GB1589433A

GB1589433A - Dielectric fluids and electric devices containing them

Info

Publication number: GB1589433A
Application number: GB27397/77A
Authority: GB
Original assignee: Dow Corning Corp
Current assignee: Dow Silicones Corp
Priority date: 1977-03-21
Filing date: 1977-06-30
Publication date: 1981-05-13
Also published as: IT1081622B; NL169117C; DE2731869B2; BE857443A; FR2385191A1; CA1091000A; JPS5613963B2; NL7707728A; DE2731869C3; JPS53116500A; AU510941B2; NL169117B; DE2731869A1; AU2660777A; FR2385191B1

Description

(54) IMPROVED DIELECTRIC FLUIDS AND ELECTRIC DEVICES CONTAINING THEM (71) We, DOW CORNING CORPORATION. a corporation organized under the laws of the State of Michigan of Midland, Michigan, United States of America; do hereby declare the invention. for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to improved dielectric fluids and to electric devices containing them.

In numerous electrical devices it is necessary to provide a liquid insulating medium which is called a "dielectric fluid" This liquid has a substantially higher breakdown strength than air and by dispiacing 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 193()'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. Patent Nos. 2.377,689 and 3,838,Q56 and U.K. Patent Nos. 899,658 and 899,661). Also, siloxane fluids containing additives have been suggested heretofore (see, for example, U.S. Patents Nos. 3,948,789 and 3,984,338). Recently, the PCB's have lost favour for environmental reasons and efforts are being made worldwide to find suitable replacements for them.

By way of illustration, corona or partial discharge is a major factor causing the 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 2() 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 a significant difference between the sensitivies of different fluids to the rate of rise in 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 a function of the intensitv of the corona before the voltage is reduced. For best results, both the CIV and the CEV;alue should be as high and as close together as possible.

We have discovered in accordance with this invention that. when certain esters of dibasic acids are incorporated into liquid polyorganosiloxanes. 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 composition which consists essentially of 80 to 99.5 percent by weight of a liquid polyorganosiloxane. and 2() to ().5 percent by weight of a dialkyl ester of carbonic acid or of an acid having the general formula: HOOC-R"-COOH wherein R"' is a divalent radical which is -(CR""@)@ -, -CR""=CR""-, -(Cr""2)

cyclohexylenc. arylenc or an halogenated arylene radical (each R"" in said divalent radicals being hydrogen. or halogen. or a methyl. ethyl. propyl. butyl or phenyl radical). y has a value of from () to 18. z has a value of from 1 to 16. and each R"' radical contains a total of not more than 18 carbon atoms.

The liquid polyorganosiloxanes useful in this invention are composed predominantly of siloxane units of the formula R9SiO and may also contain small amounts of siloxane units of the formulae KlSi(),,. RSiO3/2 and SiO1/2. Of particular interest are liquid polyorganosilo lancs of the general formula R3SiO(R2SiO)xSiR3. Also of interest are liquid polyorganosiloxanes of the general formula (R'O)R2SiO(R2SiO)xSiR2(OR'). In the foregoing formulac. 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, tolyl, xylyl. henzyl, 2-phenyl-ethyl, 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, vinvl and phenyl radicals being the most preferred.

Illustrative examples of the R' radical are alkyl radicals containing from I to 6 carbon atoms such ,is the methyl. ethyl. isopnpyl. tertiary butyl. pentyl and hexyl radicals. Preferably R' contains from I to 3 carhon atoms.

The liquid polyorganosiloxane portion of the dielectric fluid composition of this invention constitutes from S(1 to ')9.5 percent by weight. These liquid polyorganosiloxanes are well known materials to those skilled in the art of silicone chemistry.

The dielectric fluid composition of this invention also contains from 0.5 to 20 percent by weight of one or more esters of the dibasic acids specified above. Specific examples of acids useful herein include oxalic acid. malonic acid. phenvlmalonic acid. succinic acid, glutaric acid, adipic acid. pimelic acid. suberic acid. azelaic acid, sebacic acid, maleic acid, chloromaleic acid, bromomaleic acid. fumaric acid. chlorofurmaric acid, bromofumaric acid, mesaconie acid, eitrileonic acid. itaconic acid. carbonic acid. benzene dicarboxylic acids. biphenyl dicirboxylie acids. naphthalene dicarboxylic acids, cyclohexane dicarboxylie acids, tetrahydrophthalic acid and pyrotartaric acid.Each alcohol residue in this component preferably contains from I to 18 carbon atoms and most preferably from 4 to 10 carbon atoms. For example. one can use the methyl. ethyl. propyl. butyl, tertiary butyl, amyl, hexyl. octal. 2-ethyl-hexyl. decyl. dodecyl and octadecyl esters. The two alkyl groups can be the sime or difibrent. The acids and esters and their methods of preparation are well knwon to those skilled in the art of organic chemistry.Generally speaking, the most preferred esters at this time are those of the above formula where R"' is -CR""=CR""- and R"' is a hydrogen atom.

