Insulated conductor having adhesive overcoat

Disclosed is a conductor coated with a fused and cured powder applied insulating coating over which is a coating of an ultraviolet B-stageable, thermally C-stageable liquid resin. The coating is cured to the B-stage and strands of the conductor are placed side-by-side. The coating is then cured to the C-stage which fuses the strands of the conductor into a solid mass. The adhesive coated conductors may be used to form transformer coils, transposed cables, or other articles.

BACKGROUND OF THE INVENTION 
Electrical failure of transformer coils, transposed cables, and other 
electrical equipment can occur when short circuit forces or mechanical 
abuse damage insulation. The mechanical strength of insulated coils and 
cables can be increased by bonding the individual insulated electrical 
conductors together into a single mass. This has been accomplished by 
placing adhesive coated paper in between the layers of conductors. 
However, while this increases strength and reduces insulation damage, it 
also increases the cellulosic content of the electrical apparatus. 
Cellulose is undesirable if certain dielectric fluids are present because 
they react with cellulose to produce compounds which increase the 
conductivity of the fluid. Another technique for forming a single mass out 
of multiple conductors is to cover the conductors with an adhesive either 
before or after the conductors have been juxtaposed. While this technique 
has also worked, it involves an additional step, and difficulties may be 
encountered in obtaining good adhesion between the insulation and the 
adhesive. 
SUMMARY OF THE INVENTION 
We have discovered a method of preparing an adhesively coated insulated 
conductor which can be bonded to itself to form a solid mass which is 
resistant to electrical stress and mechanical abuse. The adhesively coated 
conductor according to this invention can be made in a single pass in a 
manufacturing process that requires very little space. The adhesive 
overcoat can be rapidly cured to the B-stage with ultraviolet light (UV) 
which requires less energy than a thermal cure. It can then be easily 
thermally cured to C-stage (i.e., completely cured) once the conductors 
have been formed into a coil or cable. Since the overcoat is 100% solids 
no solvent is evolved during curing and thus problems of air pollution and 
the collection and containment of vaporized solvent are avoided. The 
adhesive coated conductor has an excellent shelf life and can be stored 
for long periods of time prior to use. The overcoat does not flake and 
adds to the insulating qualities of the undercoat. 
While ultraviolet curable compositions are not meant to be cured by heat, 
we have found that such compositions can be very usefully adapted to 
producing adhesive coatings by only partially curing them with ultraviolet 
light and later completing the cure with heat. In spite of this unusual 
use of UV compositions, we have obtained excellent adherence between 
bonded conductors, and no adverse reactions have been observed that have 
lowered electrical or mechanical properties. 
Surprisingly, we have discovered that the overcoat has a synergistic 
interaction with a powder coated undercoat in that the dielectric strength 
of the overcoat on top of the undercoat is greater than the sum of the 
dielectric strengths of the two coatings by themselves. 
RELEVANT ART 
U.S. Pat. Nos. 3,619,259 and 3,911,202 disclose UV polymerization of 
continuous films which may be used for the purpose of electrical 
insulation. 
U.S. Pat. Nos. 4,184,001 and 4,268,659 disclose UV curable compositions 
specifically for use as insulation of electric wires. 
U.S. Pat. No. 4,317,858 discloses a UV curable adhesive. 
U.S. Pat. Nos. 4,032,673 and 4,239,077 disclose UV curable resins for use 
in transformer coils.

In the drawing, conductors 1 are covered with a powder-coated insulation 2 
over which has been applied a liquid resin 3 which has been B-staged with 
ultraviolet light at 4. At 5, the B-staged resin on adjacent strands of 
the conductor has been C-staged forming a solid mass 6. 
The conductor used in this invention may be of any material, though it is 
typically a metal such as copper or aluminum. The conductor may be round 
or rectangular wire or strip. 
An insulating coating is applied over the conductor. The coating can be of 
almost any resin including epoxies, polyamides, polysulfones, polyester 
amides, and other resins. The coating is preferably an epoxy because those 
resins have the best combination of electrical, chemical, and mechanical 
properties for use in transformers. (See, for example, U.S. Pat. Nos. 
