Dielectric Fluid Comprising Polysiloxane and Ketone Compound or Camphor

In numerous electrical devices it is necessary to provide a liquid insulating medium which is called a "dielectrical fluid." This liquid has a substantial higher breakdown strength than air and by displacing air from spaces between conductors in the electrical equipment or apparatus, materially raises the breakdown voltage of the electrical device. With the ever increasing sophistication of electrical equipment, the various electrical devices are operating at higher and higher voltages. This means that the dielectric fluids used in such devices are subjected to greater and greater stresses. These problems have, of course, necessitated the search for improved dielectric fluids.

With the exception of certain special applications the polychlorinated biphenyl compounds (generally known as "PCB's") have been the standard dielectric fluid in electrical devices since the 1930's when the PCB's replaced mineral oil. Various other liquids including some siloxanes have also been suggested for use as dielectric fluids. See for example U.S. Pat. Nos. 2,377,689 and 3,838,056 and British Patents Nos. 899,658 and 899,661. Recently the PCB's have lost favor in the sight of the environmentalists and, be they right or wrong, efforts are being made worldwide to find suitable replacements for the PCB's.

By way of illustration corona or partial discharge is a major factor causing deterioration and failure of capacitors or other power factor correction devices. A capacitor operating in corona will have a life of only minutes or hours instead of the expected 20 years. A capacitor properly impregnated with a suitable dielectric fluid will be essentially free of corona discharge to a voltage of at least twice the rated voltage. During use when a dielectric fluid is placed under increasing stress a point is reached where breakdown occurs. The voltage at which the capacitor will suddenly flash into corona is known in the art as the corona inception voltage (CIV). This voltage is dependent upon the rate at which the voltage is applied. There is considerable difference between the sensitivity of different fluids to the rate of rise of voltage. The corona will, however, extinguish with a reduction of voltage. The corona extinction voltage (CEV) is not a fixed value for each fluid but is a function of the intensity of corona before the voltage is reduced. For best results both the CIV and CEV should be as high and as close together as possible.

It has been discovered in accordance with this invention that when certain select ketones are incorporated into liquid polyorganosiloxanes that the resulting composition is useful as a dielectric fluid in electrical devices. It is further believed that these compositions when used as dielectric fluids provide suitable replacements for the PCB's which are currently being employed in the marketplace.

More specifically, this invention relates to an electrical device containing a dielectric fluid wherein the improvement comprises employing as the dielectric fluid a composition consisting essentially of a major amount of a liquid polyorganosiloxane and a minor amount of a compound soluble in said siloxane which compound is selected from the group consisting of R'.sub.2 C=O, R'R"C=O and camphor, wherein each R' is selected from the group consisting of hydrocarbon and halogenated hydrocarbon radicals containing from 1 to 12 carbon atoms, and R" is a radical of the formula ##STR1## wherein R' is as defined above, and R'" is a divalent hydrocarbon radical and contains from 1 to 6 carbon atoms.

This invention further relates to a dielectric fluid consisting essentially of a major amount of a liquid polyorganosiloxane and a minor amount of a compound soluble in said siloxane which compound is selected from the group consisting of R'.sub.2 C=O, R'R"C=O and camphor, wherein each R' is selected from the group consisting of hydrocarbon and halogenated hydrocarbon radicals containing from 1 to 12 carbon atoms, and R" is a radical of the formula ##STR2## wherein R' is as defined above, and R'" is a divalent hydrocarbon radical and contains from 1 to 6 carbon atoms.

The liquid polyorganosiloxanes useful in this invention will be composed predominately of siloxane units of the formula R.sub.2 SiO and may also contain small amounts of siloxane units of the formulae R.sub.3 SiO.sub.1/2 , RSiO.sub.3/2 , and SiO.sub.4/2 . Of particular interest are liquid polyorganosiloxanes of the general formula R.sub.3 SiO(R.sub.2 SiO).sub.x SiR.sub.3. In the foregoing formulae the R radicals preferably represent hydrocarbon radicals and halogenated hydrocarbon radicals. Illustrative examples of suitable R radicals are the methyl, ethyl, propyl, butyl, hexyl, decyl, dodecyl, octadecyl, vinyl, allyl, cyclohexyl, phenyl, xenyl, totyl, xylyl, benzyl, 2-phenylethyl, 3-chloropropyl, 4-bromobutyl, 3,3,3-trifluoropropyl, dichlorophenyl, and alpha,alpha,alpha-trifluorotolyl radicals. Preferably R contains from 1 to 6 carbon atoms with the methyl, vinyl and phenyl radicals being the most preferred.

The liquid polyorganosiloxane portion of the dielectric fluid composition of this invention constitutes a major portion thereof, that is to say, more than 50 percent of the composition and preferably the liquid polyorganosiloxane constitutes from 80 to 99.5 percent by weight of the dielectric fluid composition of this invention. These liquid polyorganosiloxanes are well known materials which are commercially available throughout the world.

