymium-iron-boron system magnetic powder having a grain size distribution
from 50 to 500 microns, reference 2 is anisotropic neodymium-iron-boron system magnetic
powder having grain sizes from 10 to 49 microns, and 3 is the plastic resin binder.
The anisotropic neodymium-iron-boron system magnetic powder employed in this invention
can be anisotropic neodymium-iron-boron system magnetic powder manufactured by upsetting
its overquenched melt-spun ribbon.
[0008] The magnets may also comprise, in addition, elements such as zirconium, gallium,
cobalt, praseodymium, and tin for improving temperature characteristics, anti-corrosion
properties, and magnetic properties as will be understood by those skilled in the
art.
[0009] The magnetic powder employed in the invention has excellent magnetic properties,
(magnetization 4πI, coercive force iH
c) which are substantially governed by the grain size distribution as shown in Figure
2. Thus, the magnetic characteristics of the fine powder having grain sizes of 10
to 49 microns (µm) are significantly different from those of powder of greater grain
size, as Fig. 2 illustrates.
[0010] It will be understood that the invention does not use exclusively powder having grain
sizes less than 49 microns: such magnets would have inferior magnetic properties.
Rather, the invention introduces a limited amount only of such powder into the molding
composition to improve the magnetic orientation. The magnetic orientation of the plastic
bonded magnet is highly improved by introducing magnetic powder having fine grain
sizes from 10 to 49 microns (µm) in an amount of 10 to 20% by weight. The reason for
this cannot be attributed to higher magnetic powder density, but we believe it can
be attributed to the improved flowability of the molding composition.
[0011] We prefer to exclude magnetic powder having grain sizes less than 10 microns (µm)
because of its poor magnetic characteristics. The content of fine magnetic powder
(10 to 49 microns (µm) should be limited to less than 20% by weight, but the orientation
effect cannot be attained if its content is less than 10% by weight. Although large
sized magnetic powder having grain sizes more than 500 microns (µm) has excellent
magnetic characteristics, it causes a reduction in mold density and as a result no
improvement in residual magnetic flux density B
r can be expected.
[0012] The molding composition will contain a suitable dispersing agent. Fatty acids, silicone-coupling
agents and various surface-active agents can be employed, we prefer to use a liquid
fatty acid, eg. oleic acid. This ability to function as a dispersing agent is attributed
to the higher affinity for the magnetic powder. This is not shown by any of the solid
fatty acids. The use of liquid fatty acids is particularly advantageous in compressive
molding which is performed conventionally at room temperature since it improves productivity.
[0013] Although it is important to keep the amount of fatty acid additive as low as possible,
it is preferred to introduce at least 0.8% by weight thereof (based on the weight
of the magnetic powder). An amount of fatty acid less than 0.7% by weight usually
results in an inadequate affinity to the magnetic powder resulting in reduced density
and (BH)
max. On the other hand, if an excessive amount of fatty acid is used, eg. more than 1.3%
by weight, oozing of binder tends to occur and lower strength of the molded magnet
results.
[0014] Whilst various binders can be used in the present invention, for room temperature
molding we prefer to use liquid epoxy resin to give good adhesion strength. The total
amount of binder is determined by the attainable magnetic characteristics and the
strength of the molding. The total amount of binder component used depends on the
magnetic characteristics required and the strength of the molded magnet. If an amount
of binder of less than 1.4% by weight is mixed with the magnetic powder, this would
usually result in inadequate mold strength, but if more than 3.0% by weight is used,
this would result in poor magnetic characteristics.
[0015] Any suitable curing or hardening agent can be used. Amines and acid anhydrides are
generally preferred. If an amine adduct of epoxy resin is used as a hardening agent
for epoxy resin used as a binder, a substantially extended pot-life of the molding
composition can be obtained relative to the use of a conventional amine system hardening
agent.
[0016] The molding composition is preferably prepared in a high-speed shearing machine capable
of pulverizing and mixing the magnetic powder, such as a Henschell mixer, upper-mill,
high-speed mixer, or a micro-mill grinder.
[0017] The following Examples illustrate the invention. The comparisons are not according
to the invention.
Example 1
[0018] A plastic bonded magnetic material mixture consisting of 100 weight parts of anisotropic
Nd-Fe-B system magnetic powder having grain sizes from 1 to 2 mm and 0.9 weight part
of oleic acid as a dispersion agent, are ground in a micro-mill grinder for about
five minutes, and 1.0 weight part of epoxy resin, eg. Epikote 828 (manufactured by
Yuka-Shell Epoxy Co.), is added thereto, and ground for another ten minutes.
[0019] The dispersion process is completed by adding 0.9 weight parts of acid anhydride,
eg. Kayahard MCD (manufactured by Nihon Kayaku Co.), and 0.01 weight parts of catalytic
imidazone, eg. Epikure EMI-24 (manufactured by Shikoku Kasei Kogyo Co.) thereto, and
the mixing continued for another ten minutes.
[0020] The molding composition so made is loaded into a die cavity, and is molded at room
temperature using a pressure of 6 ton/cm² (6 x 10⁸ Pa) under application of a magnetic
field of 15 KOe. After a hardening process conducted at 100°C for one hour, a plastic
bonded magnet of the invention is produced.
[0021] For purposes of analysis, a molded article before hardening is removed from a mold
and immersed in acetone to remove resin components. The magnetic powder obtained is
dried in a nitrogen atmosphere. The grain size distribution of this magnetic powder
was measured and found to be:

