Method for moistureproof coating of an electrical product

A method for moistureproof coating of an electrical product, which comprises applying an organic solution of a polyfluoroalkyl group-containing polymer dissolved in a non-combustible low boiling point organic solvent, to a surface of an electrical product, and drying it by evaporating the low boiling point organic solvent, to form a coating layer of the polyfluoroalkyl group-containing polymer on the surface of the electrical product, wherein said polyfluoroalkyl group-containing polymer is a copolymer of a polymerizable compound having a polyfluoroalkyl group with other polymerizable compound, and the copolymer has a glass transition temperature of not higher than 60.degree. C.

The present invention relates to a method for moistureproof coating which 
is useful for application to electrical products such as printed circuit 
boards for electrical or electronic devices. 
Reflecting the micro-computerization of electrical or electronic devices, 
printed circuit boards are now incorporated in various household 
electrical appliances and industrial appliances. In many cases, they are 
used under high humidity environmental conditions, and it frequently 
happens that due to the adsorption of moisture or deposition of condensed 
water on such printed circuit boards, a leakage current is generated to 
cause operational errors. 
In order to prevent such a leakage current, it is known to cover the 
printed circuit boards with a non-fluorine-type resin such as an acrylic 
resin, a urethane resin or an epoxy resin. It is also known to cover the 
boards with a fluorine-type homopolymer having polyfluoroalkyl groups from 
U.S. Pat. No. 3,931,454. It is usual to form a coating layer by applying 
such a resin in the form of an organic solution or a liquid resin, 
followed by drying or curing. 
Among conventional methods for moistureproofing, those employing 
non-fluorine type resins, require a considerable time for drying or 
curing, and thus have a problem from the viewpoint of the working 
efficiency. Further, organic solvents are in many cases combustible, and 
thus have a problem from the viewpoint of the danger of fire and labour 
hygiene. Furthermore, there is a drawback that the generation of a leakage 
current is remarkable if a substantial amount of condensed water is 
deposited on the coating layer. 
Thus, in the case of a moistureproof coating material made of a 
non-fluorine type resin such as an epoxy resin, a urethane resin or an 
acrylic resin, the moistureproof performance is not necessarily adequate, 
and it is usually required to provide a thick coating. Further, since a 
highly viscous coating material is employed, the coating layer is likely 
to have defects such as foams, and there is a problem also in the working 
efficiency, for instance, in the drying operation. With such a 
moistureproof coating material, it has been difficult to obtain an 
adequate moistureproof effect even when a thick coating is provided by 
means of a highly concentrated high viscosity coating material. 
Further, reflecting the recent miniaturization and compact assembly of 
electrical and electronic devices, the miniaturization and integration are 
in progress also in electrical products such as printed circuit boards, 
and the thick coating for moistureproof coating is problematic also from 
such a viewpoint. 
On the other hand, when a fluorine-type homopolymer is employed for such a 
coating, the moistureproof coating layer is desired to have good adhesion 
to the surface of the electrical product. The moistureproof coating layer 
of a fluorine-type homopolymer, it has a drawback that the adhesion to the 
substrate is poor. Electrical products which are required to have 
moistureproof and water resistant properties, are likely to be used under 
high temperature and high humidity environmental conditions. Under such 
environmental conditions, the adhesion properties are likely to 
deteriorate, and the coating of a fluorine-type homopolymer does not 
provide adequate adhesion. 
The present inventors have conducted extensive researches and studies with 
an aim to solve the above-mentioned problems, and have found that a 
polyfluoroalkyl group-containing copolymer such as the one obtained by 
polymerizing a methacrylate having a perfluoroalkyl group with other 
copolymerizable compound such as cyclohexyl methacrylate, is soluble in a 
non-combustible low boiling point organic solvent such as 
trichlorotrifluoroethane, and it is possible to conduct moistureproof 
coating of an electrical product smoothly and advantageously by means of 
the organic solution thereby obtained. From such an organic solution, it 
is possible to readily form a thin coating layer. Even with such a thin 
coating, a high level of moistureproof properties and water resistance can 
be imparted to the electrical product. Thus, by using such an organic 
solution, an excellent moistureproof coating layer can be formed under 
quick drying and non-combustible conditions. Furthermore, by using as the 
copolymerizable compound a polymerizable compound such as cyclohexyl 
methacrylate which gives a polymer having a glass transition temperature 
of not higher than 60.degree. C., it is possible to obtain not only the 
above-mentioned excellent properties but also good adhesion of the 
moistureproof coating layer to the surface of the electrical product. 