The dielectric fluid composition of this invention also contains a minor amount of a compound selected from the group consisting of R'.sub.2 C=O, R'R"C=O and camphor, wherein each R' is selected from the group consisting of hydrocarbon and halogenated hydrocarbon radicals containing from 1 to 12 carbon atoms, and R" is a radical of the formula ##STR3## wherein R' is as defined above, and R'" is a divalent hydrocarbon radical and contains from 1 to 6 carbon atoms. It is believed essential that these ketone compounds be soluble in the liquid polyorganosiloxane portion of the composition. In the above formulae R' can be a hydrocarbon or halogenated hydrocarbon radical containing from 1 to 12 carbon atoms. Thus, for example, R' can be a methyl, ethyl, isopropyl, butyl, hexyl, 2-ethylhexyl, dodecyl, vinyl, allyl, hexenyl, phenyl, benzyl, tolyl, chloropropyl, trifluoromethyl or dichlorophenyl radical. R" in the above formulae is a radical of the structure ##STR4## wherein R' is as defined above and R'" is a divalent hydrocarbon radical such as the --CH.sub.2 --, --(CH.sub.2).sub.2 --, --(CH.sub.2).sub.3 --, --(CH.sub.2).sub.6 -- or --CH.sub.2 CH(CH.sub.3)-- radicals. While any of the above indicated ketone compounds are useful in accordance with the present invention the R'.sub.2 C=O compounds wherein R' is a halogenated hydrocarbon radical containing from 1 to 6 carbon atoms are the preferred embodiment at this time. The ketone compounds used herein constitute a minor portion, that is, less than 50 percent of the composition of this invention. It is generally preferred, however, that these materials be employed in an amount in the range from 0.5 to 20 percent by weight of the composition.

The dielectric fluid compositions of this invention may also contain small amounts of conventional additives such as HCl scavengers, corrosion inhibitors and other conventional additives normally employed in such compositions so long as they do not have an adverse effect of the performance of the compositions of this invention.

The two most important electrical devices in which the dielectric fluids of this invention are useful are in capacitors and transformers. They are also very useful dielectric fluids in other electrical devices such as electrical cables, rectifiers, electromagnets, switches, fuses, circuit breakers and as coolants and insulators for dielectric devices such as transmitters, receivers, fly-back coils, sonar bouys, toys and military "black boxes". The methods for employing the dielectric fluids in these various applications (be they, for example, as a reservoir of liquid or as an impregnant) are well known to those skilled in the art. For best results, the viscosity of the dielectric fluid composition of this invention should be in the range of 5 to 500 centistokes at 25.degree.C. If the viscosity exceeds 500 centistokes they are difficult to use as impregnants and at less than 5 centistokes their volatility becomes a problem unless they are used in a closed system.

Now in order that those skilled in the art may better understand how the present invention can be practiced the following examples are given by way of illustration and not by way of limitation. All parts and percents referred to herein are by weight and all viscosities measured at 25.degree.C. unless otherwise specified.

A screening test for dielectric fluids was developed which it is believed correlates well with results obtained in test capacitors. The main piece of equipment required for this test is a Biddle Corona Detector with a manual Variac control. The test cell consists of a glass cylindrical container. The base of the cell is a ceramic filled plastic which has a stainless steel metal plate which is connected directly to ground. There is a stainless steel top for the container which has attached thereto a micrometer adjustable high voltage electrode with a steel phonograph needle on the end. The tip of this needle is positioned 0.025 inches (25 mils) above the grounded base. In the high voltage line attached to the electrode there is a 1.67 .times. 10.sup.8 ohm resistance. This is a current limiting resistor.

During the test a few cubic centimeters of the test fluid is placed in the container and the top set in place. As the voltage is increased, partial discharge occurs between the tip of the electrode and the ground plate. This draws current which reduces the applied voltage below the discharge level. When no current is being drawn the applied voltage is again at partial discharge potential. Current is drawn by discharges again and the process is repeated. Thus the current is in effect turned on and off very rapidly, and the total breakdown of the fluid can never occur.

In operation the applied voltage is slowly increased by adjustment of the Variac. The partial discharges are observed on the oscilloscope of the corona detector. The point at which the eliptical lissajous pattern on the screen becomes flooded with discharges, and there is a constant audible crackling from the cell, is recorded as the corona inception voltage (CIV). The rate of rise of the applied voltage is perhaps a few hundred volts per second. When the CIV has been determined, the voltage is slowly decreased until the eliptical lissajous pattern on the screen can be seen again due to the partial cessation of discharges. The point at which this occurs is recorded as the corona extinction voltage (CEV).

A number of dielectric fluid compositions were prepared which consisted essentially of a liquid trimethylsilyl endblocked polydimethylsiloxane having a viscosity of 50 cs. and various ketones in varying amounts. These compositions were tested in the screening test described above. The specific ketones employed, the amount used (the balance being the siloxane) and the test results are set forth in the table below. A number with a plus (+) behind it indicates that the test was terminated at that point and the actual value is somewhat greater than the value reported.

When the dielectric fluid compositions of this invention which are set forth in the preceding example are used as transformer fluids or to impregnate capacitors, improved performance is obtained when compared to the polydimethylsiloxane fluid per se.