AU2006260919A1

AU2006260919A1 - Oxidative stable oil formulation

Info

Publication number: AU2006260919A1
Application number: AU2006260919A
Authority: AU
Inventors: Andree Hilker; Volker Klaus Null
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: 2006-12-28
Also published as: KR20080025746A; CN101198680B; US20090082235A1; WO2006136591A1; EP1896556A1; CA2611649A1; RU2416628C2; TW200728447A; EP1896556B1; BRPI0611906B1; JP5420241B2; BRPI0611906A2; ZA200709550B; RU2008102362A; JP2008544057A; CN101198680A

Description

WO 2006/136591 PCT/EP2006/063433 1 OXIDATIVE STABLE OIL FORMULATION Field of invention The invention is related to an oxidation stable oil formulation comprising a base oil composition and additives. 5 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. 10 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 15 at 40 OC 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 20 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 25 discloses phosphorodithionates as antiwear additives. WO-A-02070629 describes a process to make iso paraffinic base oils from a wax as made in a Fischer Tropsch process. According to this publication base oils having a kinematic viscosity at 100 OC of between 2 WO 2006/136591 PCT/EP2006/063433 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 5 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 10 dipropionate. The oil had a high oxidative stability. 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 15 adverse effects on other properties than oxidation stability. Summary of the invention This aim is achieved with the following oil formulation. Oxidation stable oil formulation comprising 20 a base oil composition comprising a mineral-derived naphthenic base oil, a mineral-derived paraffinic base oil, and/or a Fischer-Tropsch derived base oil, a copper passivator and of from 0.001 to less than 0.1 wt% of an organic sulphur or phosphorus based compound. 25 Applicants found that an oil formulation is achieved having a very high oxidation stability, however not requiring a high treat rate. Brief description of the drawings Figure 1 and 2 represent the carbon distribution of 30 two Fischer-Tropsch derived base oils as used in the examples.

WO 2006/136591 PCT/EP2006/063433 3 Detailed description of the invention Applicants found that when a mineral-derived base oil of the so-called paraffinic type or naphthenic type, and/or a Fischer-Tropsch derived base oil is combined 5 with at least one copper passivator and a low content of an anti-wear additive, 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 10 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, while a Fischer-Tropsch derived base oil was derived from 15 Fischer-Tropsch synthesis products. Organic sulphur or phosphorus based compounds preferably are sulphur and phosphorus containing compounds such as sulfides, phosphides, dithiophopsphates and dithiocarbamates. More preferably, sulphur and 20 phosphorus containing compounds are used which are known to be used as an anti-wear additive in lubricating oil formulations. Yet more preferably an organic polysulphide compound is used. With polysulfide is here meant that the organic compound comprises at least one group where two 25 sulphur atoms are directly linked. A preferred polysulfide compound is a disulfide compound. Preferred polysulfide compounds are represented by the formula (I) RI-(S)a-R2 (I) wherein: 30 a is 2, 3, 4 or 5, preferably 2;

R

1 and R 2 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 WO 2006/136591 PCT/EP2006/063433 4 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 R 1 and R 2 are independently selected from a benzyl group or 5 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), which publication is incorporated by reference. Examples 10 of suitable disulfide compounds are dibenzyldi sulfide,ditertdodecyldisulfide and didodecyldisulfide. The content of the organic sulphur or phosphorus anti wear additive in the oil formulation is preferably less than formulation 800 mg/kg and even more preferably less 15 than 400 mg/kg. The lower limit is preferably 1 mg/kg more preferably 10 mg/kg, most preferably 50 mg/kg. The copper passivator or electrostatic discharge depressant, sometimes also referred as metal deactivator, may be the typical copper passivator of which 20 N-salicylideneethylamine, N,N'-di salicylideneethyl diamine, triethylenediamine, ethylenediamminetetraacetic acid, phosphoric acid, citric acid and gluconic acid. More preferred are lecithin, thiadiazole, imidazole and pyrazole and derivates thereof. Even more preferred are 25 zinc dialkyldithiophosphates, dialkyldithiocarbamates and benzotriazoles and their tetrahydroderivates. Most preferred are the compounds according to formula (II) or even more preferred the optionally substituted benzotriazole compound represented by the formula (III) WO 2006/136591 PCT/EP2006/063433 5 wherein R4 may be hydrogen or a group represented by the formula (IV) (IV) or by the formula (V /M (VT) wherein: 5 c is 0, 1, 2 or 3; R3 is a straight or branched C1_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 10 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.