4,088,809 and 4,241,101.) The coating must be applied by a powder coating 
technique such as a fluidized bed, an electrostatic fluidized bed, or an 
electrostatic spray gun. After the coating has been applied to the 
conductor, it is fully cured. The coating may be of any thickness but it 
is typically about 3 to about 8 mils. A description of a suitable powder 
coating process can be found in U.S. Pat. No. 4,127,695, herein 
incorporated by reference. Since the powder coating is typically cured by 
heat, it may be desirable to cool the coating in water or air prior to 
coating with the UV adhesive. Cooling may be desirable if the wire is to 
be wound on a spool before the UV adhesive is applied, but if the UV 
adhesive is to be applied immediately, it may be desirable to have the 
powder coated wire warm so as to aid in the flow of the viscous UV 
adhesive. However, the powder coated wire should not be so hot that it 
cures the UV adhesive to the C-stage. It is, of course, preferable to coat 
the powder coated wire with the UV adhesive immediately in order to avoid 
the extra steps of winding and unwinding the conductor. 
The adhesive overcoat may be of any liquid resin which can be B-staged with 
ultraviolet light and thermally cured to the C-stage. This can be 
accomplished with an ultraviolet curable resin by only partially curing it 
to the B-stage and then completing the cure to the C-stage using heat. 
However, it is more desirable to use a specially prepared resin which has 
two components--a UV curable component and a heat-sensitive component. A 
two-component resin is easier to work with because the ultraviolet light 
can only cure it to the B-stage and thus it is not necessary to carefully 
control the exposure to ultraviolet light as it would be if the 
ultraviolet light could cure the resin all the way to the C-stage. An 
example of a two-component resin is given in Example 1, Composition A. 
Another suitable ultraviolet curable adhesive is described by F. A. 
Sattler in U.S. Pat. No. 4,317,858. 
The adhesive overcoat must be a liquid, and it is preferably 100% solids to 
reduce air pollution and the cost of recovering solvents. Suitable resins 
include acrylated epoxies, cationic photo-initiated epoxies, thiol-polyene 
systems, and acrylo-urethanes. Acrylated epoxies are preferred as they 
have the best properties. A resin can be applied by reverse roller 
coating, by dipping and passing through a die or a wiper of leather or 
felt, or other technique. Reverse roller coating is preferred as it 
produces a thinner and more easily controlled coating. 
After the adhesive overcoat has been applied, it is cured to the B-stage. 
The B-stage is the point at which the coating becomes dry, tack free, and 
nonblocking. This occurs when the resin is cured past its gelation point. 
The cure to the B-stage is accomplished using ultraviolet light of a 
frequency and intensity which depend upon the particular composition used 
and the speed with which the conductor passes under the light. After the 
adhesive overcoat has been cured to the B-stage, the conductor can be 
wound onto reels or it can be used immediately. The B-staged coating can 
be of any thickness, but it is preferably about 0.25 to about 11/2 mils as 
a thinner coating has a poor bond strength and a thicker coating uses too 
much space. 
In the next step of this invention strands of the conductor are placed 
side-by-side. The conductors with the adhesive overcoat may be used to 
form transformer coils, motor coils, transposed cables, or other 
structures where the fusion of adjacent conductors into a solid mass would 
be desirable. 
In the final step of this invention the adhesive overcoat is heated to 
complete its cure to the C-stage. The temperature and time required to 
complete the cure will depend upon the particular adhesive overcoat 
composition that is used. 
The following examples further illustrate this invention. 
EXAMPLE 1 
A 0.114.times.0.289 inch rectangular aluminum wire was powder coated with 
an epoxy powder coating resin described in the example of U.S. Pat. No. 
4,241,101 or 4,088,809, herein incorporated by reference. The powder 
coating was cured in a wire tower at a speed of 10-50 ft/min and a tower 
temperature of 300.degree.-450.degree. C. and had a thickness of 3 to 8 
mils. After the powder coated wire had been fused and cured, short lengths 
of the powder coated wire were cut and an adhesive overcoat was brushed 
onto the wire by hand and cured to the B-stage under ultraviolet 
radiation. The following ultraviolet curable overcoats were used. 
______________________________________ 
Composition 
(parts by weight) 
Ingredients A B C 
______________________________________ 
Solid diglycidyl ether of bisphenol A 
55.3 56.4 55.7 
sold by Dow Chemical Co. as "DER 662" 
Tetraethylene glycol diacrylate 
33.0 33.7 33.6 
Triethanolamine borate in phenoxyethyl 
8.3 8.4 8.2 
acrylate sold by Westinghouse as "WT-17" 
Isopropyl benzoin ether sold by 
1.3 1.4 -- 
Stouffer as "V-10" photoinitiator 
Isobutyl benzoin ether sold by 
-- -- 1.4 
Stouffer as "V-30" photoinitiator 
Fluorinated alkyl ester sold by 3M 
2.1 -- 1.1 
as "FC-430" Surfactant 
Picric acid -- -- 0.01 
______________________________________ 
Three pieces of the adhesive coated wire were clamped together overlapping 
about 1/4 inch and were heated to 130.degree. for six hours in either air 
or in kerosene. After cooling the bonded samples were subjected to tensile 
shear testing (double lap shear testing) at temperatures from 25.degree. 
to 175.degree. C. The results are given in the following table. 
