Turbine oils with excellent high temperature oxidative stability

A turbine lubricant consisting of (A) alkylated diphenylamine and/or phenylnaphthylamines, and (B) sulfurized olefins and/or sulfurized fatty acids and/or ashless dithiocarbamates and/or tetraalkylthiuram disulfides, with the balance containing (C) base oils characterized by very low sulfur contents (<0.03 wt %) and a high level of saturates (>90 volume %), and optionally (D) neutral rust inhibitors, show superior oxidative stability and provide adequate corrosion protection and sludge control for turbine oil and R&O oil applications.

TECHNICAL FIELD
 The present invention is directed to turbine, rust and oxidation (R&O) and
 ashless hydraulic oils (hereinafter collectively referred to as "turbine
 oils") having excellent high temperature oxidative stability. A further
 object of this invention is to deliver this level of oxidation protection
 without sacrificing sludge control and without the need for phenolic
 antioxidants.
 BACKGROUND OF THE INVENTION
 Steam and gas turbine oils are top-quality rust- and oxidation-inhibited
 oils. Steam turbines employ steam that enters the turbine at high
 temperature and pressure and expands across both rotating and fixed
 blades. Only the highest-quality lubricants are able to withstand the wet
 conditions, high temperatures and long periods of service associated with
 steam turbine operation. In gas turbines, they must withstand contact with
 very hot surfaces, often with intermittent operation and periods of
 nonuse. Therefore, to be effective, both types of oil must have, in
 addition to good corrosion protection and demulsibility, outstanding
 resistance to oxidation, which includes a minimum tendency to form
 deposits in critical areas of the system.
 To achieve these desired properties, it is necessary to formulate these
 oils using a carefully balanced additive package. The nature of these
 fluids makes them very susceptible to contamination, particularly from
 other lubricants and additives. A relatively small degree of contamination
 can markedly affect the properties and expected service life of these
 lubricants. Further, to maintain effective operating conditions and to
 avoid damaging the equipment in which they are used, turbine oils should
 be kept meticulously clean and free of contaminants. Contamination is
 minimized by filtration of the turbine oils. To ensure that the turbine
 oils are substantially free of contaminants very fine filters are used.
 The ratio between power output of turbines and oil volume has increased
 considerably over the years. This has resulted in a substantial increase
 in turbine operating temperatures. Therefore, it is necessary to protect
 the lubricant from oxidative degradation. The use of more antioxidants is
 one possible solution but higher treat levels sometimes lead to other
 problems such as sludge formation and solubility difficulties. A better
 approach is the use of synergistic antioxidant combinations, such as those
 taught in the present invention, that provide improved oxidation
 performance without causing sludge formation.
 Due to the requirements of turbine oils, only a few classes of additives,
 relative to other types of lubricating compositions, are combined with the
 base oils. Generally, a finished turbine oil will contain only the base
 oil, antioxidants, rust inhibitors, demulsifiers, corrosion inhibitors and
 diluents, if necessary.
 EP 0735128 A2 discloses extended life rust and oxidation oils comprising a
 dithiocarbamate and an alkylphenyl-.alpha.-naphthylamine. This reference
 does not teach the use of Group II or higher (i.e., Group III or Group IV)
 base oils, or the advantages obtained thereby, as required by the present
 invention.
 SUMMARY OF THE INVENTION
 This invention describes the use of a two component antioxidant system that
 provides superior oxidation protection and acceptable sludge control in
 turbine oils formulated with Group II or higher base oils. The highly
 oxidatively stable lubricants of the present invention comprise (A) an
 amine antioxidant selected from the group consisting of alkylated
 diphenylamines, phenyl-naphthylamines and mixtures thereof, (B) sulfur
 containing additives selected from the group consisting of sulfurized
 olefins, sulfurized fatty acids, ashless dithiocarbamates,
 tetraalkylthiuram disulfides and mixtures thereof, and (C) a base oil
 characterized by very low sulfur contents (&lt;0.03 wt. %) and a high
 level of saturates (&gt;90 volume %). In another embodiment of the present
 invention, the highly oxidatively stable lubricants further contain (D) at
 least one rust inhibitor.

