US3714021A

US3714021A - Thermally stable insulating oil

Info

Publication number: US3714021A
Application number: US00083232A
Authority: US
Inventors: M Takahashi; A Ito; T Yamauchi; K Okuda
Original assignee: Kureha Corp
Current assignee: Kureha Corp
Priority date: 1969-10-22
Filing date: 1970-10-22
Publication date: 1973-01-30
Anticipated expiration: 1990-01-30
Also published as: DE2051934B2; GB1332509A; JPS4931279B1; DE2051934A1

Abstract

A thermally stable insulating oil comprising from 95 to 20 percent by volume of a polycyclic naphthenic hydrocarbon and from 5 to 80 percent by volume of a member selected from the group consisting of a polycyclic aromatic hydrocarbon, its lower alkyl derivative and a mixture thereof.

Description

United States Patent m1 m1 3,714,021

Takahashi et al. 1 Jan. 30, 1973 [54] THERMALLY STABLE INSULATING [56] References Cited UNITED STATES PATENTS [75] inventors: Masaaki Takahashl, Tokyo-to;

Takashi Yamauchi, TOkyo; Kens k 3,252,887 5/1966 Rizzuti ..208/l4 Okuda Akira Ito Tokyo 3" H Wynkoop el al. f Japan 3,462,358 8/1969 Mills et al ..2os |4 2,846,372 8/1958 Schneider et al..... ..208/l4 1 Asslgneer Kureha Kflgak" Kogyo Kabushlkl 3,095,366 6/]963 Schieman ..20s/19 Kaisha, Tokyo-to, Japan 22 pn Oct 22, 1970 Primary Examiner-Herbert Levine 1 pp No-183,232 Attorney-Sughrue, Rothwell, Mion, Zinn & Macpeak 57 ABSTRACT [30] Foreign Application Pnomy Data A thermally stable insulating oil comprising from 95 to Oct. 22, 1969 Japan ..44/84060 20 percent by volume of a polycyclic naphthenic hydrocarbon and from 5 to 80 percent by volume of a U-S. .i member eected from the group consisting of a polya r 208/ cyclic aromatic hydrocarbon, its lower alkyl derivative Int. Cl. ..Cl0g 37/06 and a mixture thereof 1 Field of Search ..208/l4, l9

18 Claims, No Drawings TI'IERMALLY STABLE INSULATING OIL BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an insulating oil having superior thermal stability, and more particularly, it relates to a novel electrical insulating oil comprising a mixture of a polycyclic naphthenic hydrocarbon having two or more rings in the molecule and a polycyclic aromatic hydrocarbon having two or more rings in the molecule.

2. Description of the Prior Art Conventional mineral oil-type insulating oils used hitherto as transformer oils, condenser oils, cable oils, and the like electrical insulating oils, have been prepared from middle-distilled fractions of crude oil after purification thereof by various treatments. Therefore, the performance of the resulting oil varies over a wide range depending upon the kind and nature of the crude oil used, so that the choice of a suitable crude oil has been amatter of great importance in the produc-- tion of insulating oils. 1

These known mineral oils, however, have limited thermal stability, ,i.e., they have a maximum usable temperature of at most about 100C. Many difficulties have often been encountered when using these mineral oil-type insulating oilsvin transformers, condensers, ca-

bles and like electrical equipment wherethey are used at a temperature exceeding the above maximum tem-' perature. It is of course possible to improve' the thermal stability to a certain extent by resorting to the addition of an oxidation inhibitor and similar additives, but this is rather impractical because the performance of the insulating oil abruptly decreases when the oxidation inhibitor has been used up.

The present invention is based on the discovery that the gas absorption'performanceof an insulating oil is unexpectedly improved, more so than any other electrical characteristic, and thethermal stability in the presence of oxygen is also greatly enhanced by admixing a polycyclic aromatic hydrocarbon, having two or more rings in the molecule, to a'particular hydrocarbon mixture comprising polycyclic naphthenes having two or more rings in the molecule. v

The primary object of the present invention is to provide an electrical insulating oil having improved properties.

