EP1896556B1

EP1896556B1 - Oxidative stable oil formulation

Info

Publication number: EP1896556B1
Application number: EP06777414.1A
Authority: EP
Inventors: Volker Klaus Null; Andree Hilker
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 2005-06-23
Filing date: 2006-06-22
Publication date: 2018-09-26
Anticipated expiration: 2026-06-22
Also published as: RU2416628C2; CN101198680B; WO2006136591A1; TW200728447A; RU2008102362A; BRPI0611906A2; US20090082235A1; AU2006260919A1; CA2611649A1; EP1896556A1; KR20080025746A; BRPI0611906B1; JP5420241B2; JP2008544057A; ZA200709550B; CN101198680A

Description

Field of invention

The invention is related to an oxidation stable oil formulation comprising a base oil composition and additives.

Background of the invention

US-A-6790386 describes a dielectric fluid comprising an iso-paraffin base oil and additives. The iso-paraffin base oil is prepared by hydrotreating, hydroisomerisation and hydrogenation of a paraffinic vacuum feedstock.
US-A-6214776 describes a formulation comprising a paraffinic base oil and an additive package containing a hindered phenol antioxidant and a metal deactivator, for use as load tap changer or transformer oil. According to this publication, base oils having a kinematic viscosity at 40 °C of between 5 and 20 cSt can be used as base oil in formulations such as electrical oils or transformer oils.
US-A-5241003 discloses a combination of a sulfur-containing antiwear additive and a carboxylic derivative dispersant for use as additive package for lubricants.
US-A-5773391 describes a composition comprising a polyol ester base oil, an aliphatic monocarboxylic acid mixture, and an additive package comprising an antioxidant and a metal deactivator. The document further discloses phosphorodithionates as antiwear additives.
WO-A-02070629 describes a process to make isoparaffinic base oils from a wax as made in a Fischer-Tropsch process. According to this publication base oils having a kinematic viscosity at 100 °C of between 2 and 9 cSt can be used as base oil in formulations such as electrical oils or transformer oils.
US-A-5912212 describes oxidative stable oil lubricating formulations consisting of a hydrocracked paraffinic mineral base oil and 0.1 to 5 wt% of a sulphur or phosphorus containing compound. In the examples a formulation consisting of a base oil and 3-methyl-5-tert-butyl-4-hydroxy propionic acid ester, dioctylamino-methyltolyltriazole and 0.4 wt% of dilaurylthio-dipropionate. The oil had a high oxidative stability.
US3338877 describes co-polymers of ethylene and stilbene useful as lubricating oils with excellent oxidative and thermal stability. A number of anti-scuffing agents are taught as suitable for addition thereto.
A demand is acknowledged for high oxidation resistant oil products for use as for example electrical oil, in particular as a transformer oil or a switch gear oil, preferably without high additive treat rates due to adverse effects on other properties than oxidation stability.

Summary of the invention

This aim is achieved with the oil formulation defined in claim 1.
Applicants found that an oil formulation is achieved having a very high oxidation stability, however not requiring a high treat rate.

Detailed description of the invention

Applicants found that when a mineral-derived base oil of the so-called paraffinic type and/or naphthenic type, base oil is combined with at least one copper passivator selected from optionally substituted benzotriazole compounds represented by the formula (III) and a low content of an anti-wear additive comprising an organic polysulfide represented by the formula (I), an oil product is obtained which has properties highly suitable for use as an electrical oil. It was not to be expected that the combination of the copper passivator and a small amount of an anti-wear additive would result in such an improvement in oxidative stability. A mineral-derived base oil has the meaning within the context of this specification that the base oil was obtained from a mineral oil source.
Polysulfide compounds are represented by the formula (I)

