Resin coated permanent magnet

A waterproof resin coated permanent magnet is provided. The resin coating is either a waterproof fluoroplastic resin material or a mixture of fluoroplastic resin material and at least one organic resin such as epoxy resin, polyester resin, acrylic resin, phenol resin and mixtures thereof. The magnet has superior corrosion and weathering resistance.

BACKGROUND OF THE INVENTION 
This invention relates generally to resin coated permanent magnets and in 
particular, to a permanent magnet having a waterproof organic resin 
coating to provide superior oxidation resistance and strength. 
Permanent magnets include ferrite magnets, alnico magnets and rare-earth 
magnets. The demand for rare-earth magnets has grown in proportion to the 
growing demand for smaller and higher efficiency electrical appliances for 
office automation such as computers, word processors and facsimile 
machines. 
Rare-earth magnets are grouped into three classes by method of manufacture. 
These classes include sintered magnets, bonded magnets and cast magnets. 
Typical rare-earth magnets are also grouped by composition. Specifically, 
rare-earth magnets include a rare-earth metal in combination with either 
cobalt or ferrite. 
European Patent No. 108474 issued to General Motors Corp. discloses a 
rare-earth magnet including a rare-earth metal and iron which is obtained 
by a rapid quenching method. In the rapid quenching method, a ribbon-like 
material having a thickness of 20 .mu.m is provided. The ribbon-like 
material is an aggregate of crystals having a diameter between about 0.1 
and 0.5 .mu.m, which is smaller than the critical diameter of a uniaxial 
particle. The material is pulverized to a particle size of less than about 
177 .mu.m while maintaining coercive force and the pulverized material is 
used to form a resin bonded magnet. 
Rare-earth magnets are further classified into two groups based on the 
coercive force mechanism of the magnet. One of the groups includes those 
rare-earth magnets which function in accordance with a 1-5 system magnetic 
model. These include rare-earth transition metal compounds having formulas 
such as SmCo.sub.5, CeCo.sub.5, Sm.sub.0.5 Ce.sub.0.5 Co.sub.5, YCo.sub.5, 
PrCo.sub.5 and Sm(CoCu).sub.5. Nuclear magnetic intermetallic compounds of 
at least one rare-earth metal and at least one transition metal including 
magnets based on R-Fe-B are also included in this group. 
The second type of permanent magnets function in accordance with a planning 
model of 2-17 system magnets. These two-phase separate type or analysis 
hard type magnets include rare-earth transition metal intermetallic 
compounds having formulas such as: 
EQU Sm(Co.sub.bal Cu.sub.0.05 Fe.sub.0.02 Zr.sub.0.02).sub.8.0 
EQU Sm(Co.sub.bal Cu.sub.0.06 Fe.sub.0.022 Ti.sub.0.016).sub.7.6 
EQU Sm.sub.0.8 Y.sub.0.2 (Co.sub.bal Cu.sub.0.06 Fe.sub.0.20 
Nb.sub.0.018).sub.7.8 
EQU Sm.sub.0 7 Ce.sub.0.3 (Co.sub.bal Cu.sub.0.06 Fe.sub.0.24 
Zr.sub.0.02).sub.7.8 and 
EQU Sm.sub.0 5 Pr.sub.0.5 (Co.sub.bal Fe.sub.0.3 Cu.sub.0.07 
Zr.sub.0.02).sub.7.6. 
The amount of cobalt is approximately 0.91. However, this amount is 
specified as a balance since a limited amount of impurities may be 
included. 
Rare-earth transition metal intermetallic compounds including rare-earth 
metals, transition metals and semi-metals or semiconductor elements are 
reactive with oxygen. Specifically, the magnetic surface reacts with 
atmospheric oxygen to create rust. R-Fe-B magnets cause particular 
problems. When R-Fe-B magnets are incorporated into motors, relays and the 
like, oxides produced on the surface of the magnet are removed as the 
equipment operates and cause such significant problems in the equipment 
that the magnet itself is unsuitable for practical use. 
European Patent No. 101552 issued to Sumitomo Tokushu Kinzoku Kabushiki 
Kaisha relates to rare-earth iron series permanent magnets obtained by a 
sintering method and consisting primarily of neodymium, iron and boron. 
However, the European patent does not recognize that rusting is a problem. 
Japanese Patent Laid-Open Application No. 56-81908 discloses that rust can 
be prevented by coating a resin such as an epoxy resin on a rare-earth 
magnet. However, subtle pin-holes are generated in the plating or coating 
layer and it is difficult to prevent these pin-holes. As a result, rust 
occurs when water contacts the magnet through pin-holes in the coating 
layer. 
The pin-holes are generated because the magnets do not have an entirely 
uniform planar or mirror surface. Rather, the rare-earth magnets have 
subtle uneven irregularities or spaces between magnetic particles. The 
resin can therefore not be coated uniformly. 