SUMMARY OF THE INVENTION The present invention provides an electrical insulating oil comprising a mixture of from 95- to 20 percent, by volume, of a polycyclic naphthenic hydrocarbonof two or more rings or alkyl derivatives thereof or a mixture thereof, and 5 to 80 percent, by volume, of a polycyclic aromatic hydrocarbon of two or more rings or alkyl derivatives thereof or a mixture thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The polycyclic naphthenic hydrocarbons which may be used in this invention include, for example, decaline, perhydrophenanthrene, perhydropyrene, perhydrofluorene, perhydroanthracene, perhydrochrysene, and like cyclic naphthenes having two i to five rings, or C or lower, preferably C or lower,

alkyl derivatives thereof or mixtures of two or more such hydrocarbons. Since these mixtures, however, are

hardly analyzable at present and usual ring analysis methods are not applicable, an accurate 'and detailed structure of these compounds has not been determined.

A four naphthenic rings, and is entirely new.

The general characteristics of the polycyclic naphthenic hydrocarbon are as follows:

Specific gravity d 0.85-0.99

Refractive index n l.45l.60

Viscosity (30C) cst 10-40 Hydrogen/carbon atomic ratio (hereinafter indicated by H/C) 1.75-2.00 Ignition point 1 25C Pour point 30 to 60C Dielectric constant (a) (C) Dielectric tangent tan 8 (80C) 0.00l

Volumatic resistivity (fl-cm) (80C l0 Insulation breakdown voltage KV/2.5mm 60 Total acid value mg, KOH/g 0.00l

Corrosivity' l A Thermal stability (C-75 hrs) Sludge 0.20

Total acid value mg, KOH/g 0.20

However, this conventional oil has an inferior gas absorptivity. According to the present invention, this disadvantage has been overcome by the addition of certain polycyclic aromatic compounds to the polycyclic naphthenic hydrocarbon without adversely affecting its inherent electrical properties. g

The polycyclic aromatic hydrocarbon employed as the other component in the oil of the present invention includes polycyclic aromatics having from 2 to 4 rings such as, for example, naphthalene, diphenyl, acenaphthene, fluorene, terphenyl, pyrene, chrysene, and C or lower, alkyl derivatives thereof, and mixtures of two or more such compounds. In addition, an arc matic hydrocarbon in which a portion of the ring is saturated with hydrogen, such as acenaphthene, can be used without departing from the scope of the present invention.

The aforesaid aromatic mixture, however, cannot be completely analyzed, but after various physical analysis, gas chromatographic analysis, etc., the polycyclic aromatic structure was confirmed.

The polycyclic aromatic hydrocarbon utilized in the present invention has the following characteristics:

Specific gravity d) 1.00 1.20 Refractive index n, L50 1.68 Viscosity (30C) est 10 25 Pour point 40C Ignition point 1S0C The oil composition of the present invention exhibits the following electrical properties and thermal stability:

Dielectric constant (s) (80C) 2.35 2.65 Dielectric tangent tanS (80C) 0.001 0.03 Volumatic resistance (fl-cm) (80C) 2 X10 2 X10" Insulation breakdown voltage KV/2.5mm 50 80 Thermal stability (125C 75 hrs) Sludge 0.008 0.40 Total acid value 0.05 0.60

Due to its aromatic structure, the oil of the present invention has an excellent gas absorptivity as well as excellent electrical properties.

The particular polycyclic naphthenic hydrocarbons and the polycyclic aromatic hydrocarbons used in the present invention are easily obtainable by desulfurizing and alkylating (with lower olefins) and/or nuclear hydrogenating bottom oils or tarry materials rich in aromatic compounds, the latter type of material is, for example, formed during the thermal cracking of petroleum hydrocarbons (crude oil, heavy oil, light oil, kerosene, naphtha and the like petroleum fractions) at a temperature of above 700C but below 2,300C for a period of time of 1-0.001 second to produce ethylene and/or acetylene, or an oil tar from the gasification of heavy or crude oil at a high temperature, or coal tar, or bottom oil from the dealkylation of alkylaromatics (e.g., the bottom oil from toluene dealkylation during benzene production).