R¹-(S)_a-R² (I)

wherein:

a is 2, 3, 4 or 5, preferably 2;
R¹ and R² may be the same or different and each may be straight or branched alkyl group of 1 to 22 carbon atoms, aryl groups of 6-20 carbon atoms, alkylaryl groups of 7-20 carbon atoms or arylalkyl groups of 7-20 carbon atoms. Preferred are arylalkyl groups, more preferred are optionally substituted benzyl groups. More preferably R¹ and R² are independently selected from a benzyl group or a straight or branched dodecyl group. Examples of possible sulphur and phosphorus containing compounds and the preferred compounds mentioned here are described in the aforementioned US-A-5912212 as its component (b). Examples of suitable disulfide compounds are dibenzyldisulfide,ditertdodecyldisulfide and didodecyldisulfide. The content of the anti-wear additive in the oil formulation is preferably less than formulation 800 mg/kg and even more preferably less than 400 mg/kg. The lower limit is 10 mg/kg, most preferably 50 mg/kg.

The copper passivator or electrostatic discharge depressant, sometimes also referred as metal deactivator, is the optionally substituted benzotriazole compound represented by the formula (III)
wherein R⁴ may be hydrogen or a group represented by the formula (IV)
or by the formula (V)
wherein:

c is 0, 1, 2 or 3;
R³ is a straight or branched C_1-4 alkyl group. Preferably R³ is methyl or ethyl and C is 1 or 2. R⁵ is a methylene or ethylene group; R⁶ and R⁷ are hydrogen or the same or different straight or branched alkyl groups of 1-18 carbon atoms, preferably a branched alkyl group of 1-12 carbon atoms; R⁸ and R⁹ are the same or different alkyl groups of 3-15 carbon atoms, preferably of 4-9 carbon atoms.

