Metallized polymer film capacitor having a high viscosity polyurethane oil insulating fluid

A metallized film capacitor is provided with a polyurethane oil insulating fluid. The insulating fluid has a viscosity in the range of about 500 to 3000 poise at 25.degree. C. The polyurethane oil insulating fluid is produced by reacting an organic polyisocyanate with a primary polyol selected from the group consisting of castor oil, ricinoleic acid derivatives of castor oil and mixtures thereof. The reaction may be carried out in the presence of a secondary polyol chain extender such as a hydroxy-terminated polybutadiene diol. The reaction to produce the insulating fluid is carried out under conditions wherein the ratio of NCO groups of the organic polyisocyanate to OH groups of the primary and secondary polyols, if present, is in the range of about 0.1 to 1 to about 0.6 to 1. The ratio ensures that there is no excess of unreacted NCO groups in the insulating fluid and that the reaction produces a viscous fluid, not a substantially solid elastomer.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to capacitors. More particularly 
the present invention relates to a metallized film capacitor having a high 
viscosity polyurethane oil insulating fluid surrounding at least a 
portion, and preferably all, of its capacitive element. 
2. Description of the Related Art 
Metallized film capacitors are typically used for starting and running 
motors and are also used in lighting applications. The capacitive element 
of a metallized film capacitor is typically formed by very tightly 
co-rolling two polymer films having metal layers deposited thereon around 
a core to form two spaced apart electrodes. Generally, the tightly 
co-rolled metallized polymer films are placed in a container, which is 
typically metal. Electrical leads are connected between the metal sprayed 
ends of the metallized polymer films and terminals mounted on a top. The 
container is filled with an insulating fluid and the top is sealed to the 
container. In some capacitors, insulative sheetlike material may be placed 
in the container between the co-rolled metallized polymer films and the 
metal container. Also, in some capacitors, a pressure activated circuit 
interrupter is employed. 
The insulating fluid must be compatible with the polymer film forming the 
metallized film capacitor element in the event that such fluid contacts 
the film. Compatibility with the polymer film means that the fluid must 
not act as a solvent towards the polymer film. It is believed that fluids 
that act as a solvent towards the polymer film, which is typically formed 
from a polyolefin such as polypropylene, can cause swelling and wrinkling 
of the film, which is undesirable because it can lead to premature 
capacitor failure. The insulating fluid should have good heat transfer 
capability, sufficient viscosity to impede leaking and suitable 
compatibility with outgassing byproducts caused by corona discharge and 
clearing phenomena. Also, the fluid should not attack the metal on the 
metallized polymer film. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a metallized film 
capacitor with an insulating fluid that is compatible with the materials 
forming the capacitive element of the metallized film capacitor. 
This object is accomplished, at least in part, by providing a metallized 
film capacitor formed, in part, by a container having a pair of terminals 
adapted for connection to an electrical circuit. A capacitive element, 
formed from a pair of tightly co-rolled metallized polymer films, is 
disposed in the container. A pair of electrical leads are electrically and 
operatively connected between the pair of metallized polymer films and the 
pair of terminals on the container. A polyurethane oil insulating fluid is 
disposed in the container to surround at least a portion of the capacitive 
element. The polyurethane oil insulating fluid has a viscosity in the 
range of about 500 to 3000 poise at 25.degree. C. The polyurethane oil 
insulating fluid is obtained by reacting an organic polyisocyanate with a 
polyol selected from the group consisting of castor oil, ricinoleic acid 
derivatives of castor oil, and mixtures thereof. If desired, the reaction 
can be carried out in the presence of a secondary polyol chain extender. 
The reaction to produce the insulating fluid is carried out under 
conditions wherein the ratio of NCO groups of the organic polyisocyanate 
to the OH groups of the primary and secondary polyols, if present, is in 
the range of about 0.1 to 1 to about 0.6 to 1.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention provides a metallized film capacitor having a 
polyurethane oil insulating fluid that is compatible with the polymer 
substrate and metal material forming the metallized film. Referring to 
FIGS. 1 and 2, there is shown a metallized film capacitor 10. The 
capacitor 10 is formed, in part, by a container 12 having an open top in 
which a capacitive element 14 is disposed. The container 12 may be 
cylindrically shaped as illustrated or formed to have any other shape that 
a particular application should require. As is well known in the art, the 
capacitive element 14 is formed by tightly co-rolling two polymer films 
16a, 16b that have metallized surfaces 18a, 18b, as illustrated, around a 
mandrel 19. Preferably, the polymer forming the films 16a, 16b is a 
polyolefin such as polypropylene, however, a polyester, such as Mylar.TM. 
could be used without departing from the scope of the present invention. 