The desulfurization is conducted employing conventional reaction conditions and conventional catalysts at a temperature of 350-450C, a pressure of 20-100 Kg/cm a hydrogen/high aromatic oil molar ratio of 5-20, and an LHSV of 0.5-2.0. The catalyst which may be employed includes cobalt-molybdenum, nickelmolybdenum, nickel-cobalt-molybdenum, etc., which may be supported on alumina or silica-alumina.

The alkylation is carried out in the presence of a catalyst by admixing a tarry fraction with an olefin gas.

The reaction conditions to be employed are as follows:

Reaction temperature 250 380C Pressure 1 50 Kg/crn Olefin/tar (molar ratio) 0.2 Liquid hourly space velocity (LHSV) 0.1 -3.0

The catalyst employed in the alkylation may be an acidic catalyst such as silica-alumina, zeolite, etc., but the use of a catalyst in which a Group lll-B metal, such as lanthanum, cerium or thorium, is supported on zeolite is especially effective. Under these conditions, the alkylation proceeds smoothly to give the desired product in a good yield.

The degree of alkylation can be controlled to the desired extent by modifying the olefin/tar mixing ratio or liquid hourly space velocity (LHSV).

The electrical properties of the product are not sig nificantly influenced by the type of alkyl group introduced as long as it is not larger than C Thus, the specific alkyl group employed is determined based on economics, considering the availability of the material. The degree of alkylation is suitably controlled, in

general, to such an extent that the number of alkyl groups attached per average molecular weight of the material aromatic oil ranges on the order of below 5, preferably 1-4, since no substantial fluctuation in the electrical peroperties of the product can be noticed within this range. A degree of alkylation departing from this range is not desirable from the viewpoint of the yield rather than the electrical properties of the product.

The hydrogenation reaction is conducted under conventional conditions employed in the art. Preferred hydrogenation catalysts to be used are exemplified by the oxides, sulfides, etc., of Group VI, VII and VIII metals of the Periodic Table. These catalysts may be supported on carriers such as activated carbon fullers earth, diatomaceous earth, bauxite, pumice stone, silica-alumina, etc. The reaction is ordinarily conducted at a temperature of 100-450C, under a pressure of 10-300 kglcm at a liquid hourly space velocity (LHSV) of 0.5-2.0, and at a hydrogen/alkylated high aromatic oil molar ratio of 5-20.

The polycyclic naphthen s and polycyclic aromatics should be mixed, according to the present invention, in most cases within a range of -20 percent, by volume, of the former to 5-80 percent, by volume, of the latter, although this ratio varies to some extent depending upon the type of electrical equipment to be used. An increased proportion of polycyclic naphthenes beyond the above stated range will result in a decrease in gas absorptivity, and a proportion less than the stated range will cause poorer electrical properties.

By mixing both components within the above-mentioned ratio, it is possible to attain remarkable improvements, especially with respect to the gas absorptivity and the voltage pressure resistance among other electrical properties, while still maintaining suitable control of the other properties such as tan 8, insulation resistance etc., to appropriate values for given uses. It is surprising that the thermal stability of the mixture in the presence of oxygen can be greatly improved to an extent far superior to that of either of the polycyclic naphthenes or polycyclic aromatics individually.

Although each component of the hydrocarbon oil of the present invention itself exhibits excellent electrical performance, it has now been found that surprising effects, which cannot be attained by the use of the conventional mineral type-insulating oil, have been realized by mixing both compounds together.

The insulating oil obtained according to this invention has demonstrated outstanding and excellent results, especially when used as a transformer oil, a cable oil and a condenser oil having thermal resistance.

The present invention will be further explained by reference to the following non-limiting examples.