Preferred compounds are 1-[bis(2-ethylhexyl)-aminomethyl]benzotriazole, methylbenzotriazole, dimethylbenzotriazole, ethylbenzotriazole, ethylmethylbenzotriazole, diethylbenzotriazole and mixtures thereof. Other preferred compounds include (N-Bis(2-ethylhexyl)-aminomethyl-tolutriazole, non-substituted benzotriazole, and 5-methyl-1H-benzotriazole. Examples of copper passivator additives as described above are described in US-A-5912212 , EP-A-1054052 and in US-A-2002/0109127 . These benzotriazoles compounds are preferred because they also act as an electrostatic discharge depressant, which is beneficial when the oil formulation is used as an electrical oil. Copper passivator additives as those described above are commercially available under the product names BTA, TTA, IRGAMET 39, IRGAMET30 and IRGAMET 38S from CIBA Ltd Basel Switzerland, also traded under the trade name Reomet by CIBA.
The content of the above copper passivator in the oil formulation is between 5 and 1000 mg/kg. A practical upper limit may vary depending on the specific application of the oil formulation. For example, when desiring improved dielectric discharge tendencies of the oil for use as electrical oil it may be desired to add a high concentration of the copper passivator additive. Applicants found that the advantages of the invention can be achieved at concentrations below 1000 mg/kg and more preferably below 300 mg/kg, even more preferably below 50mg/kg.
The oil formulation preferably also comprises an anti-oxidant additive. It has been found that, especially in case the base oil is a mineral paraffinic base oil, the sludge formed and total acidity both measured after the IEC 61125 C oxidation test, which properties are indicators for good oxidation stable oils, are considerably reduced when also an anti-oxidant is present. The anti-oxidant may be a so-called hindered phenolic or amine antioxidant, for example naphthols, sterically hindered monohydric, dihydric and trihydric phenols, sterically hindered dinuclear, trinuclear and polynuclear phenols, alkylated or styrenated diphenyl-amines or ionol derived hindered phenols. Sterically hindered phenolic antioxidants of particular interest are selected from the group consisting of 2,6-di-tert-butylphenol (IRGANOX TM L 140, CIBA), di tert-butylated hydroxotoluene (BHT), methylene-4,4'-bis-(2.6-tert-butylphenol), 2,2'-methylene bis-(4,6-di-tert-butylphenol), 1,6-hexamethylene-bis-(3,5-di-tert-butyl-hydroxyhydrocinnamate) (IRGANOX TM L109, CIBA), ((3,5-bis(1,1-dimethylethyl)-4-hydroxyphenyl)methyl)thio) acetic acid, C₁₀-C₁₄isoalkyl esters (IRGANOX TM L118, CIBA), 3,5-di-tert-butyl-4-hydroxyhydrocinnamic acid, C₇-C₉alkyl esters (IRGANOX TM L135, CIBA,) tetrakis-(3-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionyloxymethyl)methane (IRGANOX TM 1010, CIBA), thiodiethylene bis(3,5-di-tert-butyl-4-hydroxyhydrocinnamate (IRGANOX TM 1035, CIBA), octadecyl 3,5-di-tert-butyl-4-hydroxyhydrocinnamate (IRGANOX TM 1076, CIBA) and 2,5-di-tert-butylhydroquinone. These products are known and are commercially available. Of most particular interest is 3,5-di-tert-butyl-4-hydroxy-hydrocinnamic acid-C₇-C₉-alkyl ester.
Examples of amine antioxidants are aromatic amine anti-oxidants for example N,N'-Di-isopropyl-p-phenylenediamine, N,N'-di-sec-butyl-p-phenylenediamine, N,N'-bis(1,4-dimethyl-pentyl)-p-phenylenediamine, N,N'-bis(1-ethyl-3-methyl-pentyl)-p-phenylene-diamine, N,N'-bis(1-methyl-heptyl)-p-phenylenediamine, N,N'-dicyclohexyl-p-phenylene-diamine, N,N'-diphenyl-p-phenylenediamine, N,N'-di(naphthyl-2-)-p-phenylenediamine, N-isopropyl-N'-phenyl-p-phenylenediamine, N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine, N-(1-methylheptyl)-N'-phenyl-p-phenylenediamine, N'-cyclohexyl-N'-phenyl-p-phenylenediamine, 4-(p-toluene-sulfoamido)diphenylamine, N,N'-dimethyl-N,N'-di-sec-butyl-p-phenylenediamine, diphenylamine, N-allyldiphenylamine, 4-isopropoxy-diphenylamine, N-phenyl-1-naphthylamine, N-phenyl-2-naphthylamine, octylated diphenylamine, e.g. p,p'-di-tert-octyldiphenylamine, 4-n-butylaminophenol, 4-butyrylaminophenol, 4-nonanoylaminophenol, 4-dodecanoylaminophenol, 4-octadecanoylaminophenol, di(4-methoxyphenyl)amine, 2,6-di-tert-butyl-4-dimethyl-aminomethylphenol, 2,4'-diaminodiphenylmethane, 4,4'-diaminodiphenylmethane, N,N,N',N'-tetramethyl-4,4'-diaminodiphenylmethane, 1,2-di(phenylamino)ethane, 1,2-di[(2-methylphenyl)amino]ethane, 1,3-di-(phenylamino)propane, (o-tolyl)biguanide, di[4-(1',3'-dimethylbutyl)phenyl]amine, tert-octylated N-phenyl-1-naphthylamine, mixture of mono- and dialkylated tert-butyl-/tert-octyldiphenylamines, 2,3-dihydro-3,3-dimethyl-4H-1,4-benzothiazine, phenothiazine, N-allylphenothiazine, tert-octylated phenothiazine, 3,7-di-tert-octylphenothiazine. Also possible amine antioxidants are those according to formula VIII and IX of EP-A-1054052 , which compounds are also described in US-A-4,824,601 .
The content of the anti oxidant additive is preferably less than 2 wt% and more preferably less than 1 wt%. The content is preferably less than 0,6 wt% in certain applications, such as when the oil formulation is used as an electrical oil. The content of antioxidant is preferably greater than 10 mg/kg.