______________________________________ 
Overcoat 
Test Composition A 
Composition B 
Composition C 
Temp. (.degree.C.) 
Bld. 3.0-4.0 
Bld. 3.0-4.0 
Bld. 2.5-4.0 
______________________________________ 
Cured in 
Kerosene 
25 2663 
Cured in Air 
25 1276 1757 2000, 2708 
100 2486 
125 2351 
150 1537 
175 638 
______________________________________ 
This example shows that UV sensitive adhesives can be formulated and 
applied to powder coated conductors with good tensile shear strengths at 
temperatures as high as 175.degree. C. It also shows that bonding in 
kerosene does not adversely affect the bond strength of these adhesives. 
EXAMPLE II 
Rectangular aluminum wire (0.114.times.0.289 in) was coated with the powder 
disclosed in U.S. Pat. No. 4,241,101 in a wire tower, then cured and 
spooled. Short lengths (.about.12 in) were cut and straightened, then 
coated manually with two different UV sensitive adhesives. 
______________________________________ 
Composition A 
"DER 662" epoxy resin 47.1 pbw 
Limonene dioxide 31.3 pbw 
Methyl tetrahydrophthalic anhydride 
15.7 pbw 
Aliphatic triglycidyl ether sold by 
2.4 pbw 
Celanese as "5044" epoxy resin 
triaryl sulfonium hexafluoro phosphate 
5.1 pbw 
sold by 3M as "FC-508" photoinitiator 
Chromium acetylacetonate 0.04 pbw 
Composition B 
"DER 662" epoxy resin 45.5 pbw 
1,6-hexanediol diacrylate 
6.9 pbw 
2-ethoxyethyl acrylate 9.2 pbw 
Butyl glycidyl ether sold by Ciba 
5.0 pbw 
Geigy as "RD-1" diluent 
Diglycidyl ether of neopentyl glycol 
5.0 pbw 
Methyl tetrahydrophthalic anhydride 
15.0 pbw 
"V-30" photoinitiator 0.64 pbw 
Chromium acetylacetonate 0.04 pbw 
______________________________________ 
Wires overcoated with the above composition and B-staged were overlapped in 
pairs by a distance of 1 in. along their long axes and subjected to a 
pressure of 50 psi. The pairs were placed in a laboratory air circulating 
oven for 6 hours at 130.degree. C. to C-stage the adhesive overcoats. 
After cooling, the bond pairs were tested for lap shear strength at 
150.degree. C. The results were as follows (average of 5 samples): 
______________________________________ 
Overcoat Lap Shear Strength (psi) 
______________________________________ 
Composition A 58 
Composition B 154 
______________________________________ 
After the adhesive had been B-staged, some samples were shelf aged for a 
period of 3 months. 
The tensile shear test as described in Example I was repeated. The results 
were as follows (average of 5 samples): 
______________________________________ 
Overcoat Lap Shear Strength (psi) 
______________________________________ 
Composition A 51 
Composition B 150 
______________________________________ 
These results show that the UV adhesives of this invention can be applied 
to powder coated conductors and can retain their tensile shear strength 
(single lap shear test) after shelf aging for periods of at least 3 
months. 
EXAMPLE III 
Samples of 0.064.times.0.258 inch copper wire were coated with 4 mils of 
epoxy powder coating as in Example I. The samples were then coated with 
various adhesive overcoats including the same epoxy powder coating, 
Formvar and a UV composition which consisted of 
Acrylated epoxy--55.5% (50% phenoxyethyl acrylate); 
UV catalyst--2.5%; 
Hexamethoxymethyl melamine (sold by American Cyanamid as "Cymel 
303")--6.4%; 
Phenoxyethyl acrylate--6.4%; 
Vinyl acetate--8.4%; 
Epoxy novolac--6.4%; 
"WT-17"--6.0%; 
Benzodimethyl melamines--0.18% and blocked acrylated urethane--6.2%; 
Tetraethylene glycol diacrylate--1.8%; 
Catalyst 10-10 (manufactured by American Cyanamid)--0.03%; 
Iron or chromium acetylacetonates--0.03% 
The ultraviolet adhesive overcoat was prepared in the following manner: 
Three six inch samples were overlapped one inch and clamped together under 
a pressure of 10 psi. Beam tests were also performed on the samples. In a 
beam test, two beams 5 inches apart are placed under a stack of 5 wires 
bonded together and a third beam is pressed downward between the other two 
beams. The psi required to produce a failure are measured. The following 
table shows the results. 