EXAMPLE 1 A tarry material obtained by treating Seria crude oil at 1250C for a contact time of 0.005 second was determined by analysis to have an aromatic distribution comprising 47 percent dicyclic compounds, 33 percent tricyclic compounds, 19 percent tetracyclic compounds and 1 percent pentacyclic compounds, and determined by NMR to have a total of 7 percent side chain hydrogen comprising 4 percent CH and 3 percent CH This tarry material was desulfurized in a hydrogen gas stream by using an ordinary desulfurization catalyst consisting of cobalt-molybdenum-alumina, then alkylated with ethylene using silicaalumina as a catalyst to give a polycyclic aromatic hydrocarbon EXAMPLE 2 A bottom oil material was obtained as a by-product in the production of ethylene-propylene by the thermal cracking of Kuwait naphtha at 800C for the contact 5 mlxture time of 0.5 second. This bottom oil had an aromatic The mixture had bolhong P (calfillllated at ring distribution comprising 76 percent dicyclic comnormal P 280 pQ l P after pounds, 21 percent tricyclic compounds and 3 percent type analysis by silica gel absorption comprising 99.1 tetracyclic compounds, and total side chain hydrogen weight percent aromatics and 0.9 weight percent nonof 20 percent comprising 5% CH 5% CH and 10% aromatics, and, by Mass spectrometry analysis, com- CH --Cl-l by NMR analysis, which was an aromatic oil prising 60.1 percent dicyclic rings, 35.4 percent comalniflg relatively y Side Chaiflstricyclic rings and 4.5 percent tetracyclic rings. This oil was desulfurized in a manner similar to that A portion of (B) was completely nuclearshown in Example 1 to prepare a polycyclic aromatic hydrogenated using a conventional nickel-alumina type hydrocarbon t ,7 catalyst to form a polycyclic naphthenic hydrocarbon had the followmg Propemesi bollmg P l' mixture culated at normal pressure) 280-380C, a composi- (A) had the following properties: boiling point (cal- P 9? tyPehanalyss by 5 wfw culated at normal pressure) 290-370C, a composi- L 6 P :5 i a t tion after type analysis by silica gel absorption comprise mma a y Spe.c tome ry ana in 2 6 Wei ht cream aromatics and 97 4 Wei ht er comprising 70.5 percent dicyclic rings, 26.8 percent g g M g tricyclic rings and 2.7 tetracyclic rings. gem wf' y t speftmmgtry ana This aromatic hydrocarbon mixture (D) was 9 Pu e P dlcychc g Percent hydrogenated according to a procedure similar to that W and percent fetrafychc "P described in Example 1 to prepare a polycyclic (A)and (B) where then mixed in a ratio of 75/25, naphthenic hydrocarbon mixture 50/50 and y Volume, p (C) had the following properties boiling point (cai- The properties and electrical performances of the culated at normal pressure) =2g0 330 a i. resulting composlllons are given Table A 15 tion after type analysis by silica gel absorption comprisapparent from the table, the characteristics of these in 1,8 wei htnt aromatics d 93,2 weight mixtures after degradation were found to be very excelcent non-aromatics; and by Mass spectrometry analysis lent. comprising 44.2 percent dicyclic rings, 51.8% tricyclic Ffl'fl' "V A n iiami Feed oil A Feed oil B Thermally stable insulating oil I IS No. 2 oil (polycyclic (polycyclic (commerical naphthenes) aromatics) I(A/B =75/25 II(A/B =/50 III(A/B =25/75 mineral oil) Specific gravity d4" i 0. 983 1.002 0. 093 0. 99s 1. 002 0. 880 Retractive indeiagnguu 1.2203 IigBg 1.1823 lig tg 1.1268 1.1332 Vimsity F- {wr' CI" 514 410 414 413 411 314 Pour point, 0.-.. -42.5 010 42.5 -40.0 -45. 0 -47.5 Ignition point, o 161 104 162 163 103 140 Acidic value mg., K H 0.003 0.003 0. 003 0.003 0. 003 0. 004 Copper plate corrosion, angstroms 1 1 1 1 1 1 Dieleetr c constant (80 C.) 2. 25 2. 2. 35 2.40 2. 53 2. 25 tan a (80,) {Initial 0.001 0.07 0.01 0. 01a 0. 034 0.13 After degradation 0.20 0. 86 0. 08 0. 04 0. 08 12.2 Initial 2. 0x10 00x10 aoxio 1.3x10 20x10 50x10 Volumatlc resistance (80 C.)(ncm.) After degl'fldationn 1. 0x10 4. 0x10 2. 5x10 4. 0x10 2. 3x10 2. 0x10 Hydrogen gas absorptivity (111m) +15 180 -160 TABLE 2 M Feed oil 0 Feed oil D Thermally stable insulating oils (polycyclic (polycyclic naphthene's) aromatics) IV(C/D=7fil25) V(C/D=50/50) VI(ClD=25/75) Specific gravity, di" 0. 942 1. 024 0.065 0.983 1.001 Refractive indexfio okn 1.1902 1.2833 1. 1.1382 1 .0 many c s. 2 4.3 5.0 4. s 4. 4 Pour point. C -43. 0 45. 0 43. 5 44. 5 45. 0 Ignition point, C 151 165 152 152. 5 153 Acid valiie Ing.. KOH/g 0.003 0.003 0.003 0.003 0.003 Copper plate corroslon angstroms 1 l 1 1 1 Dielectric constant (8% 331.... 02b}; 0262; 2. :0 2655 n a... 0.00 0. 25 5 (Pelcem) {Alter degradation. 0. 20 0. 35 0. 07 0. 03 0. 000 Volumatic resistance {Initial 4.3)(10 1.0)(10 3.5)(10 4.8)(10 6. 0x10 15 C.) (ii-cm.) After degradation... 2. 5X10 8. 0X10 7.8)(10 5.0)(10 14 23x10" Hydrogen gas absorptlvity (mm.) 180 40 120 A copper coil and a 100cc sample of the oil were placed in a 200cc beaker. and the degradation test was carried out for 75 hours in a hot air dryer at l20C.