Furthermore, solvent in the plating or coating solution is volatilized when 
the layer dries even when the layer is coated as uniformly as possible. 
Pin-holes occur at the volatilized portions. For these reasons, it is 
extremely difficult to provide a coating layer on a rare-earth magnet 
without generating pin-holes. 
The generation of pin-holes is not a significant problem in prior art 
magnets such as Sm-Co magnets which include only a small amount of iron. 
In contrast, prior art magnets including a rare earth metal and a large 
amount of iron are apt to rust. When these magnets are incorporated into 
rotating machines such as motors, VCMs, speakers and relays to provide a 
magnetic circuit the rust which has been generated causes the magnetic 
performance to deteriorate for the reasons discussed. 
Accordingly, it is desirable to provide an improved permanent magnet having 
superior corrosion and weathering resistance. 
SUMMARY OF THE INVENTION 
Generally speaking, in accordance with the invention a permanent magnet 
having a waterproof coating of an organic resin yielding a permanent 
magnet of superior corrosion and weathering resistance is provided. The 
waterproof resin coating includes a fluorine resin (or fluoroplastic) or a 
mixture of fluorine resin and at least one additional organic resin, such 
as an epoxy resin, polyester resin or phenol resin. When a mixture of 
resins is used, the fluorine resin is present in an amount between about 2 
and 70% by weight. In all cases, the waterproof organic resin material is 
coated on the permanent magnet to a thickness between about 1 and 50 
.mu.m. 
The permanent magnet member to be coated in accordance with the invention 
can be of the sintered, bonded or cast type. In one preferred embodiment 
of the invention, the permanent magnet is a powder bonded permanent magnet 
which is a composite of particles of a rare-earth magnet and organic 
bonding materials. A thermosetting resin can be dispersed throughout the 
permanent magnet material prior to forming the magnet and coating on the 
permanent magnet to a thickness of between about 1 and 50 .mu.m with the 
waterproof organic resin material. 
The fluorine resins used in the waterproof organic resin coatings in 
accordance with the invention include 4-fluorinated ethylene resin (PTFE); 
a copolymer resin of 4-fluorinated ethylene and per-fluoroalkoxyethylene 
(PFA); a copolymer resin of 4-fluorinated ethylene and 6-fluorinated 
propylene (FEP); a copolymer resin of 4-fluorinated ethylene, 
6-fluorinated propylene and per-fluoroalkoxyethylene (EPE); a copolymer 
resin of 4-fluorinated ethylene and ethylene (ETFE); a copolymer resin of 
3-fluorinated ethylene chloride (PCTFE); a copolymer of 3-fluorinated 
ethylene chloride and ethylene (ECTFE); a fluorinated vinylidene resin 
(PVDF); fluorinated vinyl resin (PVE) and mixtures thereof. The resin 
coated permanent magnet does not develop pinholes in the coating layer and 
has superior corrosion and weathering resistance. 
Accordingly, it is an object of the invention to provide an improved 
rare-earth magnet having superior corrosion and weathering resistance. 
Another object of the invention is to provide a rare-earth magnet that does 
not rust. 
A further object of the invention is to provide a rare-earth magnet with a 
surface that does not deteriorate. 
Still another object of the invention is to provide a magnet that is 
resistant to damage. 
Yet another object of the invention is to provide a powder bonded permanent 
magnet including particles of a rare-earth magnet and a bonding material. 
Still a further object of the invention is to provide a powder bonded 
permanent magnet having a thermosetting resin penetrated therein. 
Yet a further object of the invention is to provide a fluoroplastic coating 
layer for a permanent magnet. 
A further object of the invention is to provide a waterproof permanent 
magnet. 
Another object of the invention is to provide a waterproof coating for a 
sintered, bonded or cast permanent magnet. 
Still other objects and advantages of the invention will in part be obvious 
and will in part be apparent from the specification. 
The invention accordingly comprises an article of manufacture possessing 
the features, properties, and the relation of elements which will be 
exemplified in the article hereinafter described, and the scope of the 
invention will be indicated in the claims. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The waterproof organic resin coated permanent magnets provided in 
accordance with the invention can include any type of permanent magnet 
member including sintered, bonded and cast permanent magnets. The 
preferred waterproof organic resins for the coating are fluorine resins to 
provide a waterproof coating which means that water is repelled under 
conditions of high humidity. 