Also, preferably, the metal for metallizing the surfaces of the polymer 
films 16a, 16b is zinc or a zinc alloy, however, aluminum may also be 
used. Metallized dielectric films suitable for forming the capacitive 
element in the present invention include: a film designated by the product 
code CM-ZTA2 produced by Toray Plastics America, Inc. of North Kingstown, 
Rhode Island; a film designated by the product code TERFOL PZXB produced 
by Tervakoski Dielectric, Ltd. of Tervakoski, Finland; and a film 
designated by the product code ZINC/HELV produced by Steinerfilm, Inc. of 
Williamstown, Mass. The metallized surfaces 18a, 18b of each of the 
polymer films 16a, 16b are laterally offset from each other so that the 
respective metallized surfaces 18a, 18b terminate at opposite edges of the 
capacitive element 14. Accordingly, each of the films 16a, 16b have metal 
free margins 20a, 20b at opposite ends of the capacitive element. The ends 
22a, 22b of the capacitive element 14 are typically covered with a metal, 
applied as a spray or solder. End 22a is in electrical contact with the 
metallized surface of film 16a while end 22b is in electrical contact with 
the metallized surface of film 16b. The capacitor 10 illustrated in FIG. 1 
also includes a lid 24 that is adapted to sealably cover the open top of 
the container 12. The lid 24 further includes a pair of electrical 
terminals 26a, 26b affixed thereto. A pair of electrical leads 28a, 28b 
are operatively connected between the pair of electrical terminals 26a, 
26b and the ends 22a, 22b of the capacitive element 14 in an ordinary 
manner. Those skilled in the art will appreciate that variation in the 
container and lid configurations may be made without departing from the 
scope of the invention. 
A polyurethane oil insulating fluid 30 is disposed in the container so as 
to surround at least a portion, and preferably all, of the capacitive 
element 14. In most applications, the quantity of fluid disposed in the 
container will be chosen to completely surround the capacitive element 14. 
The insulating fluid 30 generally has a viscosity in the range of about 
500 to 3000 poise at 25.degree. C., and preferably, the viscosity is in 
the range of about 1900 to 2500 poise at 25.degree. C. The insulating 
fluid 30 is produced by reacting a primary polyol, such as castor oil, a 
ricinoleic acid derivative thereof or a combination of both, with an 
organic polyisocyanate. The reaction may be carried out in the presence of 
a secondary polyol which acts as a chain extender for the urethane 
polymerization. Organic polyisocyanates that can be utilized to produce 
the insulating fluid include: aliphatic polyisocyanates, cycloliphatic 
polyisocyanates, aromatic polyisocyanates, polymethyleneisocyanates, 
polyphenylisocyanates, methylenediisocyanates and any organic 
polyisocyanates that are prepolymers prepared by reacting a polyisocyanate 
with any polyol in quantities such that the NCO/OH ratio is greater than 1 
to 1. A preferred secondary polyol is hydroxy-terminated polybutadiene 
diol because it demonstrates outstanding electrical and thermal expansion 
properties as well as provides structural support to the resulting 
polymeric matrix. 
Preferably, the overall NCO/OH ratio (OH groups of both primary and 
secondary polyols if present) to produce the high viscosity polyurethane 
oil will typically range from about 0.1 to 1 to about 0.6 to 1. The 
desired NCO/OH ratio and the particular polyisocyanate, primary and 
secondary polyol starting materials chosen for the reaction will dictate 
the final viscosity of the resulting polyurethane oil insulating fluid. 
Typically, any reaction done with an NCO/OH ratio higher than about 0.6 to 
1 will produce a solid elastomeric material which is unsuitable for use as 
an insulating oil in metallized film capacitors. 