The oil formulation preferably has a sulphur content of below 0,5 wt% and even more preferably below 0,15 wt%. The source of the majority of the sulphur in the oil formulation will be the sulphur as contained in the base oil component of the oil formulation according the invention.
The base oil composition preferably has a kinematic viscosity at 100 °C of less than 50 mm²/sec, more preferably between 2 and 25 mm²/sec, most preferably between 2 and 15 mm²/sec. The base oil composition preferably has a kinematic viscosity at 40 °C of between 1 and 200 mm²/sec, more preferably between 3.5 and 100 mm²/sec, most preferably between 5 and 12 mm²/sec. The viscosity of the base oil composition will also depend on the particular use of the oil formulation. If the oil formulation is used as an electrical oil its kinematic viscosity at 40 °Cis preferably between 1 and 50 mm²/sec. More preferably, if this electrical oil formulation is a transformer oil, the base oil will preferably have a kinematic viscosity at 40 °C of between 5 and 15 mm²/sec. If the electrical oil is a low temperature switch gear oil the base oil viscosity at 40°C is preferably between 1 and 15 and more preferably between 1 and 4 mm²/sec.
The flash point of the base oil composition as measured by ASTM D92 may be greater than 90 °C, preferably greater than 120 °C, yet more preferably greater than 140 °C, and even more preferably greater than 170 °C. The higher flash points are desirable for applications where peak temperatures can exceed the average oil temperature, for instance in applications under high temperature and/or with restricted heat transmission potential. Examples are electric transformers and electric engines.
The base oil composition may comprise one or more base oils selected from mineral-derived naphthenic base oils and/or mineral-derived paraffic base oils.
The base oil composition may this comprise a mineral-derived base oil of the so-called paraffinic type or naphthenic type. Such base oils are obtained by refinery processes starting from paraffinic and naphthenic crude feeds. Mineral-derived naphthenic base oils for the purpose of this invention are defined as having a pour point of below -20 °C and a viscosity index of below 70. Mineral-derived paraffin base oils are defined by a viscosity index of greater than 70, preferably greater than 90. Mineral-derived naphthenic and paraffin base oils are well known and described in more detail in "Lubricant base oil and wax processing", Avilino Sequeira, Jr., Marcel Dekker, Inc, New York, 1994, ISBN 0-8247-9256-4, pages 28-35.
Applicants found that very good oxidative stable oil formulations can be obtained when the base oil composition has a saturates content as measured by IP386 of preferably greater than 98 wt%, more preferably greater than 99 wt% and even more preferably greater than 99.5 wt% as measured on fresh base oil.
The base oil composition preferably comprises a base oil comprising a series of iso-paraffins having n, n+1, n+2, n+3 and n+4 carbon atoms and wherein n is a number between 20 and 35.
Preferably, the paraffin content in the base oil composition is greater than 80 wt%, more preferably greater than 90 wt%, yet more preferably greater than 95%, and again more preferably greater than 98%.
The base oil composition furthermore may preferably have a content of naphthenic compounds of between 1 and 20 wt%. It has been found that these base oils have a good additive response to the additives listed above when aiming to improve oxidation stability. The content of naphthenic compounds and the presence of such a continuous series of iso-paraffins may be measured by Field desorption/Field Ionisation (FD/FI) technique. In this technique the oil sample is first separated into a polar (aromatic) phase and a non-polar (saturates) phase by making use of a high performance liquid chromatography (HPLC) method IP368/01, wherein as mobile phase pentane is used instead of hexane as the method states. The saturates and aromatic fractions are then analyzed using a Finnigan MAT90 mass spectrometer equipped with a Field desorption/Field Ionisation (FD/FI) interface, wherein FI (a "soft" ionisation technique) is used for the determination of hydrocarbon types in terms of carbon number and hydrogen deficiency. The type classification of compounds in mass spectrometry is determined by the characteristic ions formed and is normally classified by "z number". This is given by the general formula for all hydrocarbon species: C_nH_2n+z. Because the saturates phase is analysed separately from the aromatic phase it is possible to determine the content of the different iso-paraffins having the same stoichiometry or n-number. The results of the mass spectrometer are processed using commercial software (poly 32; available from Sierra Analytics LLC, 3453 Dragoo Park Drive, Modesto, California GA95350 USA) to determine the relative proportions of each hydrocarbon type.
The base oil composition having the continuous isoparaffinic series as described above are preferably obtained by hydroisomerisation of a paraffinic wax, yet more preferably followed by some type of dewaxing, such as solvent or catalytic dewaxing.