The fluorine resins which may be used in accordance with the invention 
include 4-fluorinated ethylene resin (PTFE) having the structure 
EQU (--CF.sub.2 --CF.sub.2).sub.n 
wherein n is an integer; a copolymer resin of 4-fluorinated ethylene and 
perfluoroalkoxyethylene (PFA) having the structure 
##STR1## 
wherein R.sub.f is an per-fluoroalkyl group and m and n are independently 
selected integers; a copolymer resin of 4-fluorinated ethylene and 
6-fluorinated propylene (FEP) having the structure 
##STR2## 
wherein m and n are independently selected integers; a copolymer resin of 
4-fluorinated ethylene, 6-fluorinated propylene and 
per-fluoroalkoxyethylene (EPE) having the structure 
##STR3## 
wherein R.sub.f is an per-fluoroalkyl group and l, m and n are 
independently selected integers; a copolymer resin of 4-fluorinated 
ethylene and ethylene (ETFE) having the structure 
EQU (--CF.sub.2 --CF.sub.2).sub.m (--CH.sub.2 --CH.sub.2).sub.n 
wherein m and n are independently selected integers; a copolymer resin of 
3-fluorinated ethylene chloride (PCTFE) having the structure 
EQU (--CF.sub.2 --CFCl).sub.n 
wherein n is an integer; a copolymer resin of 3-fluorinated ethylene 
chloride and ethylene (ECTFE) having the structure 
EQU (--CF.sub.2 --CFCl).sub.m (--CH.sub.2 --CH.sub.2).sub.n 
wherein m and n are independently selected integers, fluorinated vinylidene 
resin (PVDF) having the structure 
EQU (--CF.sub.2 --CH.sub.2).sub.n 
wherein n is an integer; and fluorinated vinyl resin (PVE) having the 
structure 
EQU (--CHF--CH.sub.2).sub.n 
wherein n is an integer. 
The fluorine resins can be used alone to form the waterproof coating or may 
be used in combination with an additional organic resin, such as epoxy 
resin, polyester resin or phenol resin. The proportion of fluoroplastics 
should be between about 2 and 70% by weight when the fluoroplastic is used 
in combination with an organic resin. The coating layer does not have 
superior weathering properties when the proportion of fluoroplastics is 
less than about 2% by weight. It is difficult to obtain a uniform mixture 
of fluorine resin and the additional resin and the coating layer tends to 
have an uneven surface and low strength when the proportion of fluorine 
resin exceeds about 70% by weight of the coating composition. 
Desirable effects are achieved when the fluorine resin is used alone. 
However, the results are superior when the fluorine resin is mixed with a 
second waterproof organic resin such as epoxy resin, acrylic resin, phenol 
resin and the like. 
The fluorine resin or mixed layer of fluorine resin and additional organic 
resin is coated on the magnet to a thickness between about 1 and 50 .mu.m. 
It is difficult to maintain reliability of the coating layer because an 
uneven layer develops when the thickness is less than about 1 .mu.m. On 
the other hand, the time it takes to prepare a thick layer and 
consequently the cost involved is not practical for coating layers having 
thicknesses greater than about 50 .mu.m. 
A mixture of fluorine resin and additional organic resin adheres to a 
magnet better than a fluorine resin alone. This is particularly true when 
the magnet includes intermetallic compounds. In general, it is necessary 
to heat treat a magnet to a temperature between about 100.degree. and 
900.degree. C. in order to achieve adherence when the magnet is coated 
with fluorine resins alone. However, the magnetic properties are lowered 
by heat treatment which may make the magnet no longer suitable for 
practical use. By mixing an additional organic resin with the fluorine 
resin, it is possible to maintain the water repellancy of the coating and 
obtain high performance of the magnet while advantageously enhancing 
adherence and durability. 
The permanent magnets are coated with the coating material by physical or 
chemical methods. The waterproof coating layer having a thickness of 
greater than about 1 .mu.m prevents the iron in the magnet from reacting 
with water to form rust. Specifically, the substitution reaction between 
Fe and H-OH (in water) to form Fe(OH).sub.3 is prevented. 
In a preferred embodiment, the fluorine resin or mixed fluorine resin and 
additional organic resin material is coated on a powder bonded permanent 
magnet. In another preferred embodiment, the powder bonded permanent 
magnet has a thermosetting resin dispersed throughout the magnet 
composition. When a thermosetting resin is penetrated into the powder 
bonded permanent magnet, it is only necessary for the coating layer to be 
between about 1 and 50 .mu.m thick. 
The powder bonded permanent magnets can be selected from the following: 
1. Intermetallic compounds formed from a rare-earth metal and cobalt. This 
is referred to as a 1-5 system rare-earth magnet and includes compounds of 
formulas such as SmCo.sub.5, CeCo.sub.5, Sm.sub.0.5 Ce.sub.0.5 Co.sub.5, 
YCo.sub.5, PrCo.sub.5 and Sm(CoCu).sub.5 ; and 
2. Rare-earth transition metal intermetallic compounds, which are known as 
2-17 system rare-earth transition metal compounds. These compounds have 
formulas such as 
EQU Sm(Co.sub.bal Cu.sub.0.05 Fe.sub.0.02 Zr.sub.0.02).sub.8.0, 
EQU Sm(Co.sub.bal Cu.sub.0.06 Fe.sub.0.22 Ti.sub.0.016).sub.7.6, 
EQU Sm.sub.0.8 Y.sub.0.2 (Co.sub.bal Cu.sub.0.06 Fe.sub.0.20 
Nb.sub.0.018).sub.7.8, 
EQU Sm.sub.0.7 Ce.sub.0.3 (Co.sub.bal Cu.sub.0.06 Fe.sub.0.24 
Zr.sub.0.02).sub.7.8 and 
EQU Sm.sub.0.5 Pr.sub.0.5 (Co.sub.bal Fe.sub.0.3 Cu.sub.0.07 Zr0.02).sub.7.6. 
