Dielectric fluid for electrical capacitors

Di octyl phthalate (DOP) is used as a dielectric fluid for a-c electrical capcitors. An additive of an epoxidized linseed oil in an amount from 0.1% to 30% by weight of DOP stabilizes the capacitors and thus prolongs their life.

BACKGROUND OF THE INVENTION 
This invention relates to a dielectric liquid impregnant for electrical 
devices and more parrticularly to an electrical capacitor utilizing an 
improved dielectric liquid impregnant. 
Liquid impregnants for electrical capacitors should have a high dielectric 
constant, maintain a low dissipation factor, and be compatible with other 
materials utilized in the capacitor structure. At the same time, the 
impregnant must withstand elevated and fluctuating temperature, pressure, 
and voltage stress conditions with excellent electrical characteristics 
for a long operative life of the capacitor. 
The polychlorinated biphenyls as capacitor impregnants meet these 
requirements and they were eminently satisfactory for several decades. The 
polychlorinated biphenyls are broadly referred to as PCBs. The 
polychlorinated biphenyls have recently been associated with ecological 
problems, restrictive use limitations, and rising costs. These problems 
have spurred the search for a suitable replacement capacitor impregnant 
which would have some advantageous impregnant characteristics comparable 
to those of the chlorinated diphenyls, and still provide outstanding 
electrical and compatibility performance with the two most important 
present day capacitor solid dielectrics, paper and polypropylene. As a 
result, di octyl phthalate (DOP) has been used as providing good 
electrical characteristics as a dielectric fluid impregnant. 
FEATURES OR OBJECTS OF THE INVENTION 
It is a feature of the invention to increase the stability of DOP as used 
in a capacitor so as to increase the capacitor life. Another feature of 
the invention is to provide a capacitor having a dielectric fluid of DOP 
with an additive of epoxidized linseed oil. These and other features of 
the invention will become apparent from the following description taken in 
conjunction with the accompanying drawings. 
SUMMARY OF THE INVENTION 
Generally speaking, the present invention pertains to a capacitor having a 
dielectric fluid which includes DOP and an additive of epoxidized linseed 
oil.

DISCUSSION OF THE INVENTION 
Referring now to FIG. 1 of the drawing, there is illustrated a convolutely 
wound a-c capacitor body 10. The capacitor body 10 includes a pair of 
dielectric film layers 11 and 12 and overlying electrodes 13 and 14, 
respectively. Electrodes 13 and 14 are of a suitable metal such as 
aluminum for example. Margins 15 and 16 are provided for preventing 
electrical short circuit between the electrodes. As shown in FIG. 2, 
suitable terminal leads 23 and 24 are attached to the electrodes 13 and 14 
from opposite ends 21 and 22 of the capacitor body 10. Suitable lead 
material could be solder coated copper for example. 
The dielectric plastic films or layers 11 and 12 should have high 
dielectric strength, low dissipation factor to prevent thermal runaway, 
broad temperature range capabilities, resistance to the dielectric fluid, 
and freedom from pinholes and conductive particles. Suitable dielectric 
film layers 11 and 12 may be paper, a paper and polymeric film 
combination, or polymeric films. 
In FIGS. 2 and 3, a capacitor body 10 is preferably sealed to a can or 
housing 40 to form a capacitor 41. Capacitor 41 includes the metal housing 
or can 40 and terminals 42 and 43. The capacitor body 10 is contained 
within the housing 40 and terminal lead 23 makes electrical contact with 
terminal 42 and the remaining terminal lead 24 makes electrical contact 
with terminal 43. 
As noted previously, fluid or liquid dielectric impregnants for electrical 
capacitors should have a high dielectric constant, maintain a low 
dissipation factor, be compatible with the other materials used in 
capacitor structure and should add stability to the capacitor to increase 
its life. According to the present invention, the dielectric fluid which 
is used to impregnate the capacitor body 10 satisfies all of these 
requirements. The dielectric fluid of the present invention includes DOP 
with an additive of epoxidized linseed oil. Expoxidized linseed oil goes 
by the generic name Epoxol 9-5 and is obtainable in large quantities from 
American Chemical Science, Inc. The amount added is between 0.1 to 30% by 
weight of the DOP. A minor amount of a suitable antioxidant such as 
2,6-di-tert-butyl-para-cresol is also added. The antioxidant is 
generically known as CAO-1 and is produced by Sherwin-Williams Chemical 
Co. 
When used as an additive in a dielectric fluid for capacitors of the type 
described in FIGS. 1-3, the additive is mixed as a liquid with liquid DOP, 
refined through a chromatographic column using Fuller's Earth or aluminum 
oxide and then impregnated into the capacitors. Several capacitors 
impregnated with DOP and 4.7% by weight of Epoxol 9-5 as well as 0.05% by 
weight of CAO-1 were life tested for 1,000 hours. None failed.