Thus, the present invention has been accomplished on the basis of the 
above discoveries. 
The present invention provides a method for moistureproof coating of an 
electrical product, which comprises applying an organic solution of a 
polyfluoroalkyl group-containing polymer dissolved in a non-combustible 
low boiling point organic solvent, to a surface of an electrical product, 
and drying it by evaporating the low boiling point organic solvent, to 
form a coating layer of the polyfluoroalkyl group-containing polymer on 
the surface of the electrical product, wherein said polyfluoroalkyl 
group-containing polymer is a copolymer of a polymerizable compound having 
a polyfluoroalkyl group with other polymerizable compound, and the 
copolymer has a glass transition temperature of not higher than 60.degree. 
C. 
Now, the present invention will be described with reference to the 
preferred embodiments. 
In the present invention, it is important to use a polyfluoroalkyl 
group-containing copolymer as the moistureproof coating material. By using 
such a moistureproof coating material, it is possible to impart a high 
level of waterproof properties even with a thin coating. Accordingly, it 
is possible to conduct the coating by means of an organic solution having 
a low concentration and a low viscosity, which is advantageous also from 
the view point of the operation efficiency. Further, the adhesion to the 
surface of the electrical product becomes extremely good, whereby 
stabilized moistureproof properties can be attained for a long period of 
time. 
As the polymerizable compound having a polyfluoroalkyl group, the following 
unsaturated esters are preferred. For instance, the following acrylates or 
methacrylates may be mentioned as suitable ones, which may be used alone 
or in combination as a mixture of two or more. 
##STR1## 
The compound copolymerizable with the above-mentioned polymerizable 
compound having a polyfluoroalkyl group, may be selected within a wide 
range so long as it does not bring about any deterioration of the desired 
properties. It is a polymerizable compound which gives a copolymer having 
a glass transition temperature of not higher than 60.degree. C. For 
instance, a wide range of compounds may be mentioned, including ethylene, 
vinyl acetate, vinyl chloride, vinyl fluoride, a vinylidene halide, 
styrene, .alpha.-methylstyrene, p-methylstyrene, acrylic acid and its 
alkyl exters, methacrylic acid and its alkyl esters, 
poly(oxyalkylene)acrylates, poly(oxyalkylene)methacrylates, acrylamide, 
methacrylamide, diacetone acrylamide and methacrylamide, methylol-modified 
diacetone acrylamide and methacrylamide, N-methylol acrylamide and 
methacrylamide, vinyl alkyl ethers, haloalkyl vinyl ethers, vinyl alkyl 
ketones, butadiene, isoprene, chloroprene, glycidyl acrylate and 
methacrylate, benzyl acrylate and methacrylate, cyclohexyl acrylate and 
methacrylate, 2-ethylhexyl acrylate and methacrylate, maleic anhydride, 
maleic acid and its alkyl esters, tetrahydrofurfuryl acrylate and 
methacrylate, aziridyl acrylate and methacrylate, dimethylaminoethyl 
acrylate and methacrylate, a polysiloxane-containing acrylate and 
methacrylate, and N-vinylcarbazole. These compounds may be used alone or 
in combination as a mixture of two or more. 
In the present invention, an organic solution of the above-mentioned 
polyfluoroalkyl group-containing copolymer dissolved in a non-combustible 
low boiling point organic solvent, is applied and dried to form a coating 
layer on the surface of an electrical product. In order to form an 
excellent moistureproof coating layer by the application and drying, it is 
preferred to use a copolymer having good coating layer-forming properties, 
as the polyfluoroalkyl group-containing copolymer. Further, it is desired 
that the formed moistureproof coating layer has good adhesion to the 
surface of the electrical product, and the moistureproof coating layer 
itself has high strength. From the viewpoint of such desired solubility, 
coating layer-forming ability and adhesion, a compound which is capable of 
providing a polymer having a low glass transition temperature of not 
higher than 60.degree. C., preferably not higher than 40.degree. C., is 
employed as said other compound copolymerizable with the polymerizable 
compound having a polyfluoroalkyl group. For instance, as such other 
polymerizable compound, there may be mentioned as a preferred example, a 
polymerizable compound such as ethyl methacrylate, cyclohexyl methacrylate 
or n-butyl methacrylate. The glass transition temperature is preferably at 
least 20.degree. C., since it is difficult to obtain a good coating layer 
if the glass transition temperature is too low. 