The proportion of rare-earth metal in these compositions is between about 
20 and 30% by weight and therefore this type of rare earth magnet 
conserves resources as compared with the intermetallic compound magnets 
described in paragraph 1; and 
3. Intermetallic compound magnets including at least one rare-earth metal 
in an amount between about 8 and 18 atomic percent, iron or other 
transition metal in an amount between about 73 and 88 atomic percent and 
boron or other sub-metal or semiconductor element such as arsenic, 
antimony, bismuth, boron, carbon, silicon, phosphorus or selenium in an 
amount between about 4 and 9 atomic percent. These magnets have formulas 
such as Nd.sub.15 Fe.sub.77 B.sub.8, Nd.sub.15 Fe.sub.73 Co.sub.4 B.sub.8, 
Pr.sub.15 Fe.sub.77 B.sub.8 and Pr.sub.15 Fe.sub.80 B.sub.5. These magnets 
have the best performance of all of the magnets since they have a large 
saturation magnetization (4.pi.Is) and a large anisotropic magnetic field 
(Ha). 
The invention will be better understood with reference to the following 
examples. These examples are presented for purposes of illustration only 
and are not intended to be construed in a limiting sense.

EXAMPLE 1 
A composition having the atomic percentages Nd.sub.14 Fe.sub.80 B.sub.6 was 
used to form a permanent magnet material. Rapidly quenched thin fragments 
of the composition were pulverized to a particle diameter of less than 
about 177 .mu.m. An epoxy resin was admixed with the magnetic material and 
the mixture was press-molded to obtain a molded body. The molded body was 
heat treated at a temperature of 155.degree. C. for about 1 hour in order 
to obtain a powder bonded permanent magnet. The powder bonded permanent 
magnet was coated with the coating materials and in the thicknesses shown 
in Table 1. 
TABLE 1 
______________________________________ 
Sample Weight Thickness of 
No. Coating Material 
Ratio Coating Layer (.mu.m) 
______________________________________ 
1 epoxy resin/PTFE 
99:1 10 
2 epoxy resin/PTFE 
98:2 8 
3 epoxy resin/PTFE 
95:5 0.5 
4 epoxy resin/PTFE 
80:20 10 
5 epoxy resin/PTFE 
70:30 20 
6 phenol resin/PCTFE 
60:40 10 
7 phenol resin/PFA 
60:40 12 
8 phenol resin/FEP 
55:45 11 
9 phenol resin/ETFE 
60:40 9 
10 phenol resin/PCTFE 
60:40 10 
11 phenol resin/PTFE 
25:75 15 
12 none -- -- 
______________________________________ 
The magnet materials had the following magnetic properties: 
BH.sub.max =7.6 MGO.sub.e ; 
Br=5.9 kG; 
iHc=15.4 kOe; 
bHc=5.3 kOe; and 
density=6.3 g/cm.sup.3 
Each of samples 1-12 were exposed at a constant temperature of 60.degree. 
C. and a constant humidity of 95% for about 1500 hours. The magnetic 
properties and appearance of the exposed samples are shown in Table 2. 
TABLE 2 
______________________________________ 
Corro- 
Sam- sion 
ple Magnetic Properties Condi- 
No. Br(kG) bHc(kOe) iHc(kOe) 
BHmax(MGOe) 
tion 
______________________________________ 
1 5.4 4.9 14.9 6.6 Partial 
2 5.6 5.0 15.0 7.0 Minimal 
3 5.3 4.8 14.8 6.5 Minimal 
4 5.7 5.3 15.3 7.5 None 
5 5.6 5.1 15.2 7.5 None 
6 5.7 5.2 14.8 7.6 None 
7 5.7 5.1 15.1 7.4 None 
8 5.8 5.2 15.2 7.6 None 
9 5.7 5.2 15.1 7.5 None 
10 5.7 5.2 15.3 7.5 None 
11 5.4 4.8 14.8 6.5 Partial 
12 5.0 4.6 14.5 5.9 Complete 
______________________________________ 
As can be seen, samples 2-10 which had coating layers in accordance with 
the invention maintained their magnetic properties significantly better 
than samples 1, 11 and 12. In sample 1, the fluorine resin was used in an 
amount of less than about 2% by weight and in sample 11 the fluorine resin 
was used in an amount greater than about 70% by weight. Accordingly, each 
of samples 1 and 11 exhibited poor corrosion resistance. The worst 
corrosion resistance was exhibited by sample 12 which did not have a 
coating layer. 