"' A voltage of IOKV was applied at 80C under a hydrogen atmosphere, and the gas generation and gas absorption were expressed by an oil manometer.

rings and 4.0 percent tetracyclic rings.

Both hydrocarbon mixtures (C) and (D) were blended in ratios of 75/25, 50/50 and 25/75 by volume,

respectively, to form three examples of the insulating oil of this invention, the properties and electrical per- See the note in Table See the note in Table 1.

EXAMPLE 3 The material oil used in this example was a byproduct bottom oil formed in a process for the production of ethylene and propylene were Kuwait naphtha was thermally cracked in a tubular furnace at a reaction temperature of 830C for the contact time of 0.4 second. This bottom oil was hydrogenated using a nickel-molybdenum-alumina catalyst in the presence of hydrogen at a temperature of 350C, an LHSV of 0.8 and under a pressure of 40 Kglcm G. The resulting hydrogenated product was then reacted with 1.5 mole equivalents of propylene in the presence of a silica-alumina type catalyst under apressure of 10 Kg/cm, an LHSV of 1.0 and at a temperature of 200C. The oily product was distilled under reduced pressure to give a fraction (E) boiling in the range of 300-350C in a yield of 52 percent. This fraction was then determined by mass spectrometry and type analysis by adsorption on silica gel to have the following composition:

Type analysis by Silica gel adsorption:

Aromatics weight 98. Non-aromatics weight 1.

Mass spectrometry Dicyclic rings Tricyclic rings Tetracyclic rings TABLE 3 Thermally stable Thermally stable insulating oil insulating oil Vll (E/C-l-75/25 Vll (E/C=5l50) Specific gravity d, 0.982 0.961 Refractive index n,, 1.570 1.548 Viscosity (cst) 30C 16.3 17.2 75C 4.0 4.1 Pour point C 47.5 47.5 ignition point C 151 152 Acid value mgKOH/g 0.002 0.002 Copper plate corrosion 1 A 1 A Dielectric constant (80C) 2.38 2.32 tan 8 (80C) Initial 0.029 0.024

After degradation 0.05 0.04 Initial 1.3 X 10 9.5 X 10 Volumatic resis -tance (Q-cm) (80C) After degradation 4.8 X 10 3.7 X 10 Hydrogen gas absorptivity (mm) l00 60 See the note in Table 1. See the note in Table 1.

What is claimed is:

l. A thermally stable electrical insulating oil with improved gas absorption performance comprising from 5 to 80 percent by volume ofa member selected from the group consisting of one or more polycyclic naphthenic hydrocarbons, lower alkyl derivatives thereof and a mixture of such naphthenic hydrocarbons and lower alkyl derivatives, and from 95 to percent by volume of a mem lle selected from the group consisting of one or more polycyclic aromatic hydrocarbons, lower alkyl derivatives thereof and a mixture of such aromatic hydrocarbons and lower alkyl derivatives, the polycyclic naphthenic hydrocarbons being obtained by hydrogenating an aromatic fraction which is obtained by the thermal cracking of petroleum hydrocarbons at a temperature above 700C.