The delectric fluid composition of this invention may also contain small amounts of conventional additives such as HCl scavengers, corrosion inhibitors and other conventional additives normilly employed in such compositions provided they do not have an adverse effect on the performinee of the compositions of this invention.

The two most importint electrical devices in which the dielelctric 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 and circuit breakers and as coolants and insulators for dielectric devices such as transmitters, receivers, fly-back coils and sonar bouys. The methods for employing the dielectric fluids in these various applications (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 from 5 to 500 centistokes at 25 C'. If the viscosity exceeds 500 centistokes, the fluids are difficult to use as impregn;ints and at less than 5 centistokes their volatility becomes a problem unless they are used in I closed system In order that those skillcd in the art may he better unerstood 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 percentages referred to herein are by weight and all viscosities are measured at 25"C unless otherwise specified.

Example I A screening test for dielectric fluids was developed which it is believed correlates well with results obtained in test capacitors. The primary piece of equipment required for this test is a Biddle Corona Detector with a manual Variac (trade mark) 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 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 t).025 inch (().ü635 cm.) above the grounded base. In the high voltage line attached to the electrode, there is a 1.67 x 10X ohm resistance.This is a current limiting resistor.

During the test. a few cubic centimeters of the test fluid are 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 again drawn by discharge 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 elliptical Lissajou's 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 elliptical Lissajou's 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 95 percent of a liquid trimethylsilyl endblocked polydimethylsiloxane having a viscosity of 50 cs.. and 5 percent of various esters of various dibasic acids. These compositions were tested in the screening test described above together with the siloxane fluid per se as a control for comparative purposes. The specific esters employed and the test results are set forth in Table I below.

Ex(ozlple 2 A series of capacitors of approximately ().()1 microfarad were wound from two layers of ().()()127 cm. polypropylene film and one layer of ().()()102 cm. Kraft paper. A number of dielectric fluid compositions were then prepared which consisted essentially of about 90 percent of a liquid trimethylsilyl endblocked polydimethylsiloxane having a viscosity of 5 cs., and about LO percent of various esters of various dibasic acids. These dielectric fluid compositions were used to impregnate the aforementioned capacitors so that they could be evaluated for corona performance. A Biddle Corona Detector with a manual Variac control was used to apply voltage to test capacitors. Voltage was applied to the capacitors at a rate of rise of 2()()-3()() volts per second.Two corona extinction voltages (CEV's) were measured for each dielectric fluid composition. The "light" CEV is the voltage that is observed when the voltage is quickly reduced (about one second) after corona initiation. The "heavy" CEV is the voltage observed after the capacitor is allowed to become saturated with corona either by allowing it to remain at the initiation voltage for several seconds or by raising the voltage several hundred volts above the initiation value. The specific esters employed and the tests results are set forth in Table II below.

Example 3 The procedure of Example 2 was repeated using dibutyl tetrahydrophthalate as the ester in the dielectric fluid composition. This composition had a CIV of 3400 and a heavy CEV of 2000 compared to a CIV of 26()() and a heavy CEV of 400 for the siloxane fluid per se.

Example 4 Four compositions were evaluated for electrical characteristics and corona performance.

Two of the compositions were liquid methoxydimethylsilyl endblocked polydimethylsiloxanes, one having a viscosity of 7 cs. and the other 5 cs. The other two compositions consisted of 9() percent of one of the aforementioned liquids and 10 percent of dibutyl maleate. The dielectric constant (DK). dissipation factor (DF), volume resistivity (VR), CIV and heavy CEV were measured and the results are set forth in Table III below where A represents the 7 cs. siloxane per se, B represents the 5 cs. siloxane per se, C represents the 7 cs. siloxane-dibutyl maleate composition. and D represents the 5 cs. siloxane-dibutyl maleate composition.

TABLE I Ester CIV (in KV) CEV (in KV) None 16 15 Dibutyl carbonate 19 16 Dibutyl oxalate 22 18 Di(2-ethylhexyl) malonate 22 20 Di(2-ethylhexyl) phenylmalonate 24 21 Diethyl malonate 20 18 Dibutyl succinate 19 16 Dibutyl malcatc 22 18 Dibutyl adipate 18 15 Siloxane fluid per se. Included for purposes of comparison.

TABLE II Estcr CIV Light CEV Heavy CEV None' 2600 1600 400 Dibutyl malonate 2800 2400 1600 Dibutyl maleate 3000 2600 2100 Dibutyl itaconate 3200 2800 2300 Dicyclohexyl itaconate 3200 2800 2200 Dibutyl adipate 2800 2400 800 Dibutyl succinate 2600 1600 1200 @Siloxane fluid per Se. Included for purposes of comparison.