EXAMPLE 2 
A thin coating layer of Nd.sub.0.14 (Fe.sub.0.94 B.sub.0.06).sub.0.86 alloy 
obtained by a rapidly quenched thin film producing process was pulverized 
in a ball mill to yield magnetic particles having a thickness of about 177 
.mu.m. Between about 1 and 3% by weight epoxy resin bonding material was 
added to the magnetic particles and the magnetic particles and epoxy resin 
were milled. The milled magnetic particles and epoxy resin were 
press-molded to obtain a predetermined molded body. The molded body was 
cure treated at a temperature of about 150.degree. C. for one hour in 
order to harden the molded body and yield a magnet. 
The magnet was washed with trichloroethylene and PTFE was sprayed on the 
magnet. The sprayed magnet was sintered at a temperature of about 
150.degree. C. for one hour in order to obtain a magnet with a thin 
coating layer of about 5 .mu.m thickness. PTFE was sprayed on the magnet a 
second time to increase the thickness of the coating layer to about 10 
.mu.m. The magnet was maintained at a temperature of about 60.degree. C. 
and a relative humidity of about 95% for varying periods of time. Table 3 
shows the condition of the magnet and of a comparative sample having no 
thin coating film after each period of time. 
TABLE 3 
______________________________________ 
Exposure Time 
Sample 10 Hours 100 Hours 500 Hours 
______________________________________ 
Present 
Example No Corrosion 
No Corrosion 
No Corrosion 
Comparative 
Completely Completely Completely 
Example Corroded Corroded Corroded 
______________________________________ 
As can be seen from Table 3, high corrosion resistance was observed when a 
powder bonded permanent magnet was coated with a fluorine resin coating. 
The uncoated permanent magnet had no corrosion resistance. 
EXAMPLE 3 
Powder bonded magnets were produced as described in Example 1. The magnets 
were coated with the fluorine resins FEP, PCTFE and PVDF to a thickness of 
10 .mu.m. The coated magnets were exposed at a temperature of about 
60.degree. C. and relative humidity of about 95% in order to test their 
corrosion resistance. The results are shown in Table 4. 
TABLE 4 
______________________________________ 
Exposure Time 
Sample 10 Hours 100 Hours 500 Hours 
______________________________________ 
FEP No Corrosion 
No Corrosion 
No Corrosion 
PCTFE No Corrosion 
No Corrosion 
No Corrosion 
PVDF No Corrosion 
No Corrosion 
No Corrosion 
______________________________________ 
As can be seen, the fluorine resin coating on the powder bonded permanent 
magnets provided the magnets with a high degree of corrosion resistance. 
EXAMPLE 4 
Powder bonded permanent magnets were produced as described in Example 1. 
The magnets were repeatedly coated with fluorine resin to obtain coatings 
having thicknesses of 0.5 .mu.m, 1 .mu.m, 10 .mu.m, 30 .mu.m, 50 .mu.m and 
70 .mu.m, respectively. The coated magnets were exposed at a temperature 
of about 60.degree. C. and a relative humidity of about 95%. The results 
are shown in Table 5. 
TABLE 5 
______________________________________ 
Thickness of 
Exposure Time 
Coating Film 
10 Hours 100 Hours 500 Hours 
______________________________________ 
Complete Complete Complete 
0.5 .mu.m Corrosion Corrosion Corrosion 
Partial Complete Complete 
1.0 .mu.m Corrosion Corrosion Corrosion 
Partial 
10.0 .mu.m 
No Corrosion 
No Corrosion 
Corrosion 
Partial 
30.0 .mu.m 
No Corrosion 
No Corrosion 
Corrosion 
50.0 .mu.m 
No Corrosion 
No Corrosion 
No Corrosion 
70.0 .mu.m 
No Corrosion 
No Corrosion 
No Corrosion 
______________________________________ 
As can be seen from Table 5, when the thickness of the coating layer was 
less than about 1 .mu.m, it was not possible to obtain sufficient 
corrosion resistance for the coating to be practical. On the other hand, 
when the thickness of the coating layer was greater than about 50 .mu.m, 
excellent corrosion resistance was provided. However, thicknesses of 
greater than about 50 .mu.m are expensive and not practical from the point 
of view of cost. Fluorine resin coating layer thicknesses between about 1 
and 50 .mu.m are preferred in accordance with the invention. 
EXAMPLE 5 
Two solutions of fluorine resins having different densities were prepared. 
A first powder bonded permanent magnet was coated once with one of the 
solutions to provide a coating layer having a thickness of about 10 .mu.m. 
A second powder bonded permanent magnet was coated 3 times with the other 
solution to obtain a 10 .mu.m thick coating layer. A test was conducted as 
described in Example 3 and the results are shown in Table 6. 