The polyurethane oil insulating fluid 30 used in the present invention is 
not expected to provide any substantial dielectric properties to the 
capacitor as it is not intended to impregnate or otherwise penetrate into 
the capacitive element of the present invention. However, because the 
capacitive element 14 is not a hermetically sealed unit, under certain 
conditions of time, temperature and production techniques, it is possible 
that some insulating fluid 30 could migrate into the capacitive element 14 
such that the insulating fluid 30 contacts the marginal edges, and in some 
instances, the few outer layers of the tightly wound metallized polymer 
films 16a, 16b. To the extent that some polyurethane oil insulating fluid 
30 has made contact with the materials forming the capacitive element 14, 
it has not shown to have any material effects on the operation of the 
capacitor 10. 
Three dual configuration 10/25 .mu.F 370 VAC, zinc metallized polypropylene 
film capacitors were made with the high viscosity polyurethane oil 
insulating fluid as described above. The fluid was heated to about 
100.degree. C. before it was poured into the container. These capacitors 
were life tested under accelerated life test conditions. For the first 188 
hours, the capacitors were tested at 80.degree. C. and 450 VAC. After the 
first 188 hours, the capacitors were tested at 80.degree. C. and 550 VAC. 
During this time, the capacitors were operated under conditions that 
simulated a weekly duty cycle of 16 hours on and 8 hours off over a 5 day 
period and then on for 24 hours for 2 days. This cycle was repeated 
through a maximum accelerated life test duration of 1892 hours. The hours 
with voltage applied were counted as test time. At the 188 hour mark, the 
average capacitance for the 10.0 .mu.F configuration increased by about 
0.77 percent and the average capacitance of the 25 .mu.F configuration 
increased by about 1.16 percent. There was no significant change in the 
dissipation factor and none of the tested capacitors failed. At the end of 
the accelerated test, the capacitors were cut open and examined. The 
insulating fluid was not discolored, the outer turns of the rolled 
polypropylene film were not wrinkled and the metal on the film was not 
attacked. Under the same test conditions, the capacitance of the same 
capacitor configuration, but filled with Amoco H1900 polybutene insulating 
fluid instead of the high viscosity polyurethane oil, increased 0.70 and 
1.15 percent respectively. 
At the 1892 hour mark under the more accelerated testing conditions, the 
average capacitance for the 10.0 .mu.F configuration increased by about 
1.16 percent and the average capacitance of the 25 .mu.F configuration 
increased by about 1.62 percent. The two configurations of the polybutene 
filled capacitor experienced an average capacitance increase of about 0.67 
and 1.18 percent, respectively. The dissipation factor did not change 
appreciably. 
Nine dual configuration 3.0/30 .mu.F 370VAC zinc metallized polypropylene 
film capacitors were made with the high viscosity polyurethane oil 
insulating fluid as described above. The fluid was heated to about 
95.degree. C. before it was poured into the container. Also, six dual 
configuration zinc metallized polypropylene film capacitors were made with 
the Amoco H1900 polybutene insulating fluid instead of the polyurethane 
oil insulating fluid. All of these capacitors were life tested under 
accelerated life test conditions as described above up to 1396 hours. 
At the end of 1396 hours, the average capacitance for the 3.0 .mu.F 
configuration containing the high viscosity polyurethane oil insulating 
fluid increased by about 0.31. The average capacitance of the 30 .mu.F 
configuration increased by about 0.71 percent. There was no significant 
change in the dissipation factor and none of the tested capacitors failed. 
At the end of the accelerated test, the capacitors were cut open and 
examined. The insulating fluid was not discolored, the outer turns of the 
rolled polypropylene film were not wrinkled and the metal on the film was 
not attacked. 
Under the same test conditions, the capacitance of the six capacitors 
filled with Amoco H1900 polybutene insulating fluid instead of the 
polyurethane oil insulating fluid, increased by 0.40 and 0.48 percent 
respectively. There was no significant change in the dissipation factor 
during the life of the test. 
It will thus be seen that the objects and advantages set forth above and 
those made apparent from the preceding descriptions, are efficiently 
attained and, since certain changes may be made in the above construction 
without departing from the scope of the invention, it is intended that the 
matter contained in the above description or shown in the accompanying 
drawings 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.