DETAILED DESCRIPTION OF THE INVENTION
 The present invention is directed to turbine lubricating oils comprising
 (A) an amine antioxidant selected from the group consisting of alkylated
 diphenylamines, phenyl-naphthylamines and mixtures thereof, (B) sulfur
 containing additives selected from the group consisting of sulfurized
 olefins, sulfurized fatty acids, ashless dithiocarbamates,
 tetraalkylthiuram disulfides and mixtures thereof, and (C) a base oil
 characterized by very low sulfur contents (&lt;0.03 wt. %) and a high
 level of saturates (&gt;90 volume %).
 In another embodiment of the present invention, the turbine lubricating
 oils further contain (D) at least one rust inhibitor.
 Component A--Amine Antioxidants
 The amine antioxidants suitable for use in the present invention should be
 soluble in the turbine oil package. The amine antioxidant is selected from
 the group consisting of alkylated diphenylamines, phenyl-naphthylamines
 and mixtures thereof. Examples of amine antioxidants that may be used in
 this invention include, but are not limited to, diphenylamine,
 phenyl-alpha-naphthylamine, phenyl-beta-naphthylamine, butyldiphenylamine,
 dibutyldiphenylamine, octyldiphenylamine, dioctyldiphenylamine,
 nonyldiphenylamine, dinonyldiphenylamine, heptyldiphenylamine,
 diheptyldiphenylamine, methylstyryldiphenylamine mixed butyl/octyl
 alkylated diphenylamines, mixed butyl/styryl alkylated diphenylamines,
 mixed nonyl/ethyl alkylated diphenylamines, mixed octyl/styryl alkylated
 diphenylamines, mixed ethyl/methylstyryl alkylated diphenylamines, octyl
 alkylated phenyl-alpha-naphthylamine, mixed alkylated
 phenyl-alpha-naphthylamines, and combinations of these at varying degrees
 of purity that are commonly used in the petroleum industry. Examples of
 commercial diphenylamines include, but are not limited to, Irganox.RTM.
 L06, Irganox.RTM. L57, and Irganox.RTM. L67 from Ciba Specialty Chemicals;
 Naugalube.RTM. AMS, Naugalube.RTM. 438, Naugalube.RTM. 438R,
 Naugalube.RTM. 438L, Naugalube.RTM. 500, Naugalube.RTM. 640,
 Naugalube.RTM. 680, and Naugard.RTM. PANA from Uniroyal Chemical Company;
 Goodrite.RTM. 3123, Goodrite.RTM. 3190X36, Goodrite.RTM. 3127,
 Goodrite.RTM. 3128, Goodrite.RTM. 3185X1, Goodrite.RTM. 3190X29,
 Goodrite.RTM. 3190X40, and Goodrite.RTM. 3191 from BFGoodrich Specialty
 Chemicals; HiTEC.RTM. 569 antioxidant and HiTEC.RTM. 4793 antioxidant
 available from Ethyl Corporation; Vanlube.RTM. DND, Vanlube.RTM. NA,
 Vanlube.RTM. PNA, Vanlube.RTM. SL, Vanlube.RTM. SLHP, Vanlube.RTM. SS,
 Vanlube.RTM. 81, Vanlube.RTM. 848, and Vanlube.RTM. 849 from R. T.
 Vanderbilt Company, Inc. These amine antioxidants are generally
 characterized by their nitrogen content and TBN as determined by ASTM D
 2896. It is preferred that the nitrogen content of the amine antioxidants
 be between 3.0 and 7.0 wt % and the TBN be between 100 and 250 mg KOH/g of
 the neat, i.e. undiluted, additive concentrate.
 The concentration of amine antioxidants in the finished oil can vary
 depending upon the basestock used, customer requirements and applications,
 and the desired level of antioxidant protection required for the specific
 turbine oil. Typically, the amine antioxidant is present in the finished
 turbine oil in an amount of from 0.04 wt % to 0.5 wt %, preferably, 0.05
 wt % to 0.3 wt. %.