Comparison 1
[0022] For comparison purpose, the procedure of Example 1 is repeated except that magnetic
powder having grain sizes from 300 to 500 um is used as starting material. The grain
size distribution in this case was measured and was found to be:

Comparison 2
[0023] Example 1 is repeated except that magnetic powder having grain sizes from 3 to 4
mm is used. The grain size distribution was found to be:

Comparison 3
[0024] Example 1 is repeated except that this time the mixture is ground for 12 hours in
a ball-mill using acetone as a solvent and then the acetone is evacuated. The grain
size distribution was found to be:

Example 2
[0025] The general procedure of Example 1 is followed except that an electromagnetic steel
plate made of soft magnetic material is inserted in the die before the molding composition
is injected. The molding is then conducted at room temperature under a pressure of
6 ton/cm² (6 x 10⁸ Pa) and a magnetic field of 15 KOe. After a hardening process at
100°C for two hours, a plastic bonded magnet having an integrated electromagnetic
steel plate made of soft magnetic material, is obtained. The adhesion between the
soft steel plate and plastic bonded magnet was found adequately high.
[0026] The determined densities, magnetic characteristics and magnetic orientations of the
plastic magnets obtained in Example 1 and in Comparisons 1,2 and 3 are set out in
Table 1.
[0027] The degree of magnetic orientation is defined as B
r(//)/ (B
r(//) + B
r(┴)]. This represents the ease of movement of magnetic powder under application of
an external magnetic field, and a higher value means a higher magnetic orientation.
B
r(//) and B
r(┴) represent a residual magnetic flux density along the applied magnetic field and
a residual magnetic flux density perpendicular to the applied magnetic field, respectively.