TABLE 6 
______________________________________ 
Exposure Time 
10 Hours 100 Hours 500 Hours 
______________________________________ 
Single layer Partial Partial 
coating No Corrosion 
Corrosion Corrosion 
Three layer 
coating No Corrosion 
No Corrosion 
No Corrosion 
______________________________________ 
As can be seen from Table 6, a coating layer obtained by repeated coating 
processes had superior corrosion resistance when two different coating 
layers having the same thickness were compared. This is due to generation 
of pin-holes in the coating layer resulting from vaporization of the 
solvent during the drying process. However, the pin-holes were filled when 
the coating process was repeated several times. 
EXAMPLE 6 
Several types of rare-earth magnets were prepared and coated with layers of 
epoxy resin, PTFE or PFA. The coated magnets were maintained at a 
temperature of about 40.degree. C. and a relative humidity of about 95% 
for 500 hours and the condition of the magnets was observed. The results 
are shown in Table 7. 
TABLE 7 
__________________________________________________________________________ 
Sample Thickness of 
40.degree. C. .times. 95% RH .times. 
No. Magnet Composition Layer (.mu.m) 
500 Hours 
__________________________________________________________________________ 
S-1 Sintered SmCo.sub.5 
12 No Corrosion 
Sintered 
S-2 Sm(Co.sub.bal Cu.sub.0.06 Fe.sub.0.16 Zr.sub.0.01).sub.7.6 
14 No Corrosion 
Sintered 
S-5 Sm.sub.6.6 Ce.sub.0.4 (Co.sub.bal Cu.sub.0.06 Fe.sub.0.18 Zr.sub.0. 
012).sub.7.4 16 No Corrosion 
S-7 Sintered Nd.sub.15 Fe.sub.78 B.sub.7 
15 No Corrosion 
S-12 Bonded Sm(Co.sub.bal Cu.sub.0.08 Fe.sub.0.02 Zr.sub.0.028).sub.8.3 
15 No Corrosion 
S-15 Bonded SmCo.sub.5 14 No Corrosion 
Comparative Complete 
Test Sintered Nd.sub.15 Fe.sub.78 B.sub.7 
None Corrosion 
Comparative Partial 
Test Bonded Sm(Co.sub.bal Cu.sub.0.08 Fe.sub.0.02 Zr.sub.0.028).sub.8.3 
None Corrosion 
__________________________________________________________________________ 
As can be seen from Table 7, no difference in corrosion resistance was 
observed as a function of whether the magnets were sintered or powder 
bonded or as a function of the composition of the magnet. Superior effects 
were obtained by coating the powder bonded permanent magnets with organic 
resins for interrupting the flow of air and gasses to the magnet. Rust 
generation was prevented and the surface of the magnet was not damaged by 
loss of particles. 
EXAMPLE 7 
Rapidly quenched thin ribbon fragments of an Nd.sub.13 Fe.sub.77 Co.sub.4 
B.sub.6 composition were pulverized to a particle diameter of less than 
about 100 .mu.m. An epoxy resin was mixed with the pulverized fragments 
and the mixture was press-molded to obtain a molded body. The molded body 
was heat treated at a temperature of about 125.degree. C. for about 1 hour 
to obtain powder bonded permanent magnets. Then, epoxy resin is mixed with 
the permanent magnets. The powder bonded permanent magnets were coated 
with the coating materials shown in Table 8 to the thicknesses shown. 
TABLE 8 
______________________________________ 
Sample Thickness of 
No. Coating Material 
Ratio Coating Layer (.mu.m) 
______________________________________ 
21 epoxy resin/PTFE 
99:1 10 
22 epoxy resin/PTFE 
98:2 8 
23 epoxy resin/PTFE 
95:5 0.8 
24 epoxy resin/PTFE 
80:20 10 
25 epoxy resin/PCTFE 
70:30 18 
26 phenol resin/PTFE 
60:40 10 
27 phenol resin/PFA 
50:50 12 
28 phenol resin/FEP 
60:40 20 
29 phenol resin/ETFE 
55:45 9 
30 phenol resin/PCTFE 
60:40 11 
31 phenol resin/PTFE 
25:75 15 
32 none -- -- 
______________________________________ 
The magnets had the following magnetic properties: 
BH.sub.max =11.0 MGOe; 
Br=7.2 kG; 
iHc=9.8 kOe; 
bHC=5.0 kOe; and 
density=6.4 g/cm.sup.3 
Each of samples 21-32 were exposed at a constant temperature of 60.degree. 
C. and a constant relative humidity of 95% for about 1500 hours. The 
magnetic properties and appearance of the exposed samples are shown in 
Table 9. 