The above described base oil composition may preferably be obtained by hydroisomerisation of a paraffinic wax, preferably followed by a dewaxing treatment, such as a solvent or catalytic dewaxing treatment. The paraffinic wax may be a highly paraffinic slack wax.
The oil formulation may comprise a single type of base oil or blends of the above-described base oils as base oil composition. The base oil composition comprises at least 80% by weight of the total formulation of a mineral-derived naphthenic base oil; and to formulations wherein the base oil comprises at least 80% by weight of a mineral-derived paraffinic base oil.
Also further base oils and other synthetic base oil components may be present in the oil formulation, such as for example esters, poly alpha olefins, as preferably obtained by oligomerisation of an olefinic compound, poly alkylene glycols and the like.
Additional additives next to the ones described above may also be present in the formulation. The type of additives will depend on the specific application. Without intending to be limiting, examples of possible additives are dispersants, detergents, viscosity modifying polymers, hydrocarbon or oxygenated hydrocarbon type pour point depressants, emulsifiers, demulsifiers, antistaining additives and friction modifiers. Specific examples of such additives are described in for example Kirk-Othmer Encyclopedia of Chemical Technology, third edition, volume 14, pages 477-526. Suitably the dispersant is an ashless dispersant, for example polybutylene succinimide polyamines or Mannic base type dispersants. Suitably the detergent is an over-based metallic detergent, for example the phosphonate, sulfonate, phenolate or salicylate types as described in the above referred to General Textbook. Suitably the viscosity modifier is a viscosity modifying polymer, for example polyisobutylenes, olefin copolymers, polymethacrylates and polyalkylstyrenes and hydrogenated polyisoprene star polymer (Shellvis). Examples of suitable antifoaming agents are polydimethylsiloxanes and polyethylene glycol ethers and esters.
The oil formulation may find use as turbine oil, gasoline engine oil, diesel engine oil, automotive and industrial gear oils, for example automatic and manual transmission and differential oils, hydraulic machine oil, refrigerator oil, plastic processing oil for rolling, press, forging, sqeezing, draw, punch and the like operations, thermal treating oil, discharge processing oil, slide guide oil, rust proofing oil and heat medium. A preferred use of the oil formulation is as electrical oil. Examples of applications are switch gears, transformers, regulators, circuit breakers, power plant reactors, cables and other
electrical equipment. A problem often encountered when using an electrical oil based on a naphthenic base oil is that the kinematic viscosity at -30 °C is too high. When such an oil would be used in application which have to start up at low temperatures, especially at temperatures below 0 °C, the higher viscosity will have a negative effect on the required heat dissipation of the electrical oil. Overheating of the equipment can result. In order to improve the gassing tendency of the oil formulation it is preferred to add between 0.05 and 10 wt%, preferably between 0.1 and 5 wt% of an aromatic compound. Preferred aromatic compounds are for example tertrahydronaphthalene, diethylbenzene, diisopropylbenzene, a mixture of alkylbenzenes as commercially obtainable as "Shell Oil 4697" or "Shellsol A 150" both "Shell" products obtainable from Shell Deutschland GmbH. Another preferred mixture of aromatic compounds is comprised in a mixture of 2,6-di-t-butyl phenol and 2,6-di-t-butyl cresol. Preferably the oil formulation comprises between 0.1 and 3 wt% of 2,6-di-t-butyl phenol and 0.1 to 2 wt% of 2,6-di-t-butyl cresol in a weight ratio of between 1:1 and 1:1.5.
The oil formulation, preferably comprising the anti-wear additive, is preferably subjected to an additional clay treatment. Clay treatment is a well know treatment to remove polar compounds from the oil formulation. It is performed in order to further improve the color, chemical and thermal stability of the oil formulation. It may be performed prior to adding the additives mentioned in this description on a, partly, formulated oil formulation. Clay treatment processes are for example described in Lubricant base oil and wax processing, Avilino Sequeira, Jr., Marcel Dekker, Inc, New York, 1994, ISBN 0-8247-9256-4, pages 229-232. Preferably the copper passivator and optional anti-oxidant are added after the clay treatment.
The electrical oil as described above may find use in applications which have to start up regularly, especially more than 10 times per year at a temperature of below 0 °C, more preferably below -5 °C, wherein the temperature of the oil when the application is running is above 0 °C. Examples of such applications are as low temperature switch gear oils, transformers, regulators, circuit breakers, power plant reactors, switch gear, cables, electrical equipment. Such applications are well known to the skilled person and described for example in Lubricants and related products, Dieter Klamann, Verlag Chemie GmbH, Weinhem, 1984, pages 330-339.