 Component B--Sulfur-containing Compound
 The sulfur-containing compounds of the present invention are selected from
 the group consisting of sulfurized olefins, sulfurized fatty acids,
 ashless dithiocarbamates, tetraalkylthiuram disulfides and mixtures
 thereof. The sulfurized olefins suitable for use in the present invention
 may be prepared by a number of known methods. They are characterized by
 the type of olefin used in their production and their final sulfur
 content. High molecular weight olefins (e.g., those having an average
 molecular weight (Mn) of from about 112 to about 351 g/mole) are
 preferred. Examples of olefins that may be used include alpha-olefins,
 isomerized alpha-olefins, branched olefins, cyclic olefins, polymeric
 olefins and mixtures thereof. Examples of alpha olefins that may be used
 include 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, 1-nonene,
 1-decene, 1-undecene, 1-dodecene, 1-tridecene, 1-tetradecene,
 1-pentadecene, 1-hexadecene, 1-heptadecene, 1-octadecene, 1-nonadecene,
 1-eicosene, 1-heneicosene, 1-docosene, 1-tricosene, 1-tetracosene,
 1-pentacosene and mixtures thereof. Alpha olefins may be isomerized before
 the sulfurization reaction or during the sulfurization reaction.
 Structural and/or conformational isomers of the alpha olefins that contain
 internal double bonds or branching may also be used. For example,
 isobutylene is the branched olefin counterpart of the alpha olefin
 1-butene.
 Sulfur sources that may be used in the sulfurization reaction can include,
 for example, elemental sulfur, sulfur monochloride, sulfur dichloride,
 sodium sulfide, sodium polysulfide, and mixtures thereof added together or
 at different stages of the sulfurization process.
 Unsaturated fatty acids and oils, because of their unsaturation, may also
 be sulfurized and used in this invention. Examples of fatty acids that may
 be used include lauroleic acid, myristoleic acid, palmitoleic acid, oleic
 acid, elaidic acid, vaccenic acid, linoleic acid, linolenic acid, gadoleic
 acid, arachidonic acid, erucic acid, and mixtures of these. Examples of
 oils or fats that may be used include corn oil, cottonseed oil, grapeseed
 oil, olive oil, palm oil, peanut oil, rapeseed oil, safflower seed oil,
 sesame seed oil, soybean oil, sunflower oil, sunflower seed oil, and
 combinations thereof.
 The ashless dithiocarbamates and tetraalkylthiuram disulfides suitable for
 use in the present invention are preferably soluble in the turbine oil
 package. Examples of ashless dithiocarbamates that may be used include,
 but are not limited to, methylenebis(dialkyldithiocarbamate),
 ethylenebis(dialkyldithiocarbamate), and isobutyl
 disulfide-2,2'-bis(dialkyldithiocarbamate), where the alkyl groups of the
 dialkyldithiocarbamate can preferably have from 1 to 16 carbons. Examples
 of preferred ashless dithiocarbamates are
 methylenebis(dibutyldithiocarbamate), ethylenebis
 (dibutyldithiocarbamate), and isobutyl
 disulfide-2,2'-bis(dibutyldithiocarbamate). Examples of preferred
 tetraalkylthiuram disulfides that may be used include tetrabutylthiuram
 disulfide and tetraoctylthiuram disulfide.
 The concentration of Component B in the finished turbine oil can vary
 depending upon the customers' requirements and applications, and the
 desired level of antioxidant protection required for the specific turbine
 oil. An important criteria for selecting the concentration of Component B
 used in the turbine oil is the sulfur content. Component B should deliver
 between 0.005 wt. % and 0.07 wt. % of sulfur to the finished turbine oil.
 For example, a sulfurized olefin containing 12 wt. % sulfur content should
 be used between 0.04 wt. % and 0.58 wt. % to deliver between 0.005 wt. %
 and 0.07 wt. % sulfur to the finished turbine oil. An ashless
 dithiocarbamate containing 30 wt. % sulfur content should be used between
 0.02 wt. % and 0.23 wt. % to deliver between 0.005 wt. % and 0.07 wt. %
 sulfur to the finished oil.
 Another criterion useful for selecting Component B is the material's
 content of active sulfur as determined by ASTM D 1662. The presence of
 high levels of active sulfur can lead to significant corrosion and sludge
 problems in the finished turbine oil. In a preferred embodiment of the
 present invention, the level of active sulfur in Component B is below 1.5
 wt. % as determined by ASTM D 1662.
