Ferromagnetic resin composition containing polymeric surface precoated magnetic rare earth cobalt powders

A ferromagnetic resin composition obtained by filling a thermoplastic resin with 70 to 97% by weight of rare earth-cobalt powder, the surface of which has been coated with a thermosetting resin or a thermoplastic resin. The composition gives a plastic magnet excellent in impact resistance.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a ferromagnetic resin composition obtained by 
subjecting rare earth-cobalt powder, which is a ferromagnetic powder, to 
oxidation-inhibiting treatment, and then filling a thermosetting resin 
with said powder in an amount of 70 to 97% by weight. 
2. Description of the Prior Art 
With the development of electronic and electric industries, the performance 
characteristics of magnets have been improved, and the use thereof has 
greatly been broadened and the amounts thereof have greatly been 
increased. Magnets which are most general and much used are sintered 
ferrite magnets produced by a powder metallurgy method. Their 
characteristics, when expressed in terms of maximum energy product 
(BH).sub.max, are approximately 1 MGOe in the case of isotropic magnets 
and 2 to 4 MGOe in the case of anisotropic magnets, and sintered ferrite 
magnets are markedly characterized in that they are very inexpresive as 
compared with other magnets. In addition, Alnico magnets are often used, 
and show such excellent characteristics as compared with ferrite magnets 
that their maximum energy products are 5 to 8 MGOe. However, they are 
disadvantageous in that they are expensive owing to the sudden rise in 
prices of raw material cobalt, tend to lose magnetic force because of a 
small coercive force Hc, and hence, the use thereof is limited. Further, 
recently, rare earth-cobalt magnets have come to be noticed in various 
fields because they have excellent magnetic characteristics. Although rare 
earth-cobalt magnets themselves are considerably expensive because rare 
earth elements per se and cobalt are both expensive, they are fairly often 
used in smallsized parts in which they can effectively exhibit their 
excellent characteristics. 
The magnets described above are disadvantageous in that they are low in 
impact resistance and tend to crack because they are produced by casting 
or sintering. In recent years, there have been developed and widely used 
plastic magnets obtained by filling plastics with ferrite powder in order 
to improve the impact resistance. These magnets have a lower magnetic 
force than sintered magnets because they contatin a large amount of a 
plastic material which is a substance irrelevant to magnetism. In order to 
supply this deficiency, it has been tried to improve a technique for 
conversion to anisotropic by which the easy axes of magnetization of 
ferrite powder are aligned in one direction, and it has become possible to 
enhance the (BH).sub.max values of plastic magnets to 1.7 MGOe which is 
higher than those of isotropic sintered ferrite magnets. However, most of 
magnets having a (BH).sub.max of 2.0 MGOe or higher are anisotropic 
sintered ferrite magnets, Alnico magnets or rare earth magnets, which are 
brittle and hence said to be not usable in a considerable number of 
fields. In recent years, there have been invented plastic magnets obtained 
by impregnating rare earth-cobalt powder with epoxy resin powder, as 
magnets which can have a (BH).sub.max in the range described above and 
possess improved impact resistance. However, they are not yet sufficient 
in impact resistance and are disadvantageous also in that they cannot be 
recycled at all and hence become expensive after all. 
SUMMARY OF THE INVENTION 
In order to produce a ferromagnetic resin composition, which has a further 
improved impact resistance and whose magnetic force covers all the ranges 
from the range of magnetic force of sintered ferrite magnets to that of 
Alnico magnets and rare earth-cobalt magnets, by filling a thermoplastic 
resin capable of being recycled with 70 to 97% by weight of rare 
earth-cobalt powder, we have conducted research to accomplish this 
invention. 
This invention relates to a resin magnet capable of generating a magnetic 
force in terms of (BH).sub.max of 2.0 to 15 MGOe which is obtained by 
coating the surface of rare earth-cobalt powder with a resin in order to 
prevent its oxidative deterioration, filling a thermoplastic resin with 
the rare earth-cobalt powder in an amount of 70 to 97% by weight, and then 
subjecting the thermoplastic resin to injection molding in a magnetic 
field.

DETAILED DESCRIPTION OF THE INVENTION 
The particle size of the rare earth element to be used is 2 to 10.mu., 
preferably 5 to 8.mu.. When it is less than 2.mu., the resulting 
composition is greatly inferior in ability as ferromagnetic substance 
because the domain is broken. When it is more than 10.mu., the magnetic 
force decreases because the degree of orientation becomes low. 