The invention will be illustrated with the following non-limiting examples. In the examples use has been made of four different types of base oils. One Fischer-Tropsch derived base oil, referred to as GTL BO, two naphthenic type of base oils, referred to as naphthenic-1 and naphthenic-2, and a mineral paraffinic base oil. The properties of these base oils are listed in Table 1.

Table 1

Base Oil			GTL BO -1	GTL BO-2	GTL BO-3	Naphthenic-1	Naphthenic-2	Paraffinic -1	Paraffinic -2
Vk @ 100°C	ASTM D445	mm²/s	2,4	4.0	7.8	2,1	2,1	2,2	8,3
Vk @ 40°C	ASTM D445	mm²/s	7,9			8,8	7,8	8,0	75,1
VI	ASTM D2270		126	135	148	<0	47	88	73
Pour Point	ASTM D5950	°C	-51	-30	-24	-60	-60	-15	-18
Flash point	ASTM D92	°C	192	228	274	147	154	186	232
Paraffins by FD/FI technique		(wt%)	90.7	92.3	90.8
Carbon distribution			See Figure 1(*)		Figure 2(*)
Basic Nitrogen	ISO 3771mod	mg/kg				4	<1	1	3
Sulphur	ISO 14596	%m	<0,001			0,075	0,001	0,015	0,021
Colour	ASTM D2049		L0.5			L0.5	L0.5	L0.5	L1.5
Biodegradation after 28 days	ISO 14593	%	60

(*) Carbon distribution per carbon number as measured by Field desorption/Field Ionisation (FD/FI) technique, wherein Z=2 represents the iso and normal paraffins, Z=0 the 1-ring naphthenic compounds, Z=-2 the 2-ring naphthenic compounds, Z=-4 the 3-ring naphthenic compounds etc.

Example 1

In Example 1 two formulations A and B were prepared of which the base oil component consisted of 95 wt% of the naphthenic-2 base oil and for 5 wt% of the paraffinic-1 base oil. To these mixtures 10 mg/kg of 1-[bis(2-ethylhexyl)aminomethyl]benzotriazole (Reomet38S) was added. To mixture A 200 mg/kg of Dibenzyldisulfide was added and to mixture B 200 mg/kg of Di-n-dodecyldisulfid was added. Oil mixtures A and B were tested with the IEC 61125 C Oxidation test 164h/120 °C test to measure the acidity of the oil phase. The acidity of the oil phase of mixture A was 0.26 mg KOH/g and the acidity of the oil phase of mixture B was 0.94 mg KOH/g. Both values are very low and illustrate an excellent oxidative stability. The values for Mixture A show that even more excellent results are obtained when the preferred Dibenzyldisulfide additive is used as the an organic polysulphide anti-wear additive. It is surprising that the choice of a particular anti-wear additive can improve the oxidation stability in the manner here illustrated.