 An example of a commercial sulfurized olefin that may be used in this
 invention is HiTEC.RTM. 7188 sulfurized olefin, which contains
 approximately 12 wt. % total sulfur content and &lt;1 wt. % active sulfur,
 available from Ethyl Corporation. Examples of commercial sulfurized fatty
 oils or mixtures of sulfurized fatty oils and olefins, that may be used in
 this invention include Additin.RTM. R 4410 which contains approximately
 9.5 wt. % sulfur content and 1 wt. % active sulfur, Additin.RTM. R 4412 F
 which contains approximately 12.5 wt. % sulfur content and 1.5 wt. %
 active sulfur, and Additin.RTM. RC 2810-A which contains approximately 10
 wt. % sulfur content and &lt;1 wt. % active sulfur, all from Rhein Chemie
 Corporation. An example of a commercial ashless dithiocarbamate that may
 be used in this invention is Vanlube.RTM. 7723 which contains
 approximately 30 wt. % sulfur from R. T. Vanderbilt Company. From a
 practical standpoint Component B should contain a minimum of 8.0 wt %
 sulfur in order to minimize the amount of additive needed to deliver the
 required amount of sulfur. This is desired in order to control cost of the
 turbine oil package.
 Mixtures of sulfurized olefins, ashless dithiocarbamates and
 tetraalkylthiuram disulfides, in varying proportions, may also be used, as
 long as the desired total sulfur content, and active sulfur content are
 satisfied.
 Component C--Base Oil
 The base oils suitable for use in the present invention are characterized
 by very low sulfur contents (&lt;0.03 wt. %) and a high level of saturates
 (&gt;90 volume %).
 Group II and Group III basestocks are particularly suitable for use in the
 present invention, and are typically prepared from conventional feedstocks
 using a severe hydrogenation step to reduce the aromatic, sulfur and
 nitrogen content, followed by dewaxing, hydrofinishing, extraction and/or
 distillation steps to produce the finished base oil. Group II and III
 basestocks differ from conventional solvent refined Group I basestocks in
 that their sulfur, nitrogen and aromatic contents are very low. As a
 result, these base oils are compositionally very different from
 conventional solvent refined basestocks. The American Petroleum Institute
 has categorized these different basestock types as follows: Group I,
 &gt;0.03 wt. % sulfur, and/or &lt;90 vol % saturates, viscosity index
 between 80 and 120; Group II, .ltoreq.0.03 wt. % sulfur, and .gtoreq.90
 vol % saturates, viscosity index between 80 and 120; Group III,
 .ltoreq.0.03 wt. % sulfur, and .gtoreq.90 vol % saturates, viscosity index
 &gt;120; Group IV, all polyalphaolefins. Hydrotreated basestocks and
 catalytically dewaxed basestocks, because of their low sulfur and
 aromatics content, generally fall into the Group II and Group III
 categories. Polyalphaolefins (Group IV basestocks) are synthetic base oils
 prepared from various alpha olefins and are substantially free of sulfur
 and aromatics. Polyalphaolefins may also be used as Component C of this
 invention. Furthermore, blends of Group II, Group III and/or Group IV base
 oils may also be used as Component C of this invention. Further, the base
 oils suitable for use in the present invention may contain some Group I
 basestocks provided that the total base oil composition contains &lt;0.03
 wt. % sulfur and &gt;90 volume % saturates.
 There is no limitation as to the chemical composition of the various
 basestocks used in component C. For example, the proportions of aromatics,
 paraffinics, and naphthenics in the various Group II and Group III oils
 can vary substantially. This composition is generally determined by the
 degree of refining and the source of the crude used to produce the oil. It
 is preferred to have a basestock that is high in paraffinic content, i.e.
 &gt;60 vol %.
 The base oil (C), of the present invention, is present in an amount of from
 about 90 to 99.75 wt. % based on the total weight of the turbine
 lubricating oil.
 Component D--Rust Inhibitor(s)
 If present, any type of rust inhibitor may be used in this invention.
 Suitable acidic rust inhibitors for use in the present invention include
 the reaction products obtained by reacting a monocarboxylic acid, a
 polyalkylene polyamine and an alkenyl succinic anhydride, such as those
 taught in U.S. Pat. No. 4,101,429, hereby incorporated by reference. When
 compatibility in the presence of water and contaminants is required, the
 use of neutral rust inhibitors is preferred over acidic rust inhibitors
 because it has been found that they provide improved filterability. The
 concentration of the rust inhibitor(s) can vary from 0.02 to 0.5 wt. %.