TABLE III Composition DK (100 Hz) DF (100 Hz) VR CIV Heavy CIV A@ 2.80 0.00011 1.3 x 1014 2700 500 B@ 2.59 0.00004 7.0 x 1014 2600 600 C 3.14 0.00265 1.4 x 1012 2900 2000 D 2.87 0.00052 1.8 x 1013 3200 2000 Included for comparison WHAT WE CLAIM IS: 1.An electrical device containing a dielectric fluid composition which consists essentially of 80 to 99.5 percent by weight of a liquid polyorganosiloxane, and 0.5 to 20 percent by weight of a dialkyl ester of carbonic acid or of an acid having the general formula: wherein R" is a divalent radical which is -(CR""2)v-, -CR""=CR""-,

cyclohexylene, arvlene or halogenated arylene (each R"" in the aforesaid divalent radical being hydrogen or halogen, or a methyl, ethyl, propyl, butyl or phenyl radical), y has a value not exceeding 18, z has a value from 1 to 16, and said R"' radicals contain a total of not more than 18 carbon atoms.

2. An electrical device as claimed in claim 1 wherein the dialkyl ester is a dialkyl ester of oxalic acid, malonic acid, phenyl malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, maleic acid, chloromaleic acid, bromomaleic acid, fumaric acid, chlorofumaric acid, bromofumaric acid, mesaconic acid, citraconic acid, itaconic acid, carbonic acid, a benzene dicarboxylic acid, a diphenyl

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

A represents the 7 cs. siloxane per se, B represents the 5 cs. siloxane per se, C represents the 7 cs. siloxane-dibutyl maleate composition. and D represents the 5 cs. siloxane-dibutyl maleate composition.

TABLE I Ester CIV (in KV) CEV (in KV) None 16 15 Dibutyl carbonate 19 16 Dibutyl oxalate 22 18 Di(2-ethylhexyl) malonate 22 20 Di(2-ethylhexyl) phenylmalonate 24 21 Diethyl malonate 20 18 Dibutyl succinate 19 16 Dibutyl malcatc 22 18 Dibutyl adipate 18 15 Siloxane fluid per se. Included for purposes of comparison.

TABLE II Estcr CIV Light CEV Heavy CEV None' 2600 1600 400 Dibutyl malonate 2800 2400 1600 Dibutyl maleate 3000 2600 2100 Dibutyl itaconate 3200 2800 2300 Dicyclohexyl itaconate 3200 2800 2200 Dibutyl adipate 2800 2400 800 Dibutyl succinate 2600 1600 1200 @Siloxane fluid per Se. Included for purposes of comparison.

TABLE III Composition DK (100 Hz) DF (100 Hz) VR CIV Heavy CIV A@ 2.80 0.00011 1.3 x 1014 2700 500 B@ 2.59 0.00004 7.0 x 1014 2600 600 C 3.14 0.00265 1.4 x 1012 2900 2000 D 2.87 0.00052 1.8 x 1013 3200 2000 Included for comparison WHAT WE CLAIM IS: 1.An electrical device containing a dielectric fluid composition which consists essentially of 80 to 99.5 percent by weight of a liquid polyorganosiloxane, and 0.5 to 20 percent by weight of a dialkyl ester of carbonic acid or of an acid having the general formula: wherein R" is a divalent radical which is -(CR""2)v-, -CR""=CR""-,

cyclohexylene, arvlene or halogenated arylene (each R"" in the aforesaid divalent radical being hydrogen or halogen, or a methyl, ethyl, propyl, butyl or phenyl radical), y has a value not exceeding 18, z has a value from 1 to 16, and said R"' radicals contain a total of not more than 18 carbon atoms.
2. An electrical device as claimed in claim 1 wherein the dialkyl ester is a dialkyl ester of oxalic acid, malonic acid, phenyl malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, maleic acid, chloromaleic acid, bromomaleic acid, fumaric acid, chlorofumaric acid, bromofumaric acid, mesaconic acid, citraconic acid, itaconic acid, carbonic acid, a benzene dicarboxylic acid, a diphenyl

dicarboxylic acid, a naphthalene dicarboxylic acid, a cyclohexane dicarboxylic acid, tetrahydrophthalic acid or pyrotartaric acid.
3. An electrical device as claimed in claim 1 or 2, wherein the dielectric fluid composition has a viscosity in the range from 5 to 500 cs. at 25"C., and each alkyl group in the ester contains from 1 to 18 carbon atoms.
4. An electrical device as defined in claim 3 which is a transformer.
5. An electrical device as defined in claim 3 which is a capacitor.
6. An electrical device according to claim 1 substantially as herein described with reference to any one of the specific examples.