Example 2

Starting with the mineral-derived naphthenic-1, mineral-derived paraffin base oil and the GTL base oil-1 of Table 1 five different oil mixtures according to the additivation schemes 1-5 of table 2 were made. For all of these oil mixtures the Sludge Formation was measured according to the Oxidation Test IEC 61125 C at 164h/120°C. The lower the value the less sludge is found. The results are also presented in Table 2.

From Table 2 it can be seen that the combination of the organic polysulphide anti-wear additive and the copper passivator result in a remarkable low sludge Formation for mineral naphthenic and mineral paraffinic base oils. Especially for the mineral paraffin base oils the presence of an anti-oxidant further reduces the sludge formation significantly.

Table 2

Sludge formation according to IEC 61125 C
Additivation scheme		1	2	3	4	5
Dibenzyldisulfide	mg/kg	-	-	200	200	200
1-[bis(2-ethylhexyl)-aminomethyl]benzotriazole (Reomet38S)	mg/kg	-	10	-	10	10
Antioxidant BHT	%m	-	-	-	-	0,08
Naphthenic base oil	Sludge	1,700	1,530	0,561	0,281	0,295
Paraffinic base oil-1	Sludge	3,340	2,440	0,209	0,086	<0,006
GtL base oil-1	Sludge	0,085	0,023	0,043	0,071	0,006

For all of these oil mixtures according to additivation schemes 1-5 of above also the Total Acidity using the Oxidation Test IEC 61125 C at 164h/120 °C was measured. The lower the value the less acid compounds are formed and the more oxidative stable the oil formulation is. The results are presented in Table 3.

Table 3

Total Acidity formation according to IEC 61125 C
Additivation scheme		1	2	3	4	5
Dibenzyldisulfide	mg/kg	-	-	200	200	200
1-[bis(2-ethylhexyl)aminomethyl]-benzotriazole	mg/kg	-	10	-	10	10
Antioxidant BHT	Wt%	-	-	-	-	0,08

Total acidity according to IEC 61125 C
Naphthenic base oil-1	Mg KOH/g	4,14	3,87	1,59	0,83	1,02
Paraffinic base oil-1	Mg KOH/g	9,12	6,78	0,78	0,38	0,02
GTL Base Oil-1	Mg KOH/g	13,67	10,55	12,65	12,57	0,10

Example 3

4 oil mixtures were prepared according to the scheme as presented in Table 4. Two oil mixtures were subjected to a clay treatment using Tonsil 411 clay as obtainable from Sued Chemie, Muenchen (D). The anti-oxidant and copper passivator additives were added after the clay treatment. The properties of the oil mixtures were measured and the oil mixtures were subjected to the IEC OXIDATION TEST at 500h/120 °C.

Table 4

Sample Identification			U	V	X	Y	Z	W
GTL base oil-1	Wt%		99,61	99,3	99,68	-	94,68	-
Naphthenic-1	Wt%				-	99,68		94,68
Mineral paraffinic base oil-1	Wt%				-	-	5,00	5,00
Dibenzyldisulfid	Wt%		0,09	0,4	0,02	0,02	0,02	0,02
Clay treatment (Tonsil)	%				-	-	1	1
1-[bis(2-ethylhexyl)aminomethyl]-benzotriazole	mg/kg		10	10	10	10	10	10
Antioxidant BHT	Wt%		0,30	0,30	0,30	0,30	0,30	0,30
Properties of the oil mixtures			U	V	X	Y	Z	W
FLASH POINT		ISO 2719			160	145	160	145
POUR POINT	°C	DIN ISO 3016			<-60	<-60	-51	-54
KIN.VISCOSITY -30 °C	mm²/s	DIN 51562			341	1140	368	1210
KIN.VISCOSITY 40 °C	mm²/s	DIN 51562			8	8,7	8	9
KIN.VISCOSITY 100 °C	mm²/s	DIN 51562			2,4	2,2	2,4	2,2
BREAKDOWN VOLTAGE	kV	VDE 0370-5					84
DIELECTR. DISSIPATION FACTOR 90°C	VDE	VDE 0370-1					0,0002
KORRO. SULFUR Ag/100°C		DIN 53 353	Fail (*)	Fail (**)			pass	pass
IEC OXIDATION TEST 500h/120°C: IEC 61125/C
- Total acidity after 500h/120°C test	mgKOH/ g	IEC 61125/C			<0,01	0,69	0,02	0,41
- Sludge after 500h/120°C test	m %	IEC 61125/C			<0,006	0,202	<0,006	0,043
- Dielectr. Dissip. F. 90°C after 500h/120°C test		IEC 61125/C			0,0015	0,102 1	<0,003 5	0,1017