 The term "neutral rust inhibitors", in the present invention, means rust
 inhibitors that are essentially free of a --COOH functional group.
 The neutral rust inhibitors, suitable for use in the present invention,
 include any rust inhibitors that are essentially free of a --COOH
 group(s). Preferably, the neutral rust inhibitors are hydrocarbyl esters
 of the formula: R (COOR').sub.n, wherein R and R' are hydrocarbyl groups,
 or hydroxyhydrocarbyl groups, containing 1 to about 40 carbon atoms,
 preferably 8 to 20 carbon atoms, and n is 1 to about 5. The esters contain
 at least one, and preferably from 1 to 5 hydroxy groups in the molecule.
 They may all be attached to R or R' or they may be attached to R and R' in
 varying proportions. Further, the hydroxy groups can be at any position or
 positions along the chain of R or R'. It will be appreciated that the
 maximum number of groups COOR' that are present on the hydrocarbyl or
 hydroxyhydrocarbyl group R will vary depending on the number of carbon
 atoms in R.
 The hydrocarbyl esters can be prepared by conventional esterification
 procedures from a suitable alcohol and an acid, acid halide, acid
 anhydride or mixtures thereof. In addition, the esters of the invention
 can be prepared by conventional methods of transesterification.
 Typically, the neutral rust inhibitors will have a TAN of less than 10 mg
 KOH/g. Preferred esters include, but are not limited to, octyloleyl
 malate, dioleyl malate, pentaerythritol monooleate and glycerol
 monooleate.
 By "essentially free", it is meant that the starting acids, acid halides,
 acid anhydrides or mixtures thereof used in preparing the neutral rust
 inhibitors are reacted with an amount of alcohol sufficient to
 theoretically convert the --COOH groups to esters.
 Another class of preferred neutral rust inhibitors includes aspartic acid
 diesters of a 1-(2-hydroxyethyl)-2-heptadecenyl imidazoline. This
 imidazoline is primarily a mixture of diester of L-aspartic acid and an
 imidazoline based on the reaction between oleic acid and
 aminoethanolamine. Esters of this type are commercially available from
 Mona Industries, Inc. as Monacor.RTM. 39.
 Succinimide and succinamide compounds represented by the formulae (I) may
 also be used as rust inhibitors in the present invention. These compounds
 may be used alone or in combination with one or more neutral or acidic
 rust inhibitors described above:
 ##STR1##
 wherein Z is a group R.sub.1 R.sub.2 CH--, in which R.sub.1 and R.sub.2 are
 each independently straight- or branched-chain hydrocarbon groups
 containing from 1 to 34 carbon atoms and the total number of carbon atoms
 in the groups R.sub.1 and R.sub.2 is from 11 to 35.
 In formulae (I), the radical Z may be, for example, 1-methylpentadecyl,
 1-propyltridecenyl, 1-pentyltridecenyl, 1-tridecylpentadecenyl or
 1-tetradecyleicosenyl. Preferably, the number of carbon atoms in the
 groups R.sub.1 and R.sub.2 is from 16 to 28 and more commonly 18 to 24. It
 is especially preferred that the total number of carbon atoms in R.sub.1
 and R.sub.2 is about 20 to 22. The preferred compound represented by
 formulae (I) is the succinimide shown, the preferred succinimide being a
 3-C.sub.18-24 alkenyl-2,5-pyrrolidione. A more preferred embodiment of
 this succinimide contains a mixture of alkenyl groups having from 18 to 24
 carbon atoms.
 In one aspect of the invention, the compounds represented by formulae (I)
 have a titratable acid number (TAN) of about 80 to about 140 mg KOH/g,
 preferably about 110 mg KOH/g. The TAN is determined in accordance with
 ASTM D 664.
 These compounds are commercially available or may be made by the
 application or adaptation of known techniques (see for example
 EP-A-0389237).