In the present invention, the polyfluoroalkyl group-containing copolymer 
preferably contains from 30 to 90% by weight, more preferably from 60 to 
80% by weight of the polymerizable compound having a polyfluoroalkyl 
group. Usually, the other compound copolymerizable with the 
polyfluoroalkyl group-containing polymerizable compound is incorporated in 
an amount of from 70 to 10% by weight, preferably from 40 to 20% by 
weight. 
As the non-combustible low boiling point organic solvent, there may be 
employed a solvent which is capable of dissolving the above-mentioned 
polyfluoroalkyl group-containing copolymer. Usually, a chlorine-type 
solvent such as methylchloroform or a fluorine-containing organic solvent 
is employed. Particularly preferred is a fluorine-containing organic 
solvent such as 1,1,2-trichloro-1,2,2-trifluoroethane or 
1,1-difluorotetrachloroethane having a boiling point of a level of from 
40.degree. to 100.degree. C. These organic solvents may be used as a 
solvent mixture as the case requires. Further, in order to improve the 
solubility of the polyfluoroalkyl group-containing copolymer, other 
organic solvents may be employed together so long as the non-combustion 
properties can be maintained. 
In the present invention, there is no particular restriction as to the 
concentration of the organic solution of the polyfluoroalkyl 
group-containing copolymer. However, the concentration is usually selected 
within a range of from 0.1 to 40% by weight, preferably from 3 to 15% by 
weight. If the concentration is too high, the viscosity of the solution 
tends to be excessive, whereby the coating operation will be difficult, 
and it will be difficult to obtain a uniform coating. On the other hand, 
if the concentration is too low, it will be difficult to obtain good 
moistureproof properties, and there will be a disadvantage also from the 
viewpoint of the efficiency of the drying operation for the recovery of a 
great amount of the solvent. 
In the method of the present invention, there is no particular restriction 
as to the means and conditions for coating and drying the organic 
solution, and there may be employed various means and conditions. For 
instance, a dipping method or a spraying method may be employed for the 
application of the organic solution to the surface of an electrical 
product, and the drying temperature may be at a level of from 10.degree. 
to 50.degree. C. 
Thus, a coating layer of the polyfluoroalkyl group-containing copolymer 
will be formed on the surface of the electrical product. The thickness of 
the coating layer is not particularly restricted, but is usually from 0.1 
to 50 .mu.m, preferably from 0.5 to 15 .mu.m, whereby it is possible to 
impart superior moistureproof properties as compared with the conventional 
thick coating layer for moistureproofing. 
The method of the present invention is applicable to the moistureproof 
coatings of various electrical products. For instance, it can be applied 
to a wide range of products including printed circuits boards, hybrid 
integrated circuits, resisters, capacitors, switches or connectors. 
In the present invention, the polyfluoroalkyl group-containing copolymer 
forms a coating layer on the surface of an electrical product such as a 
printed circuit board after the evaporation of the non-combustible low 
boiling point organic solvent, and thus serves for the protection from the 
moisture or water. The polyfluoroalkyl groups contained in such a 
copolymer, serve to impart waterproof properties to the coating layer and 
at the same time to improve the solubility to the non-combustible low 
boiling point organic solvent such as 
1,1,2-trichloro-1,2,2-trifluoroethane. The compound copolymerizable with 
the polymerizable compound containing a polyfluoroalkyl group, serves to 
improve the strength of the coating layer and the adhesion of the coating 
layer to the surface of the electrical product.

Now, the present invention will be described in further detail with 
reference to Examples. However, it should be understood that the present 
invention is by no means restricted by such specific Examples. 
EXAMPLES 1 to 8 and COMATIVE EXAMPLES 1 to 7 
A glass epoxy plate of 5.times.5 cm formed with comb-shaped electrodes with 
21 teeth at intervals of 0.7 mm, was dipped in a test solution as 
identified in Table 1, and then withdrawn and dried at room temperature to 
obtain a test piece. By using this test piece, the moistureproof 
properties and the waterproof properties of the coating layer were 
examined. The results are shown in Table 1. The methods for evaluation 
were as follows: 
Moistureproof properties: 
The test piece was left to stand in a constant temperature and constant 
moisture chamber at 60.degree. C. under a relative humidity of 95% for 500 
hours, and then the insulation resistance was measured. 
Waterproof properties: 
The test piece was immersed in distilled water at 25.degree. C. for 24 
hours, and then the insulation resistance was measured. 