WO 2006/136591 PCT/EP2006/063433 6 Preferred compounds are 1-[bis(2-ethylhexyl) aminomethyl]benzotriazole, methylbenzotriazole, dimethylbenzotriazole, ethylbenzotriazole, ethylmethylbenzotriazole, diethylbenzotriazole and 5 mixtures thereof. Other preferred compounds include (N Bis(2-ethylhexyl)-aminomethyl-tolutriazole, non substituted benzotriazole, and 5-methyl-lH-benzotriazole. Examples of copper passivator additives as described above are described in US-A-5912212, EP-A-1054052 and in 10 US-A-2002/0109127, which publications are hereby incorporated by reference. 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 15 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. 20 The content of the above copper passivator in the oil formulation is preferably above 1 mg/kg and more preferably above 5 mg/kg. A practical upper limit may vary depending on the specific application of the oil formulation. For example, when desiring improved 25 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. This concentration may be up to 3 wt%, preferably however in the range of from 0.001 to 1 wt%. Applicants found that 30 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 50 mg/kg.

WO 2006/136591 PCT/EP2006/063433 7 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 or a Fischer-Tropsch derived base oil, the sludge formed and 5 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 10 example naphthols, sterically hindered monohydric, dihydric and trihydric phenols, sterically hindered dinuclear, trinuclear and polynuclear phenols, alkylated or styrenated diphenylamines or ionol derived hindered phenols. Sterically hindered phenolic antioxidants of 15 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 20 butyl-hydroxyhydrocinnamate) (IRGANOX TM L109, CIBA), ((3,5-bis(1,1-dimethylethyl)-4-hydroxyphenyl)methyl)thio) acetic acid, C 10

-C

14 isoalkyl esters (IRGANOX TM L118, CIBA), 3,5-di-tert-butyl-4-hydroxyhydrocinnamic acid, C 7 C 9 alkyl esters (IRGANOX TM L135, CIBA,) tetrakis-(3-(3,5 25 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 30 butylhydroquinone. These products are known and are commercially available. Of most particular interest is 3,5-di-tert-butyl-4-hydroxy-hydrocinnamic acid-C 7

-C

9 alkyl ester.

WO 2006/136591 PCT/EP2006/063433 8 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' 5 bis(l-ethyl-3-methyl-pentyl)-p-phenylene-diamine, N,N' bis(l-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 10 phenylenediamine, N-(1,3-dimethylbutyl)-N'-phenyl-p phenylenediamine, N-(l-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, 15 diphenylamine, N-allyldiphenylamine, 4-isopropoxy diphenylamine, N-phenyl-l-naphthylamine, N-phenyl-2 naphthylamine, octylated diphenylamine, e.g. p,p'-di tert-octyldiphenylamine, 4-n-butylaminophenol, 4-butyrylaminophenol, 4-nonanoylaminophenol, 20 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, 25 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 30 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 WO 2006/136591 PCT/EP2006/063433 9 of EP-A-1054052, which compounds are also described in US-A-4,824,601, which publications are hereby incorporated by reference. The content of the anti oxidant additive is 5 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. 10 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 15 invention. The base oil composition preferably has a kinematic viscosity at 100 OC of less than 50 mm 2 /sec, more preferably between 2 and 25 mm 2 /sec, most preferably between 2 and 15 mm 2 /sec. The base oil composition 20 preferably has a kinematic viscosity at 40 *C of between 1 and 200 mm 2 /sec, more preferably between 3.5 and 100 mm 2 /sec, most preferably between 5 and 12 mm 2 /sec. The viscosity of the base oil composition will also depend on the particular use of the oil formulation. If the oil 25 formulation is used as an electrical oil its kinematic viscosity at 40 oCis preferably between 1 and 50 mm 2 /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 mm 2 /sec. 30 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 mm 2 /sec.