[0028] Table 1 shows that plastic bonded magnets having higher densities, degrees of magnetic
orientation and (BH)
max values can be obtained by the present invention.
[0029] Changes in grain configurations of magnetic powder are shown in Figs. 3,4 and 5.
Figure 3 shows a schematic configuration of raw magnetic powder material, and Figure
4 is a schematic showing the rounded-off grains of powder after pulverization and
mixing conducted by a high-speed shearing machine which is employed to prepare the
magnet of Example 1. Figure 5 is a schematic diagram showing grain configuration of
magnetic powder after kneading and dispersion of the magnetic powder of Comparison
3, showing more squarish and smaller grain sizes compared with those shown in Figure
4.
[0030] Thus, as shown in Figure 1, higher magnetic orientation and higher density, and higher
(BH)
max, can be realized by the grain size changes in accordance with the invention, for
example produced by the pulverization and dispersion process using a high-speed shearing
machine, as described. Whilst we do not wish to be bound by theory, we believe that
the reasons for these improvements can be explained as follows. Whereas the magnet
of Example 1 contains 14% by weight magnetic powder having grain sizes from 10 to
49 um, the amounts in Comparisons 1,2 and 3 are 30% by weight, 10% by weight, and
78% by weight, respectively. Those improvements of magnetic orientation in Example
1 and Comparison 2 are due to the containment of 10 to 20% by weight magnetic powder
having grain sizes from 10 to 49 µm. While the magnetic orientation is 0.70 and 0.68
in Example 1 and Comparison 2, respectively, it is only 0.65 in Comparison 3.
[0031] This improvement of magnetic orientation is attributed to the improved flowability
of the compressive molding composition containing fine magnetic powder having grain
sizes from 10 to 49 µm in amounts from 10 to 20% by weight. Hitherto, the use of this
type of magnetic powder had been avoided because of its lower magnetic characteristics.
[0032] Furthermore, as shown in Example 1, the improvement of (BH)
max is attributed to the higher magnetic orientation and improved density due to using
magnetic powder which is rounded-off during said high-speed shearing process. The
density of the magnet of Comparison 2 is 6.15 which is low because it contains magnetic
powder having grain sizes from 1 to 2mm, and thus, the (BH)
max is low also.
[0033] This means that the improvements of both orientation and density contribute to the
improved (BH)
max, achieved by the present invention. Further, in the plastic bonded magnet containing
an integrated soft magnetic plate, described in Example 2, the bond strength between
these parts was found satisfactory even without using any adhesives, and thus, a simplification
in the conventional manufacturing process can be realized.
Example 3
[0034] An anisotropic Nd-Fe-B system magnetic powder having an average grain size of 1mm
and oleic acid are mixed at a weight ratio of 100 to 0.9, and this mixture is kneaded
for 10 minutes in a high-speed mixer under a nitrogen atmosphere.
[0035] Then, 1.6 weight parts of an epoxy resin, eg. Epikote-828 manufactured by Yuka-Shell
Epoxy Co., is added thereto and the mixture is kneaded for another ten minutes. A
latent hardening agent, eg. Amicure PN-23, manufactured by Ajinomoto Co., is then
added in an amount of 0.4 weight part, and the mixture kneaded for another 10 minutes
before the process is complete. The resulting molding composition is then loaded into
a cavity for applying a compressive pressure of 6 ton/cm² with an applied magnetic
field of 15 KOe. The anisotropic plastic bonded magnet is produced after hardening
for one hour at 100°C.
Comparison 4
[0036] Example 3 is repeated except that the materials are as follows:

Comparison 5
[0037] Example 3 is repeated except that the materials used were:

[0038] The initial magnetic characteristics, (BH)
max, of the magnets, and the pot-life of the molding compositions are shown in Table
2. The pot-life was determined as follows. The molding composition is left still at
room temperature, and samples are taken out every one hour and used to mold a sample
plastic bonded magnet under the above-described conditions. The elapsed time at which
a sample of the composition forms a magnet which shows a magnetic characteristic which
is lower by 5 % than that of the initial magnet is defined as the pot-life of the
composition.

[0039] The plastic bonded magnet shown in Example 3 wherein a latent hardening agent is
used, showed a value of (BH)
max as high as 15 MGOe and a pot-life as long as 20 hours, which are very useful features.
The use of a high-speed shearing machine minimizes the possibility of stability damage
of binder system due to the local heat produced at kneading.
[0040] On the other hand, the magnet produced by using an aliphatic amine hardening agent,
shown in Comparison 4, had a value of (BH)
max of 12 MGOe and a pot life of only 3 hours, and this has little practical value. Although
using an aromatic amine hardening agent as shown in Comparison 5, gave a value of
(BH)
max of 15 MGOe which is reasonably high, the pot life of 6 hours is still considered
inadequate for practical purposes.
[0041] As mentioned above, the (BH)
max of the magnet, and the pot-life of the composition are influenced by the type of
hardening agent. The difference between magnetic characteristics of the magnets can
be attributed in part to the difference between the affinities of binders for the
magnetic powder, which results in the difference between dispersibilities.
[0042] The difference between pot-lives can be attributed to the difference between thermal
stabilities of the binder systems against local heat generation. The latent hardening
agent employed, i.e. a hardening agent which does not start hardening until a certain
time is elapsed after it is mixed with a primary epoxy agent, is much preferred. This
hardening system has a high thermal stability, and is thus considered highly useful
to extend the pot-life.