Adhesion: 
The coating layer on the test piece surface was cut crosswise by a sharp 
blade with cut lines reaching the substrate surface and defining 100 
squares of 1 mm.sup.2, and then an adhesive tape was pressed on the 
coating layer under a prescribed pressure and immediately peeled, 
whereupon the number of squares of the coating layer remained was counted. 
TABLE 1 
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Copolymer composition 
(Numerical values 
Glass transition 
Concentration 
Moistureproof 
Waterproof 
represent % by weight 
temperature (.degree.C.) 
Solvent 
(% by weight) 
properties (.OMEGA.) 
properties 
Adhesion) 
__________________________________________________________________________ 
Example 1 
FA/MMA (60/40) 
54 R-113 
2.0 7 .times. 10.sup.10 
1 .times. 10.sup.12 
70/100 
Example 2 
FA/EMA (70/30) 
33 R-113 
2.0 2 .times. 10.sup.11 
8 .times. 10.sup.11 
100/100 
Example 3 
FA/BMA (80/20) 
20 R-113 
2.0 6 .times. 10.sup.10 
2 .times. 10.sup.11 
100/100 
Example 4 
FA/B.sub.z MA (70/30) 
30 R-113 
2.0 6 .times. 10.sup.10 
1 .times. 10.sup.12 
100/100 
Example 5 
FA/CyHMA (80/20) 
29 R-113 
2.0 7 .times. 10.sup.11 
2 .times. 10.sup.12 
100/100 
Example 6 
FA/CyHMA/DMAEMA 
28 R-113 
1.0 4 .times. 10.sup.11 
1 .times. 10.sup.12 
100/100 
(80/18/2) 
Example 7 
FA/CyHMA/DMAEMA 
28 R-113 
2.0 7 .times. 10.sup.11 
2 .times. 10.sup.12 
100/100 
(80/18/2) 
Example 8 
FA/CyHMA/DMAEMA 
28 R-113 
5.0 7 .times. 10.sup.11 
2 .times. 10.sup. 
100/100 
(80/18/2) 
Comparative 
FA homopolymer 
20 R-113 
2.0 1 .times. 10.sup.11 
2 .times. 10.sup.12 
50/100 
Example 1 
Comparative 
BMA homopolymer 
20 Toluene 
25.0 3 .times. 10.sup.9 
1 .times. 10.sup.9 
100/100 
Example 2 
Comparative 
EMA homopolymer 
65 Toluene 
30.0 7 .times. 10.sup.9 
4 .times. 10.sup.9 
50/100 
Example 3 
Comparative 
CyHMA/BMA (50/50) 
43 Toluene 
2.0 3 .times. 10.sup.9 
4 .times. 10.sup.9 
60/100 
Example 4 
Comparative 
CyHMA/BMA (50/50) 
43 Toluene 
30.0 1 .times. 10.sup.10 
1 .times. 10.sup.10 
70/100 
Example 5 
Comparative 
FA/MMA (50/50) 
65 R-113 
2.0 2 .times. 10.sup.10 
8 .times. 10.sup.11 
50/100 
Example 6 
Comparative 
Test piece with no coating 8 .times. 10.sup.7 
7 .times. 10.sup.7 
-- 
Example 7 
__________________________________________________________________________ 
FA . . . CH.sub.2 .dbd.CHCOOC.sub.2 H.sub.4 C.sub.9 F.sub.19 
MMA . . . Methyl methacrylate 
EMA . . . Ethyl methacrylate 
BMA . . . nButyl methacrylate 
B.sub.z MA . . . Benzyl methacrylate 
CyHMA . . . Cyclohexyl methacrylate 
DMAEMA . . . Dimethylaminoethyl methacrylate 
R113 . . . 1,1,2trichloro-1,2,2-trifluoroethane 
The moistureproof coating composition in the present invention has desired 
properties such as quick drying properties and non-combustible properties, 
and exhibits excellent characteristics also in the moistureproof 
properties and the waterproof properties. Further, the copolymer 
constituting the coating layer is soluble selectively in a solvent 
composed mainly of e.g. 1,1,2-trichloro-1,2,2-trifluoroethane, and hardly 
soluble or hardly eroded by other organic solvents. Further, the coating 
layer surface exhibits high water repellency, and even if condensed water 
is deposited, it is likely to be readily removed from the printed circuit 
board. Furthermore, the coating layer of the copolymer of the present 
invention, has extremely good adhesion.