WO 2006/136591 PCT/EP2006/063433 10 The flash point of the base oil composition as measured by ASTM D92 may be greater than 90 OC, preferably greater than 120 oC, yet more preferably greater than 140 OC, and even more preferably greater 5 than 170 oC. 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 10 transformers and electric engines. The base oil composition may comprise one or more base oils selected from mineral-derived naphthenic base oils, mineral-derived paraffic base oils, or Fischer Tropsch derived base oils. 15 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 20 purpose of this invention are defined as having a pour point of below -20 OC 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 25 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 30 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 WO 2006/136591 PCT/EP2006/063433 11 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, 5 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 10 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 15 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 20 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 25 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 30 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: CnH2n+z. Because the saturates phase WO 2006/136591 PCT/EP2006/063433 12 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 5 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 iso 10 paraffinic 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 15 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. More preferably the paraffinic wax is a 20 Fischer-Tropsch derived wax, because of its purity and even higher paraffinic content. The base oils as derived from a Fischer-Tropsch wax as here described will be referred to in this description as Fischer-Tropsch derived base oils. 25 Examples of Fischer-Tropsch processes which for example can be used to prepare the above-described Fischer-Tropsch derived base oil are the so-called commercial Slurry Phase Distillate technology of Sasol, the Shell Middle Distillate Synthesis Process and the 30 "AGC-21" Exxon Mobil process. These and other processes are for example described in more detail in EP-A-776959, EP-A-668342, US-A-4943672, US-A-5059299, WO-A-9934917 and WO-A-9920720. Typically these Fischer-Tropsch synthesis WO 2006/136591 PCT/EP2006/063433 13 products will comprise hydrocarbons having 1 to 100 and even more than 100 carbon atoms. This hydrocarbon product will comprise normal paraffins, iso-paraffins, oxygenated products and unsaturated products. If base oils are one 5 of the desired iso-paraffinic products it may be advantageous to use a relatively heavy Fischer-Tropsch derived feed. The relatively heavy Fischer-Tropsch derived feed has at least 30 wt%, preferably at least 50 wt%, and more preferably at least 55 wt% of compounds 10 having at least 30 carbon atoms. Furthermore the weight ratio of compounds having at least 60 or more carbon atoms and compounds having at least 30 carbon atoms of the Fischer-Tropsch derived feed is preferably at least 0.2, more preferably at least 0.4 and most preferably at 15 least 0.55. Preferably the Fischer-Tropsch derived feed comprises a C 20 + fraction having an ASF-alpha value (Anderson-Schulz-Flory chain growth factor) of at least 0.925, preferably at least 0.935, more preferably at least 0.945, even more preferably at least 0.955. Such a 20 Fischer-Tropsch derived feed can be obtained by any process, which yields a relatively heavy Fischer-Tropsch product as described above. Not all Fischer-Tropsch processes yield such a heavy product. An example of a suitable Fischer-Tropsch process is described in 25 WO-A-9934917. The Fischer-Tropsch derived product will contain no or very little sulphur and nitrogen containing compounds. This is typical for a product derived from a Fischer Tropsch reaction, which uses synthesis gas containing 30 almost no impurities. Sulphur and nitrogen levels will generally be below the detection limits, which are currently 5 mg/kg for sulphur and 1 mg/kg for nitrogen respectively.