The rare earth-cobalt powder includes A-Co.sub.5 and A.sub.2 -Co.sub.17, 
wherein A is a rare earth element showing crystal magnetic anisotropy 
which includes yttrium (Y), cerium (Ce), praseodymium (Pr), neodymium 
(Nd), samarium (Sm), gadolinium (Gd), misch metal (M-M) which is a mixture 
of various rare earth metals, etc. In kneading rare earth-cobalt powder 
together with a resin, the most caseful attention must be directed to 
oxidative deterioration due to water and adsorption of oxygen. 
Particularly at the time of kneading and molding, heat and pressure 
applied are intense, so that oxidative deterioration tends to take place. 
Further, the starting materials often come in contact with oxygen in the 
production process, and hence, are liable to be deteriorated before the 
formation of a molded article. Therefore, an ingot after heat treatment is 
subjected to wet grinding by using an organic solvent and at the same 
time, the surface of rare earth element powder is coated with a 
thermosetting resin having an oxygen- and water-barrier property or a 
thermoplasic resin which has previously dissolved therein in an amount of 
0.1 to 5% by weight, preferably 0.5 to 2% by weight. As the resin used for 
the coating, there are thermosetting resins such as phenolic resins, eopxy 
resins, urea resins, melamine resins, unsaturated polyesters, alkyd 
resins, urethane resins, and the like. Although these resins may finally 
be cured, prepolymers thereof may as such be used in the uncured state. As 
the thermoplastic resins, there may be used, for example, olefinic resins 
such as polyethylenes, polypropylenes, EVA, ionomers, polybutenes, 
olefinic copolymers and the like and polyamide resins. In coating the 
powder with any of these resins, the use of the resin in an amount of 0.5% 
by weight or less does not make it possible to prevent oxygen from being 
adsorbed on the powder. When the amount exceeds 2% by weight, it is so 
large that the magnetic force is lowered. 
Subsequently, a thermoplastic resin is filled with the coated rare 
earth-cobalt powder in an amount of 70 to 97% by weight. As the resin, 
there may be used olefinic resins such as polyethylenes, polypropylenes 
and the like, polyamide resins such as nylon-6, 12, 6--6 and the like, 
polycarbonate resins, modified PPO, polyacetals, PBT, polyacrylate resins, 
engineering plastics such as PPS, PS, PES and the like, etc. The magnetic 
force can be adjusted by properly selecting the mixing ratio between these 
resins and the treated rare earth-cobalt powder. As one example, there is 
shown in FIG. 1 the maximum energy product (BH).sub.max of a molded 
article obtained by filling, as the 1-5 type powder, 1-5 type 
samarium-cobalt powder into polypropylene, and subjecting the 
polypropylene to injection molding in a magnetic field. As can be seen 
from FIG. 1, the magnetic force increases suddenly from a filled amount of 
about 90% by weight and reaches a saturation point at a filled amount of 
97% by weight. If the filled amount is more than 97% by weight, the 
magnetic force decreases on the contrary. The reason for this is that when 
a large amount of rare earth element powder is filled, the resulting 
composition has a lowered fluidity and a lowered degree of orientation. 
When the physical properties of a composition obtained by kneading the 
rare earth element powder subjected to the above-mentioned treatment are 
measured, the composition has an improved strength as compared with 
sintered product, but the strength as a resin composition is in a low 
range. This is because the bonding strength between the rare earth element 
powder and the resin is insufficient. In order to supply this deficiency, 
a surface-treating agent is added in an amount of 0.1 to 2% by weight 
based on the weight of the rare earth element powder. The surface-treating 
agent to be added includes organosilanes such as epoxy silanes, amino 
silanes, vinyl silanes, chloro silanes, and the like, and is selected 
depending upon the resin used. 
DESCRIPTION OF PREFERRED EMBODIMENTS 
Example 1 
Into 300 g of toluene having dissolved therein 3 g of an epoxy resin 
(EPB-27 manufactured by Nihon Soda Co., Ltd.) was poured 300 g of 1-5 type 
samarium-cobalt powder, and the resulting mixture was subjected to 
stirring to coat the powder with the resin, after which the coated powder 
was dried under reduced pressure to obtain an epoxy resin-coated 1-5 type 
samarium-cobalt powder. Ten grams of each of this powder and untreated 1-5 
type samarium cobalt powder as a comparative example was allowed to stand 
in air, and the change with lapse of time of the amount of oxygen adsorbed 
was measured for each powder to obtain the results shown in Table 1. 