(*) light grey discolouration
(**) grey discolouration

Claims

Oxidation stable oil formulation comprising a base oil composition, said base oil composition comprising at least 80% by weight of a mineral-derived naphthenic base oil or at least 80% by weight of a mineral-derived paraffinic base oil, a content of between 5 and 1000 mg/kg of a copper passivator selected from optionally substituted benzotriazole compounds represented by the formula (III)
wherein R4 may be hydrogen or a group represented by the formula (IV)
or by the formula (V)
wherein:
c is 0, 1, 2 or 3;

R³ is a straight or branched C_1-4 alkyl group; R⁵ is a methylene or ethylene group; R⁶ and R⁷ are hydrogen or the same or different straight or branched alkyl groups of 1-18 carbon atoms, preferably a branched alkyl group of 1-12 carbon atoms; R⁸ and R⁹ are the same or different alkyl groups of 3-15 carbon atoms; from 0.001 wt% to less than 0.1 wt% of an anti-wear additive is selected from organic polysulfides represented by the formula

R¹-(S)a-R² (I)

wherein:
a is 2, 3, 4 or 5;

R¹ and R² are independently selected from the group consisting of optionally substituted, straight or branched, saturated or unsaturated C₁-C₂₅ hydrocarbon groups.
Formulation according to claim 1, wherein R¹ and R² are independently selected from the group consisting of optionally substituted, straight or branched, aromatic or aliphatic C₄-C₂₀, preferably C₆-C₁₄ hydrocarbon groups.
Formulation according to claim 1 or 2, wherein R¹ and R² are independently selected from straight or branched dodecyl and benzyl.
Formulation according to one or more of the preceding claims, wherein the content of anti-wear additive in the formulation is less than 800 mg/kg, more preferably less than 400 mg/kg.
Formulation according to any preceding claim, wherein R³ is methyl or ethyl and C is 1 or 2.
Formulation according to any one of claims 1-5, wherein the formulation has a sulphur content of below 0.5 wt%.
Formulation according to any one of claims 1-6 wherein the formulation also contains greater than 10 mg/kg to less than 0.6 wt% of an antioxidant additive wherein the antioxidant additive is a phenolic or amine antioxidant.
Formulation according to claim 7, wherein the anti-oxidant additive is ditert.-butylated hydroxotoluene.
Formulation according to any one of claims 1-8, wherein the kinematic viscosity at 40 °C of the base oil composition is between 1 and 4 mm²/sec.
Formulation according to any one of claims 1-8, wherein the kinematic viscosity at 40 °C of the base oil composition is between 5 and 15 mm²/sec.
Process to prepare a formulation according to any one of claims 1-6, wherein a mixture of the base oil composition and the organic sulphur anti-wear additive is subjected to a clay treatment and wherein the copper passivator is added after performing the clay treatment.
Use of the formulation according to one or more of the preceding claims as an electrical oil.
Use of the formulation according to claim 12 as a transformer oil.