WO 2006/136591 PCT/EP2006/063433 14 The process will generally comprise a Fischer-Tropsch synthesis, a hydroisomerisation step and an optional pour point reducing step, wherein said hydroisomerisation step and optional pour point reducing step are performed as: 5 (a) hydrocracking/hydroisomerisating a Fischer-Tropsch product, (b) separating the product of step (a) into at least one or more distillate fuel fractions and a base oil or base oil intermediate fraction. 10 If the viscosity and pour point of the base oil as obtained in step (b) is as desired no further processing is necessary and the oil can be used as the base oil according the invention. If required, the pour point of the base oil intermediate fraction is suitably further 15 reduced in a step (c) by means of solvent or preferably catalytic dewaxing of the oil obtained in step (b) to obtain oil having the preferred low pour point. The desired viscosity of the base oil may be obtained by isolating by means of distillation from the intermediate 20 base oil fraction or from the dewaxed oil the a suitable boiling range product corresponding with the desired viscosity. Distillation may be suitably a vacuum distillation step. The hydroconversion/hydroisomerisation reaction of 25 step (a) is preferably performed in the presence of hydrogen and a catalyst, which catalyst can be chosen from those known to one skilled in the art as being suitable for this reaction of which some will be described in more detail below. The catalyst may in 30 principle be any catalyst known in the art to be suitable for isomerising paraffinic molecules. In general, suitable hydroconversion/hydroisomerisation catalysts are those comprising a hydrogenation component supported on a WO 2006/136591 PCT/EP2006/063433 15 refractory oxide carrier, such as amorphous silica alumina (ASA), alumina, fluorided alumina, molecular sieves (zeolites) or mixtures of two or more of these. One type of preferred catalysts to be applied in the 5 hydroconversion/hydroisomerisation step in accordance with the present invention are hydroconversion/ hydroisomerisation catalysts comprising platinum and/or palladium as the hydrogenation component. A very much preferred hydroconversion/hydroisomerisation catalyst 10 comprises platinum and palladium supported on an amorphous silica-alumina (ASA) carrier. The platinum and/or palladium is suitably present in an amount of from 0.1 to 5.0% by weight, more suitably from 0.2 to 2.0% by weight, calculated as element and based on total weight 15 of carrier. If both present, the weight ratio of platinum to palladium may vary within wide limits, but suitably is in the range of from 0.05 to 10, more suitably 0.1 to 5. Examples of suitable noble metal on ASA catalysts are, for instance, disclosed in WO-A-9410264 and EP-A-0582347. 20 Other suitable noble metal-based catalysts, such as platinum on a fluorided alumina carrier, are disclosed in e.g. US-A-5059299 and WO-A-9220759. A second type of suitable hydroconversion/ hydroisomerisation catalysts are those comprising at 25 least one Group VIB metal, preferably tungsten and/or molybdenum, and at least one non-noble Group VIII metal, preferably nickel and/or cobalt, as the hydrogenation component. Both metals may be present as oxides, sulphides or a combination thereof. The Group VIB metal 30 is suitably present in an amount of from 1 to 35% by weight, more suitably from 5 to 30% by weight, calculated as element and based on total weight of the carrier. The non-noble Group VIII metal is suitably present in an WO 2006/136591 PCT/EP2006/063433 16 amount of from 1 to 25 wt%, preferably 2 to 15 wt%, calculated as element and based on total weight of carrier. A hydroconversion catalyst of this type, which has been found particularly suitable, is a catalyst 5 comprising nickel and tungsten supported on fluorided alumina. The above non-noble metal-based catalysts are preferably used in their sulphided form. In order to maintain the sulphided form of the catalyst during use 10 some sulphur needs to be present in the feed. Preferably at least 10 mg/kg and more preferably between 50 and 150 mg/kg of sulphur is present in the feed. A preferred catalyst, which can be used in a non sulphided form, comprises a non-noble Group VIII metal, 15 e.g., iron, nickel, in conjunction with a Group IB metal, e.g., copper, supported on an acidic support. Copper is preferably present to suppress hydrogenolysis of paraffins to methane. The catalyst has a pore volume preferably in the range of 0.35 to 1.10 ml/g as 20 determined by water absorption, a surface area of preferably between 200-500 m 2 /g as determined by BET nitrogen adsorption, and a bulk density of between 0.4-1.0 g/ml. The catalyst support is preferably made of an amorphous silica-alumina wherein the alumina may be 25 present within wide range of between 5 and 96 wt%, preferably between 20 and 85 wt%. The silica content as SiO 2 is preferably between 15 and 80 wt%. Also, the support may contain small amounts, e.g., 20-30 wt%, of a binder, e.g., alumina, silica, Group IVA metal oxides, 30 and various types of clays, magnesia, etc., preferably alumina or silica. The preparation of amorphous silica-alumina microspheres has been described in Ryland, Lloyd B., WO 2006/136591 PCT/EP2006/063433 17 Tamele, M.W., and Wilson, J.N., Cracking Catalysts, Catalysis: volume VII, Ed. Paul H. Emmett, Reinhold Publishing Corporation, New York, 1960, pp. 5-9. The catalyst is prepared by co-impregnating the 5 metals from solutions onto the support, drying at 100-150 *C, and calcining in air at 200-550 OC. The Group VIII metal is present in amounts of about 15 wt% or less, preferably 1-12 wt%, while the Group IB metal is usually present in lesser amounts, e.g., 1:2 to about 10 1:20 weight ratio respecting the Group VIII metal. A typical catalyst is shown below: Ni, wt% 2.5-3.5 Cu, wt% 0.25-0.35 A1 2 0 3 -SiO 2 wt% 65-75 15 A1 2 0 3 (binder) wt% 25-30 Surface Area 290-325 m 2 /g Pore Volume (Hg) 0.35-0.45 ml/g Bulk Density 0.58-0.68 g/ml Another class of suitable hydroconversion/ 20 hydroisomerisation catalysts are those based on zeolitic materials, suitably comprising at least one Group VIII metal component, preferably Pt and/or Pd, as the hydrogenation component. Suitable zeolitic and other aluminosilicate materials, then, include Zeolite beta, 25 Zeolite Y, Ultra Stable Y, ZSM-5, ZSM-12, ZSM-22, ZSM-23, ZSM-48, MCM-68, ZSM-35, SSZ-32, ferrierite, mordenite and silica-aluminophosphates, such as SAPO-11 and SAPO-31. Examples of suitable hydroisomerisation/ hydroisomerisation catalysts are, for instance, described 30 in WO-A-9201657. Combinations of these catalysts are also possible. Very suitable hydroconversion/ WO 2006/136591 PCT/EP2006/063433 18 hydroisomerisation processes are those involving a first step wherein a zeolite beta or ZSM-48 based catalyst is used and a second step wherein a ZSM-5, ZSM-12, ZSM-22, ZSM-23, ZSM-48, MCM-68, ZSM-35, SSZ-32, ferrierite, 5 mordenite based catalyst is used. Of the latter group ZSM-23, ZSM-22 and ZSM-48 are preferred. Examples of such processes are described in US-A-20040065581, which disclose a process comprising a first step catalyst comprising platinum and zeolite beta and a second step 10 catalyst comprising platinum and ZSM-48. Combinations wherein the Fischer-Tropsch product is first subjected to a first hydroisomerisation step using the amorphous catalyst comprising a silica-alumina carrier as described above followed by a second 15 hydroisomerisation step using the catalyst comprising the molecular sieve has also been identified as a preferred process to prepare the base oil to be used in the present invention. More preferred the first and second hydroisomerisation steps are performed in series flow. 20 Most preferred these two steps are performed in a single reactor comprising beds of the above amorphous and/or crystalline catalysts. In step (a) the feed is contacted with hydrogen in the presence of the catalyst at elevated temperature and 25 pressure. The temperatures typically will be in the range of from 175 to 380 oC, preferably higher than 250 OC and more preferably from 300 to 370 OC. The pressure will typically be in the range of from 10 to 250 bar and preferably between 20 and 80 bar. Hydrogen may be 30 supplied at a gas hourly space velocity of from 100 to 10000 Nl/l/hr, preferably from 500 to 5000 Nl/l/hr. The hydrocarbon feed may be provided at a weight hourly space velocity of from 0.1 to 5 kg/l/hr, preferably higher than WO 2006/136591 PCT/EP2006/063433 19 0.5 kg/l/hr and more preferably lower than 2 kg/l/hr. The ratio of hydrogen to hydrocarbon feed may range from 100 to 5000 Nl/kg and is preferably from 250 to 2500 Nl/kg. The conversion in step (a) as defined as the weight 5 percentage of the feed boiling above 370 0C which reacts per pass to a fraction boiling below 370 OC, is at least 20 wt%, preferably at least 25 wt%, but preferably not more than 80 wt%, more preferably not more than 65 wt%. The feed as used above in the definition is the total 10 hydrocarbon feed fed to step (a), thus also any optional recycle of a high boiling fraction which may be obtained in step (b). In step (b) the product of step (a) is preferably separated into one or more distillate fuels fractions and 15 a base oil or base oil precursor fraction having the desired viscosity properties. If the pour point is not in the desired range the pour point of the base oil is further reduced by means of a dewaxing step (c), preferably by catalytic dewaxing. In such an embodiment 20 it may be a further advantage to dewax a wider boiling fraction of the product of step (a). From the resulting dewaxed product the base oil and oils having a desired viscosity can then be advantageously isolated by means of distillation. Dewaxing is preferably performed by 25 catalytic dewaxing as for example described in WO-A-02070629, which publication is hereby incorporated by reference. The final boiling point of the feed to the dewaxing step (c) may be the final boiling point of the product of step (a) or lower if desired. 30 The oil formulation may comprise a single type of base oil or blends of the above-described base oils as base oil composition. Preferably, the present invention further relates to formulations wherein the base oil WO 2006/136591 PCT/EP2006/063433 20 composition comprises at least 80% by weight of the total formulation of a mineral-derived naphthenic base oil; to formulations wherein the base oil comprises at least 80% by weight of a mineral-derived paraffinic base oil; and 5 to formulations wherein the base oil composition comprises at least 80% by weight of a Fischer-Tropsch derived base oil. Also further base oils and other synthetic base oil components may be present in the oil formulation, such as 10 for example esters, poly alpha olefins, as preferably obtained by oligomerisation of an olefinic compound, poly alkylene glycols and the like. Possible base oil compositions preferably include mineral-derived paraffinic base oils and Fischer-Tropsch derived base 15 oils, mineral-derived naphthenic base oils and Fischer Tropsch derived base oils, and mixtures of the three base oil components. However, it has been found especially advantageous to use a Fischer-Tropsch derived base oil as the 20 substantially the sole base oil component. With substantially is here meant that more than 80 wt%, more preferably more than 90 wt% and most preferably 100 wt% of the base oil component in the oil formulation is a Fischer-Tropsch derived base oil as described in detail 25 above. 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 30 additives are dispersants, detergents, viscosity modifying polymers, hydrocarbon or oxygenated hydrocarbon type pour point depressants, emulsifiers, demulsifiers, antistaining additives and friction modifiers. Specific WO 2006/136591 PCT/EP2006/063433 21 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 5 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 10 viscosity modifier is a viscosity modifying polymer, for example polyisobutylenes, olefin copolymers, poly methacrylates and polyalkylstyrenes and hydrogenated polyisoprene star polymer (Shellvis). Examples of suitable antifoaming agents are polydimethylsiloxanes and 15 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 20 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 25 electrical oil. It has further been found that when the base oil component of the oil formulation comprises substantially of the Fischer-Tropsch derived base oil an electrical oil formulation is obtained which has good oxidative stability, as expressed by low acid formation 30 and/or low sludge formation and also excellent low temperature viscosity values. Examples of applications are switch gears, transformers, regulators, circuit breakers, power plant reactors, cables and other WO 2006/136591 PCT/EP2006/063433 22 electrical equipment. A problem often encountered when using an electrical oil based on a naphthenic base oil is that the kinematic viscosity at -30 OC is too high. When such an oil would be used in application which have to 5 start up at low temperatures, especially at temperatures below 0 OC, the higher viscosity will have a negative effect on the required heat dissipation of the electrical oil. Overheating of the equipment can result. Applicants have found that when a Fischer-Tropsch base oil having a 10 kinematic viscosity at 40 OC of between 1 and 15 mm 2 /sec and a pour point of below -30 OC, more preferably below -40 OC an electrical oil can be obtained having the above desired properties. In order to improve the gassing tendency of the oil 15 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, di isopropylbenzene, a mixture of alkylbenzenes as 20 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 25 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 30 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 WO 2006/136591 PCT/EP2006/063433 23 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, 5 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 oil formulations comprising a Fischer-Tropsch 10 derived base oil as described above show a very low dielectric dissipation factor, even after prolonged testing at elevated temperature. The low dissipation factor is indicative for a low loss of electric power in the application wherein the electrical oil is used. 15 Because the dissipation factor does not significantly increase over time, especially when compared to the naphthenic based electrical oil formulations, a very efficient application of the oil results. The electrical oil as described above may find use in 20 applications which have to start up regularly, especially more than 10 times per year at a temperature of below 0 0 C, more preferably below -5 0 C, wherein the temperature of the oil when the application is running is above 0 *C. Examples of such applications are as low 25 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 30 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 WO 2006/136591 PCT/EP2006/063433 24 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.