TABLE 9 
______________________________________ 
Corro- 
Sam- sion 
ple Magnetic Properties Condi- 
No. Br(kG) bHc(kOe) iHc(kOe) 
BHmax(MGOe) 
tion 
______________________________________ 
21 6.8 4.6 9.4 8.6 Partial 
22 6.8 4.8 9.7 8.9 Minimal 
23 6.9 4.8 9.6 9.2 Minimal 
24 7.1 5.0 9.8 10.7 None 
25 7.2 4.9 9.7 10.9 None 
26 7.1 5.1 9.9 11.0 None 
27 7.1 4.8 10.0 10.9 None 
28 7.2 4.9 9.8 10.9 None 
29 7.2 4.8 9.8 10.8 None 
30 7.2 5.0 9.8 10.9 None 
31 6.9 4.8 9.5 8.5 Partial 
32 6.5 4.5 9.2 7.9 Complete 
______________________________________ 
As can be seen from Table 9, Sample 21 which had less than about 2% by 
weight fluoroplastic and Sample 31 which had greater than about 70% by 
weight fluoroplastic in the coating composition had poor corrosion 
resistance. Sample 32 which had no coating also had no corrosion 
resistance. Samples 22-30 having between about 2 and 75% by weight organic 
resin in the coating composition performed well even after 1500 hours. 
EXAMPLE 8 
An Nd.sub.0.14 (Fe.sub.0.89 Co.sub.0.05 B.sub.0.06).sub.0.86 alloy 
composition obtained by a rapidly quenched thin film producing process was 
pulverized in a ball mill to obtain magnetic particles having a thickness 
of about 90 .mu.m. Between about 1 and 3% by weight epoxy resin was added 
to the magnetic particles and the mixture was milled. The milled magnetic 
particles were press-molded to yield a molded body. The molded body was 
cure treated at a temperature of about 150.degree. C. for about one hour 
in order to harden the body and obtain a magnet. 
The magnets were washed with trichloroethylene and sprayed with PTFE. The 
PTFE coated magnets were sintered at a temperature of about 150.degree. C. 
for about one hour and a thin coating layer having a thickness of about 5 
.mu.m was obtained on the magnet. PTFE was sprayed a second time to 
provide a thin coating layer having a thickness of about 10 .mu.m. The 
magnet was compared with a comparative sample which did not have a coating 
layer. The coated magnet and comparative sample were maintained at a 
temperature of about 60.degree. C. and relative humidity of about 95%. The 
results are shown in Table 10. 
TABLE 10 
______________________________________ 
Exposure Time 
Sample 10 Hours 100 Hours 500 Hours 
______________________________________ 
Present 
Example No Corrosion 
No Corrosion 
No Corrosion 
Comparative 
Completely Completely Completely 
Example Corroded Corroded Corroded 
______________________________________ 
As can be seen from Table 10, a high degree of corrosion resistance was 
observed when a powder bonded permanent magnet was coated with PTFE. 
EXAMPLE 9 
Powder bonded permanent magnets were produced as described in Example 7. 
The magnets were coated with FEP, PCTFE and PVDF, respectively, to a 
thickness of 10 .mu.m. The coated magnets were exposed at a temperature of 
about 60.degree. C. and a relative humidity of about 95% in order to test 
the corrosion resistance. The results are shown in Table 11. 
TABLE 11 
______________________________________ 
Exposure Time 
Sample 10 Hours 100 Hours 500 Hours 
______________________________________ 
FEP No Corrosion 
No Corrosion 
No Corrosion 
PCTFE No Corrosion 
No Corrosion 
No Corrosion 
PVDF No Corrosion 
No Corrosion 
No Corrosion 
______________________________________ 
As can be seen from Table 11, high corrosion resistance was observed when 
magnets were coated with FEP, PCTFE or PVDF. 
EXAMPLE 10 
Powder bonded permanent magnets were produced as described in Example 7. 
The magnets were repeatedly coated with fluorine resin to obtain fluorine 
resin coating layers having thicknesses of 0.5 .mu.m, 1 .mu.m, 10 .mu.m, 
30 .mu.m, 50 .mu.m and 70 .mu.m, respectively. The magnets were exposed at 
a temperature of about 60.degree. C. and a relative humidity of about 95%. 
The results are shown in Table 12. 
TABLE 12 
______________________________________ 
Thickness of 
Exposure Time 
Coating Film 
10 Hours 100 Hours 500 Hours 
______________________________________ 
Complete Complete Complete 
0.5 .mu.m Corrosion Corrosion Corrosion 
No Partial Complete 
1.0 .mu.m Corrosion Corrosion Corrosion 
10 .mu.m No Corrosion 
No Corrosion 
No Corrosion 
Partial 
30 .mu.m No Corrosion 
No Corrosion 
Corrosion 
50 .mu.m No Corrosion 
No Corrosion 
No Corrosion 
70 .mu.m No Corrosion 
No Corrosion 
No Corrosion 
______________________________________ 
As can be seen from Table 12, when the thickness of the coating layer was 
less than or equal to about 1 .mu.m, good corrosion resistance was not 
observed. When the thickness of the coating layer was greater than about 
50 .mu.m, good corrosion resistance was observed, but thicknesses greater 
than about 50 .mu.m are not practical from the point of view of 
manufacturing cost. 