TABLE 1 
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(Amount of oxygen adsorbed (PPM)) 
Standing period (day) 
1 3 7 14 28 
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Untreated powder (PPM) 
30 60 83 88 90 
Coated powder (PPM) 
10 15 20 21 22 
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It can be seen that as is evident from Table 1, the coated powder of this 
invention is difficult to oxidize as compared with the untreated powder. 
The magnetic forces of molded articles obtained from a composition prepared 
by filling nylon-12 with 95% by weight of each of the powders after 
allowing the powder to stand in air for 28 days, were measured to find 
that in the untreated powder case, BH.sub.max was 3 MGOe and in the case 
of the powder of this invention, BH.sub.max was 9 MGOe. 
Example 2 
Into 300 g of toluene having dissolved therein 0.3, 1.5, 3, 6 or 9 g of a 
nylon copolymer was poured 300 g of 2-17 type samarium-cobalt powder, 
after which the resulting mixture was subjected to stirring to coat the 
powder with the resin. The thus coated powder was then dried under reduced 
pressure to obtain nylon-coated 2-17 type samarium-cobalt powder. The 
powders thus obtained were allowed to stand in air, and the change with 
lapse of time of the amount of oxygen adsorbed was measured for each 
powder to obtain the results shown in Table 2. 
TABLE 2 
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(Amount of oxygen adsorbed (PPM)) 
Standing period (day) 
1 3 7 14 28 
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0.1% coated powder 
30 48 59 65 70 
0.5% coated powder 
12 18 21 25 28 
1% coated powder 
10 14 16 20 24 
2% coated powder 
8 15 17 19 19 
3% coated powder 
7 14 17 19 20 
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It can be seen that as shown in Table 2, considerable adsorption of oxygen 
was observed in the case of the 0.1% coated powder, and that the amount of 
oxygen adsorbed becomes smaller in the case of 0.5% or more coated 
powders, but substantially no difference is observed in the case of the 2% 
or more coated powders. 
Subsequently, the results of measuring the magnetic forces of molded 
articles obtained from a composition prepared by filling nylon-12 with 93% 
by weight of each of the powders allowed to stand in air for 28 days are 
shown in FIG. 2. It can be seen that as shown in FIG. 2, the magnetic 
force was lowered considerably in the case of the 0.1% coated powder. In 
the case of the 3% coated powder, the magnetic force showed a tendency to 
lower slightly owing to an increase of the total amount of the resins. 
Example 3 
Into 940 g of toluene having dissolved therein 9.4 g of an epoxy resin 
(EBT-27 manufactured by Nihon Soda Co., Ltd.) was poured 940 g of 1-5 type 
samarium-cobalt powder, after which the resulting mixture was subjected to 
stirring to coat the powder with the resin. The thus coated powder was 
then dried under reduced pressure to obtain an epoxy resin-coated 1-5 type 
samarium-cobalt powder. The powder was divided into two parts, and one 
part was kneaded together with 30 g of nylon-12 (P3014B of Ube Kosan Co., 
Ltd.), while the other part was kneaded together with 30 g of nylon-12 
(P3014B of Ube Kosan Co., Ltd.) and 2.35 g of an aminosilane (A-1160 
manufactured by Nihon Unica Co., Ltd.), and the physical properties of the 
thus obtained compositions were measured. The results obtained are shown 
in Table 3, in which the former composition is represented by the symbol 
"A" and the latter composition by the symbol "B". 
TABLE 3 
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Test item Test method 
Unit A B 
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Specific ASTM-D-792 5.56 5.56 
gravity 
Rockwell ASTM-D-785 105 110 
hardness 
(R scale) 
Tensile ASTM-D-638 kg/cm.sup.2 
230 450 
strength 
Izod impact 
ASTM-D-256 kg-cm/cm.sup.2 
4.4 4.8 
strength 
Flexural ASTM-D-790 kg/cm.sup.2 
380 540 
strength 
Flexural ASTM-D-790 kg/cm.sup.2 
1.1 .times. 10.sup.5 
1.3 .times. 10.sup.5 
elastic 
modulus 
Heat- ASTM-D-648 .degree.C. 
135 138 
deformation 
temperature 
Residual JIS-K-2501 G 6350 6330 
magnetic 
flux density 
(Br) 
Coercive " Oe 5100 5110 
force (Hc) 
Maximum " .times. 10.sup.6 G.Oe 
8.8 8.8 
energy 
product 
(BH).sub.max 
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As can be seen from Table 3, the composition B containing the aminosilane 
had an improved strength as compared with the composition A which did not 
have it. Further, it was confirmed that no lowering of the magnetic force 
was caused by the addition of the aminosilane.