WO 2006/136591 PCT/EP2006/063433 25 u 44 1N -n r rd r H N InN H 44-I 0 Io n w H D )C) ' 0) H CD C 0 r H 1 N N v CIl 0) H- 0 0 N IJ (N m cco 0~ 0 r. (N N r NN H I 0) H ( (N L 0~U 00n K4K 0) 0 H C) ri 0 F: : 0i-i ) 0 P4 -oU 0 H 4 4 1 P C) 4 4 H r4 0 o4 r- a 4 - 0 O 0d : Q 4) U) ;J (d - U 4 M U d4H 0 - d r ) r -H o H 0) P © @> >l P4 r14 - ,, P4 4-) *Hr P WO 2006/136591 PCT/EP2006/063433 26 0 4.) Hf HZ 4J 0 0 N 0 1 .H Cl) H ( o0 E HO0 -T4 -H 0 -H m rd H C) H- 41) ci) i) ci) rX4 4-) CD 'd NO0 '4 ai) E I-c Ea 0 CD -H r 4-cU ) I-c -H) r-I 4c i 04 0 '-0 Qj rd o ;: 0 ty) r d I-c t- 0 wi o\O-H 4 .c.0) ) c 0 ' 00 El)4. E- CD H f H 4- P4) Qd 09 -H m i 0 HH H4 0 CD U) CN cci 9 cc u ad) 0 NH ccd 4-) . q ci)4 I-I- 4 CDl I- 0 c En c j b cc >. ccd -H . 0O. 0 0 ciC i MU 4.) I-c 04 0 'dO 0 d 10 4-) H- H 0 0 Wci P4 -H ; 0 -H- *Hi CC (d.0 c z . MU ImP 4.) N - F: WO 2006/136591 PCT/EP2006/063433 27 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 5 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 10 tested with the IEC 61125 C Oxidation test 164h/120 OC 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 15 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 20 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 25 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/1200C. The lower the value the less sludge is found. 30 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 WO 2006/136591 PCT/EP2006/063433 28 formation. Especially for the mineral paraffin base oils and the Fischer-Tropsch derived base oil the presence of an anti-oxidant further reduces the sludge formation significantly.