2. The thermally stable insulating oil of claim 1 wherein the one or more polycyclic naphthenic hydrocarbons, lower alkyl derivatives thereof and mixtures thereof, have a specific gravity d of from 0.85 to 0.99, a refractive index n of from 1.45 to 1.60, a viscosity (30C.) of from 10 to 40 cst, a hydrogen/carbon atomic ratio of from 1.75 to 2.00, an ignition point of above 125C., and a pour point of from --30 to 60 C.

3. The thermally stable insulating oil of claim 1 wherein the one or more polycyclic aromatic hydrocarbons, lower alkyl derivatives thereof and mixtures thereof, have a specific gravity d of from 1.00 to 1.20, a refractive index n of from 1.50 to 1.68, a viscosity (30C.) of from 10 to 25 est, an ignition point of above 150C., and a pour point of below -40C.

4. The thermally stable insulating oil of claim 1 wherein more than one polycyclic naphthenic hydrocarbon is present as a mixture of polycyclic naphthenic hydrocarbons, each having from two to five rings, their lower alkyl derivatives or a mixture thereof.

5. The thermally stable insulating oil of claim 1 wherein the polycyclic aromatic hydrocarbon is a mixture of polycyclic hydrocarbons each having from two to four rings, their lower alkyl derivatives or a mixture thereof.

6. The thermally stable insulating oil of claim 1 wherein the polycyclic naphthenic hydrocarbons, lower alkyl derivatives thereof and mixtures thereof are obtained by desulfurization, alkylation and hydrogenation of an aromatic fraction which is obtained by the high temperature cracking of petroleum hydrocarbons.

7. The thermally stable insulating oil of claim 6 wherein the alkylation is carried out with butylene, propylene or ethylene.

8. The thermally stable insulating oil of claim 1 wherein the polycyclic aromatic hydrocarbons are obtained by desulfurization and alkylation of an aromatic fraction which is obtained by the high temperature thermal cracking of the petroleum hydrocarbons.

9. The thermally stable insulating oil of claim 8 wherein the alkylation is carried out with butylene, propylene or ethylene.

10. The thermally stable insulating oil of claim 1 wherein the alkyl portion of the lower alkyl derivatives contains less than eight carbon atoms.

11. The thermally stable insulating oil of claim 10 wherein the alkyl portion of the alkyl derivative contains four or less carbon atoms.

12. The thermally stable insulating oil of claim I having the following properties:

Dielectric constant (a) (C) 2.35 2.65

Dielectric tangent tan 8 (80C) 0.001 0.03

Volumatic resistance (Q-cm) (80C) 2 X 10 2 X Insulation breakdown voltage KV/2.5mm 50 80 Thermal stability (C 75 hrs) Sludge 0.008 0.40

Total acid value 0.05 0.60.

13. The thermally stable insulating oil of claim 1 wherein the polycyclic hydrocarbons are obtained by desulfurization and alkylation of an aromatic fraction obtained by the high temperature cracking of the petroleum hydrocarbons at a temperature greater than 700C. to yield the polycyclic aromatic hydrocarbons, and a portion of the polycyclic aromatic hydrocarbons is thereafter hydrogenated to' yield the polycyclic naphthenic hydrocarbons, the polycyclic naphthenic and the polycyclic aromatic hydrocarbons thereafter being blended to yield the thermally stable insulating oil.

14. The thermally stable insulating oil of claim 13 wherein the thermal cracking is at a temperature of above 700C but below 2,300C for a period of time above from 1 to 0.001 seconds.

- 15. The thermally stable insulating oil of claim 14 wherein the material cracked is selected from the group consisting of crude oil, heavy oil, light oil, kerosene, naphtha, an oil tar from the gasification of heavy or crude oil at a high temperature, a coal tar or a bottom oil from the dealkylation of alkyl aromatics.