EXAMPLE 11 
Rapidly quenched thin fragments having the composition Nd.sub.13 Fe.sub.74 
Co.sub.7 B.sub.6 were pulverized to a particle diameter of less than about 
120 .mu.m. The particles were mixed with an epoxy resin and pressmolded to 
obtain a molded body. The molded body was heat treated at a temperature of 
160.degree. C. for about 1 hour to obtain a powder bonded permanent 
magnet. The powder bonded permanent magnet was coated with the coating 
materials shown in Table 13. 
TABLE 13 
______________________________________ 
Sample Weight Thickness of 
No. Coating Material Ratio Coating Layer (.mu.m) 
______________________________________ 
41 polyester resin/PTFE 
99:1 12 
42 polyester resin/PTFE 
98:2 7 
43 polyester resin/PTFE 
95:5 0.6 
44 polyester resin/PTFE 
80:20 10 
45 polyester resin/PCTFE 
70:30 15 
46 polyester resin/PTFE 
60:40 11 
47 phenol:epoxy(50:50)/PFA 
60:40 13 
48 polyester:epoxy(50:50)/ 
FEP 60:40 20 
49 polyester:phenol(40:60)/ 
ETFE 60:40 8 
50 phenol:polyester:epoxy 
(20:30:50)/PCTFE 60:40 10 
51 phenol:polyester:epoxy 
(20:30:50)/PTFE 25:75 13 
52 none -- -- 
______________________________________ 
The magnets had the following magnetic properties: 
BH.sub.max =11.5 MGOe; 
Br=7.4 kG; 
iHc=9.4 kOe; 
bHc=4.8 kOe; and 
density=6.6 g/cm.sup.3. 
Each of samples 41 to 52 was exposed at a constant temperature of about 
60.degree. C. and constant relative humidity of about 95% for about 1500 
hours. The magnetic properties and appearance of the exposed samples are 
shown in Table 14. 
TABLE 14 
______________________________________ 
Corro- 
Sam- sion 
ple Magnetic Properties Condi- 
No. Br(kG) bHc(kOe) iHc(kOe) 
BHmax(MGOe) 
tion 
______________________________________ 
41 6.9 4.7 8.5 9.6 Partial 
42 7.0 4.7 8.9 10.0 Minimal 
43 6.9 4.7 8.8 10.4 Minimal 
44 7.4 4.9 9.1 11.4 None 
45 7.3 4.8 9.0 11.5 None 
46 7.4 4.8 8.9 11.3 None 
47 7.4 4.9 9.2 11.3 None 
48 7.4 4.9 9.1 11.2 None 
49 7.4 4.8 9.1 11.4 None 
50 7.4 4.8 9.2 11.4 None 
51 7.1 4.7 8.9 10.5 Partial 
52 6.6 4.7 8.0 8.9 Complete 
______________________________________ 
As can be seen from Table 14, Sample 41 had an amount of polyester resin in 
the coating composition of less than about 2% by weight and Sample 51 had 
an amount of phenol:polyester:epoxy resin of greater than about 70% by 
weight. Both of these samples exhibited poor corrosion resistance. Sample 
52 which had no coating layer also had no corrosion resistance. The amount 
of epoxy resin in the coating composition is preferably between about 2 
and 70% by weight. 
Permanent magnets coated with waterproof organic coating compositions in 
accordance with the invention have a high degree of corrosion resistance 
and are suitable for practical use. The magnet material can be powder 
bonded, sintered or cast and the organic resin coating preferably includes 
a fluorine resin which can be used alone or mixed with an additional 
organic resin such as an epoxy, polyester or phenol resin. If the fluorine 
resin is mixed with an additional organic resin, the amount of fluorine 
resin should be between about 2 and 70% by weight of the total coating 
mixture. 
It is possible to achieve a high degree of reliability for an extended 
period of time when these permanent magnets are used in speakers, motors, 
meters and the like. Stability is also achieved. It is also possible to 
provide magnetic circuits having a high degree of accuracy and high 
efficiency. 
Magnets provided in accordance with the invention have a high temperature 
and corrosion resistance and accordingly, a broad range of applications. 
The organic coating compositions prepared in accordance with the invention 
prevent magnetic particles from being dislodged from the magnet and 
prevent cracking of the magnet. Additionally, the stability and resistance 
of the magnet to heat as well as the strength of a device in which such 
magnets are utilized can be enhanced. 
It will thus be seen that the objects set forth above, among those made 
apparent from the preceding description, are efficiently attained and, 
since certain changes may be made in the above article without departing 
from the spirit and scope of the invention, it is intended that all matter 
contained in the above description shall be interpreted as illustrative 
and not in a limiting sense. 
It is also to be understood that the following claims are intended to cover 
all of the generic and specific features of the invention herein described 
and all statements of the scope of the invention which, as a matter of 
language, might be said to fall therebetween. 
Particularly it is to be understood that in said claims, ingredients or 
compounds recited in the singular are intended to include compatible 
mixtures of such ingredients wherever the sense permits.