WO 2006/136591 PCT/EP2006/063433 29 0 CC) n w w 0 0 In oo v 0 0 0 c OD N j40 I N0 0 o 0 0 0 w~ 0 Nv m0 1 I n (N 0 o 0 0 0 0 rn) (N I H I I 'I 0 H- (N 0 0 0 In H- m 0 (N H H U 4J 0 0 0 (d u 4 x0 0 00 0 M HWH4 k -ri -Hi w I 0 41 Ho rI Hr -H E90 X ' 00 10 Ur -H EQ rl Hr rll 09 4J HO Q . -H I004 ro- -H 09 E~ Uo I- l 'I I (d I~ -H -H 04 WO 2006/136591 PCT/EP2006/063433 30 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 OC was measured. The lower the value the less acid compounds are 5 formed and the more oxidative stable the oil formulation is. The results are presented in Table 3.

WO 2006/136591 PCT/EP2006/063433 31 co c'q CN 0 Lfl 0 0 0 0 H NH - - 0 H 00 0 00m co ,: 0 C o Co m 00 I N( 00 m N H 0n H U - co L oN 0co r- L H-- - - - - --- - (N z CN H O .H On m U H O H H I) r 0 0 H 1(N -a ,. -H 0 H 4-Q oo 00 -H O - -a O \ -I- - 4J -0 -H U H m e > - U H 0 (dI -H H 0 a) 0 H -0 H N H ,d 5 r - -H H H :d a) H 0 E rH 4 w) a) X 0 0 0 . r ' a) L u-H ,r H a) a) U1) 44I H Ca ) H H- >1 (I) mn rIS d I .H- 0 m2 41~ 0 -I-1 rO -H -H al) N F: U U -H 4J rO I rd (d * -H U rd H (4 -H '0 r- r-, (n > > - -i -Hi w -H -HA N m/ 4I-) x r, 44I (t (10 HF4 : -Hi 0 0 -1-) 4-4 PQ H d -Hj () ,Q N -HI Ir. rtd Q 4-) rd ,Q - r. -W) Q-4 rd 0 '0 -H 1 0) mr n H H p I il IQ H .i q I I a41 WO 2006/136591 PCT/EP2006/063433 32 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 5 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.