16. The thermally stable insulating oil of claim 1 wherein the polycyclic naphthenic hydrocarbon is selected from the group consisting of decaline, perhydrophenanthrene, perhydropyrene, perhydrofluorene, perhydroanthracene and perhydrochrysene, alkyl derivatives thereof wherein the alkyl group contains eight or less carbon atoms, and mixtures thereof.

17. The thermally stable insulating oil of claim 1 wherein the polycyclic aromatic hydrocarbon has from 2 to 4 rings and is selected from the group consisting of naphthalene, diphenyl, acenaphthene, fluorene, terphenyl, pyrene, chrysene, alkyl derivatives thereof wherein the alkyl group has eight or less carbon atoms and mixtures thereof. I

Claims

1. A thermally stable electrical insulating oil with improved gas absorption performance comprising from 5 to 80 percent by volume of a member selected from the group consisting of one or more polycyclic naphthenic hydrocarbons, lower alkyl derivatives thereof and a mixture of such naphthenic hydrocarbons and lower alkyl derivatives, and from 95 to 20 percent by volume of a member selected from the group consisting of one or more polycyclic aromatic hydrocarbons, lower alkyl derivatives thereof and a mixture of such aromatic hydrocarbons and lower alkyl derivatives, the polycyclic naphthenic hydrocarbons being obtained by hydrogenating an aromatic fraction which is obtained by the thermal cracking of petroleum hydrocarbons at a temperature above 700*C.

2. The thermally stable insulating oil of claim 1 wherein the one or more polycyclic naphthenic hydrocarbons, lower alkyl derivatives thereof and mixtures thereof, have a specific gravity d415 of from 0.85 to 0.99, a refractive index nD20 of from 1.45 to 1.60, a viscosity (30*C.) of from 10 to 40 cst, a hydrogen/carbon atomic ratio of from 1.75 to 2.00, an ignition point of above 125*C., and a pour point of from -30* to -60*C.

3. The thermally stable insulating oil of claim 1 wherein the one or more polycyclic aromatic hydrocarbons, lower alkyl derivatives thereof and mixtures thereof, have a specific gravity d415 of from 1.00 to 1.20, a refractive index nD20 of from 1.50 to 1.68, a viscosity (30*C.) of from 10 to 25 cst, an ignition point of above 150*C., and a pour point of below -40*C.

12. The thermally stable insulating oil of claim 1 having the following properties: Dielectric constant ( epsilon ) (80*C) 2.35 - 2.65 Dielectric tangent (%) tan delta (80*C) 0.001 - 0.03 Volumatic resistance ( Omega -cm) (80*C) 2 X 1014- 2 X 1016 Insulation breakdown voltage KV/2.5mm 50 - 80 Thermal stability (125*C - 75 hrs) Sludge % 0.008 - 0.40 Total acid value 0.05 - 0.60.

13. The thermally stable insulating oil of claim 1 wherein the polycyclic hydrocarbons are obtained by desulfurization and alkylation of an aromatic fraction obtained by the high temperature cracking of the petroleum hydrocarbons at a temperature greater than 700*C. to yield the polycyclic aromatic hydrocarbons, and a portion of the polycyclic aromatic hydrocarbons is thereafter hydrogenated to yield the polycyclic naphthenic hydrocarbons, the polycyclic naphthenic and the polycyclic aromatic hydrocarbons thereafter being blended to yield the thermally stable insulating oil.

14. The thermally stable insulating oil of claim 13 wherein the thermal cracking is at a temperature of above 700*C but below 2, 300*C for a period of time above from 1 to 0.001 seconds.

15. The thermally stable insulating oil of claim 14 wherein the material cracked is selected from the group consisting of crude oil, heavy oil, light oil, kerosene, naphtha, an oil tar from the gasification of heavy or crude oil at a high temperature, a coal tar or a bottom oil from the dealkylation of alkyl aromatics.

17. The thermally stable insulating oil of claim 1 wherein the polycyclic aromatic hydrocarbon has from 2 to 4 rings and is selected from the group consisting of naphthalene, diphenyl, acenaphthene, fluorene, terphenyl, pyrene, chrysene, alkyl derivatives thereof wherein the alkyl group has eight or less carbon atoms and mixtures thereof.