______________________________________ 
Test Temperature 
Adhesive R. T. 120.degree. C. 
______________________________________ 
Powder Coated Epoxy 480 650 
Formvar* 830 430 
UV 872 736 
______________________________________ 
*a wire enamel which contains polyvinyl formyl as a base resin. Other 
resins such as phenols, blocked isocyanates, and melamine formaldehyde ar 
added, depending on the supplier. 
These results show that correctly formulated UV adhesives have beam shear 
strengths comparable to those of either powdered or solvent based 
adhesives when applied over powder coated conductors. 
EXAMPLE V 
A further benefit of using UV sensitive overcoats is a marked improvement 
in electric strength. A spool of 0.064.times.0.258 copper rectangular wire 
was coated with powder manufactured by HYSOL and finely ground. 
Example I was repeated and the dielectric strength of samples with and 
without UV overcoat were tested. The following table gives the results on 
0.064 by 0.258 inch rectangular copper wire. 
__________________________________________________________________________ 
Dielectric (K. Volts) 
1st sample 2nd sample 
Undercoat Overcoat 
#1 #2 #3 #4 #5 #6 #7 #8 Build (Mils) 
__________________________________________________________________________ 
A diglycidyl ether 
None 4.2 
3.5 
4.1 
2.2 
3.8 
3.8 
4.6 
3.4 
Side 1 - 4.5 Powder Thickness 
of bisphenol A epoxy 
avg. 
3.5 
Kv, avg. Kv/mil = 0.82 
Side 2 - 4.0 Powder Thickness 
power cured with 
trimellitic anhydride 
UV Adhesive 
4.8 
4.5 
4.9 
4.2 
3.7 
4.2 
4.3 
4.5 
Side 1 - 4.0 Total Thickness 
sold by 3M Company 
avg. 
4.4 
Kv, avg. Kv/mil = 1.11 
Side 2 - 3.9 Total Thickness 
Diglycidyl ether 
None 2.8 
0.3 
2.0 
4.8 
3.9 
0.6 
4.7 
3.9 
Side 1 - 3.0 
of bisphenol A avg. 
3.0 
Kv, avg. Kv/mil = 0.89 
Side 2 - 3.7 
epoxy power 
(See U.S. Pat. No. 
UV Adhesive 
4.5 
4.5 
5.0 
5.4 
4.9 
5.4 
5.0 
3.6 
Side 1 - 3.5 
4,241,101) avg. 
4.9 
Kv, avg. Kv/mil = 1.30 
Side 2 - 4.1 
__________________________________________________________________________ 
This experiment shows that the addition of a UVsensitive adhesive overcoa 
increases the electric strength in Kv/mil of a powder coated conductor by 
at least 20%. This is believed to be due to the initially liquid UV 
sensitive filling any voids or thinner areas in the powder coating. 
EXAMPLE VI 
Three samples of rectangular aluminum wire coated with the same powder used 
in Example I were dipped into a UV sensitive resin comprised of: 
______________________________________ 
"DER 662" epoxy resin 502.5 g 
"WT-17" 75.0 
Tetraethylene glycol diacrylate 
200.0 
Ethyl hexyl acrylate 150.0 
2-hydroxy ethyl acrylate 
37.5 
"V-10" photoinitiator 3.75 
Tert-butyl perbenzoate 3.75 
______________________________________ 
Excess resin was wiped off and the coating was irradiated for 8 minutes 
under a source of ultraviolet light. 
After the irradiation, the coating was dry to the touch and measured 3.5 
mil (addition to the thickness). 
The three samples were pressed together under nominal spring pressure (from 
a bulldog clip) at 150.degree. C. for 11/2 hours. 
A double lap-shear test gave a test value of 184 lbs. on the two adhered 
areas of 0.350.times.0.258 in., equivalent to 1020 psi.