WO 2006/136591 PCT/EP2006/063433 33 U) 0 0 H1 Hn 0n oN 0 0 H Lf 0~ 0 0 0D 0N0 In r- - H (o N 00 0 0 H D' N 0 0 m0 0 0) - ) C) 0 0 LO H m N N (N I I H0 m) m) m)N (N U U mU 0 0 0 z z z z ri 4-) 41) 4-) 41) t- 4U E > - 0 H 0 H- Q) :j0 -Hi 5 4-3 p Q) 5 -H S 0 CD - -H U U 0 .r- H 0 0 U o H 4-) C) 0 >1 0 -H H xrU 0 0 w -H H -H H 4)I H H 44~ H- LI-4 4-4 4-) HH C) 1H 4-4 H ~ >- Q) CQ 44I >1 >-I4 m~ H: Hi rr- I) i 0 H- H H OH -H I 4 0I 4) 0 4) E- H H H a) 0 C) rd -Hi 41) ) N r4 CA Z H C/) W/ Wl P4 -H (' d I t m w) H Z 0 0 0 H )~H U) (Nq -H- 'd -H- 0H u U u N 4 CD co U) H-- >i~ l '- 4 -H 4-) a4 0 Cl) Cl) U) 0 H U) rd .EI rd N 4) CO 4J-) >4 -j 124 H H HU a) Hi Q 4-) I r_ -H1 0 0 a) r H- X! Q) a) >1 N -H- Q.4 C) 04 .* P4 rS Q (d~~ 4-) 0 D Z Z Z i~ (15H -Hi -H H I $4 1 0 OH H H P~H H 0 1 z I IQ u IH ,q I - _ WO 2006/136591 PCT/EP2006/063433 34

C.

C/2 11 H rd I 0 02 N 0 0 Nq r2 0 0D 0 L 040 0 0 v v (N (Nq 0 0 o q H H o t o i o on 0 0 H 0 C C o o v v 0 Ln L) Ln LO U U U N (N N H H H If)| HY r-ij- r x 0 O S o .1 o -) LI-4 . N 2 rd U H a U 0 4-)~ U 0 (N 0 0 4J H- 0 0 m rd Lfl 0 (4 71 0 U 0 4 H 4 0 H 0 0 H 4-) 0 0 i-L j 0 rd -H 0 4- 0 4 u -4 4-) H E (d o -H U) $ (d C) -t -H 0 O tN H 2 rO H u W >4 m m Oo . H E - 4 ) - - O 0 LI-I - U)> 2 zH - H 4 H r '1 ) U ) 0 r H U u (d H - 0 w _q p -, rd -) u '0 t ) U af Q) ~ L) o H Cl) Q (N1 H w U) 0),~U H H rd H (N 0 U) x H -W- U -H U) H 0 0 w 0 H- -H - H H4 p~ 41- ml Q All .Q E p: U U 02 0- *~ I 0 W I I C * H C)o 1 H H 4L) _- WO 2006/136591 PCT/EP2006/063433 35 Table 4 shows that the oil formulation based on the Fischer-Tropsch derived base oil has a low viscosity at -30 OC in combination with excellent oxidative stability properties. The gassing tendency of the Mixture Z of 5 Table 4 can be improved by adding an aromatic solvent as illustrated in Table 5. Table 5 Sample Identification Z Z' GTL base oil-1 Wt0 94,68 94,18 naphthenic base oil-1 Wt% Mineral Paraffinic base Wt% 5,00 5,00 oil-1 Dibenzyldisulfid (Antiwear Wt% 0,02 0,02 additive) Clay treatment (Tonsil) Wt% 1 1 1-[bis(2-ethylhexyl)amino Mg/kg 10 10 methyl]benzotriazole Shellsol A 150 (aromatic Wt% none 0,5 hydrocarbon solvent) Antioxidant BHT Wt% 0,30 0,30 GASSING TENDENCY measured according to BS 5797 mm 3 /min > 0 -8,9 Example 4 Three oil formulations A-C were made using the GTL Base Oils 1, 2 and 3 of Table 1 according to the 10 formulation as listed in Table 6. The oil formulations A-C were subjected to a clay treatment using Tonsil 411 clay as obtainable from Sued Chemie, Manchen (D) .The anti-oxidant and copper passivator additive were added after the clay treatment.

WO 2006/136591 PCT/EP2006/063433 36 The oils were tested with the test methods listed in Table 6. The results show that excellent oils for use as electrical oils.

WO 2006/136591 PCT/EP2006/063433 37 Table 6 Oil properties Oil A Oil B Oil C Formulation GTL BO-1 Wt% 94,7 GTL BO-2 Wt% 98,7 GTL BO-3 Wt% 98,7 Paraffinic-base oil 1 Wt% 5,0 Paraffinic-base oil 2 wt% 1,0 1,0 Dibenzyldisulfide mg/kg 200 200 200 1-[bis(2-ethylhexyl) aminomethyl]benzo- mg/kg 10 10 10 triazole Ionol 861805 % 0,3 0,3 0,3 Test results TEST DIMENS.METHODE FLASH POINT 0 C ISO 2592 160 226 263 DIN ISO POUR POINT 0 C 3016 -51 -30 -18 Not DIN measure KIN.VISCOSITY 40 OC mm 2 /s 51562 7,8 17,5 d DIN KIN.VISCOSITY 100 OC mm 2 /s 51562 2,4 4,1 7,8 IEC OXIDATION TEST 500h/1200C IEC 61125/C - Total acidity mgKOH/g 0,02 0,02 0,04 - Sludge Gew.% < 0,006 <0,008 < 0,007 - Dielectr. Dissip. F. 90 0 C 0,0035 0,0004 0,0004

Claims

1. Oxidation stable oil formulation comprising a base oil composition comprising a mineral-derived naphthenic base oil, a mineral-derived paraffinic base oil, and/or a Fischer-Tropsch derived paraffinic base oil, a copper 5 passivator and at most 0.1 wt% of an organic sulphur or phosphorus anti-wear additive.

2. Formulation according to claim 1, wherein the anti wear additive comprises an organic polysulfide represented by the formula 10 RI-(S)a-R 2 (I) wherein: a is 2, 3, 4 or 5; R 1 and R 2 are independently selected from the group consisting of optionally substituted, straight or 15 branched, saturated or unsaturated Cl-C 25 hydrocarbon groups.

3. Formulation according to claim 2, wherein R 1 and R2 are independently selected from the group consisting of optionally substituted, straight or branched, aromatic or 20 aliphatic C 4 -C 20 , preferably C 6 -C 14 hydrocarbon groups.

4. Formulation according to claim 3, wherein R, and R 2 are independently selected from straight or branched dodecyl and benzyl.

5. Formulation according to one or more of the preceding 25 claims, wherein the content of organic sulphur or phosphorus anti-wear additive in the formulation is less than 800 mg/kg, more preferably less than 400 mg/kg.

6. Formulation according to one or more of the preceding claims, wherein the copper passivator is a compound WO 2006/136591 PCT/EP2006/063433 39 according to formula (II) or an optionally substituted benzotriazole compound represented by the formula (III) wherein R4 may be hydrogen or a group represented by the formula (IV) 1~/ (IV) 5 or by the formula (V I (V) wherein: c is 0, 1, 2 or 3; R3 is a straight or branched C1_4 alkyl group; R5 is a methylene or ethylene group; R6 and R7 are hydrogen or 10 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. WO 2006/136591 PCT/EP2006/063433 40

7. Formulation according to claim 6, wherein R 3 is methyl or ethyl and C is 1 or 2.

8. Formulation according to any one of claims 1-6, wherein the content of the copper passivator additive is 5 between 5 and 1000 mg/kg.

9. Formulation according to any one of claims 1-7, wherein the formulation has a sulphur content of below 0.5 wt%.

10. Formulation according to any one of claims 1-8 also 10 containing an anti-oxidant additive.

11. Formulation according to claim 10, wherein the anti oxidant additive is a phenolic or amine antioxidant.

12. Formulation according to claim 11, wherein the anti oxidant additive is ditert.-butylated hydroxotoluene. 15

13. Formulation according to any one of claims 1-12, wherein the base oil composition comprises at least 80% by weight of a mineral-derived naphthenic base oil.

14. Formulation according to any one of claims 1-12, wherein the base oil composition comprises at least 80% 20 by weight of a mineral-derived paraffinic base oil.

15. Formulation according to any one of claim 1-12, wherein the base oil composition comprises at least 80% by weight of a Fischer-Tropsch derived base oil.

16. Formulation according to any one of claims 1-15, 25 wherein the kinematic viscosity at 40 OC of the base oil composition is between 1 and 4 mm 2 /sec.

17. Formulation according to any one of claims 1-15, wherein the kinematic viscosity at 40 *C of the base oil composition is between 5 and 15 mm 2 /sec. 30

18. Process to prepare a formulation according to any one of claims 1-17, wherein a mixture of the base oil composition and the organic sulphur or phosphorus anti wear additive is subjected to a clay treatment and WO 2006/136591 PCT/EP2006/063433 41 wherein the copper passivator is added after performing the clay treatment.

19. Use of the formulation according to one or more of the preceding claims as an electrical oil. 5

20. Use of the formulation according to claim 19 as a low temperature switch gear application.

21. Use of the formulation according to claim 19 as a transformer oil.