 Typically, the additive components of this invention (A, B, and D, when
 present) are added to the base oil (C) in the form of an additive package
 concentrate. The total amount of additive components in the concentrates
 generally varies from 20 to 95 wt. % or more, with the balance being
 diluent oil. The diluent oil may be the Group II or higher base oils of
 this invention, conventional Group I base oils, as defined above, or a
 hydrocarbon, preferably aromatic, solvent or mixtures thereof. The
 concentrates may contain other additives. Examples of other additives
 include demulsifiers, copper corrosion inhibitors, ashless antiwear
 additives and supplemental antioxidants such as hindered phenolics.
 Examples of hindered phenolic antioxidants that may be used include
 2,6-di-t-butylphenol, 2,4,6-tri-t-butylphenol,
 4,4'-methylenebis(2,6-di-t-butylphenol), methylene bridged t-butylphenol
 mixtures, isooctyl 3,5-di-t-butyl-4-hydroxyhydrocinnamate, and
 thiodiethylenebis(3,5-di-t-butyl-4-hydroxy)hydrocinnamate. Typically, the
 additive package concentrates are added to the base oil (C) in an amount
 sufficient to provide from 0.25 to 2.0 wt. % of components (A), (B) and
 (D), if present, to the finished oil.
 In a preferred embodiment of the present invention, the turbine lubricating
 oils are prepared without the addition of hindered phenolic antioxidants.
 There are a number of problems that may be associated with the use of
 hindered phenolics. There are toxicity issues related to the use of
 hindered phenolics that contain low levels of free phenol. Further,
 hindered phenolics under high temperatures can dealkylate and produce free
 phenol. Water extractability of certain water soluble phenolics is another
 potential problem. Thus a phenolic-free formulation may be desired.
 The present invention is also directed to a method of improving the
 oxidative stability of a base oil, wherein said method comprises adding to
 a base oil having a sulfur content of less than 0.03 wt. % and greater
 than 90 volume % saturates (A) an amine antioxidant selected from the
 group consisting of alkylated diphenylamines, phenyl-naphthylamines and
 mixtures thereof; and (B) a sulfur containing additive selected from the
 group consisting of sulfurized olefins, sulfurized fatty acids, ashless
 dithiocarbamates, tetraalkylthiuram disulfides and mixtures thereof.
 The turbine oils of the present invention may be used in other applications
 including circulating systems, compressors, ashless hydraulic systems, and
 other equipment where oxidation stability is of primary importance.
 EXAMPLES
 It is important to note that the use of sulfur containing additives (those
 defined in Component B) in finished turbine oils can be limited due to
 corrosion and significant increases in sludge during oxidation of the oil.
 Suitable oils for turbine applications are required to pass certain tests
 demonstrating acceptable corrosion and sludge control.
 The following Examples show the superior oxidation stability of the turbine
 oils of this invention as well as adequate sludge and corrosion control.
 Example I
 A series of 32 oils were blended using the components, concentrations, and
 basestocks indicated in Table I. The oils were blended by combining all
 components with the oils and heating the oils at 50.degree. C., with
 adequate mixing, for 1 hour. The components used were as follows:
 Corrosion Inhibitor--Derivatized tolyltriazole corrosion inhibitor.
 Ashless DTC--Methylenebis(di-n-butyl-dithiocarbamate) containing
 approximately 30 wt % sulfur. This additive represents component B of the
 lubricant composition.
 Sulfurized Olefin--A C16-C18 sulfurized olefin containing approximately 12
 wt. % sulfur. This additive represents component B of the lubricant
 composition.
 Acidic Rust Inhibitor--HiTEC.RTM. 536 rust inhibitor, a derivatized acidic
 rust inhibitor available from Ethyl Corporation.
 PANA--Phenyl-alpha-naphthylamine containing approximately 6.6 wt. %
 nitrogen. This additive represents component A of the lubricant
 composition.
 2,6-DTBP--2,6-di-tert-butylphenol.
 DPA--A styryl octyl alkylated diphenylamine containing approximately 4.3
 wt. % nitrogen. This additive represents component A of the present
 lubricant composition.
 Neutral Rust Inhibitor--Pentaerythritol monooleate neutral rust inhibitor.
 This additive represents component D of the lubricant composition.
 100 N Group II--A basestock containing approximately 0.01 wt % sulfur and a
 viscosity index of 99. This represents C of the lubricant composition.
 100 N Group I--A basestock containing approximately 0.15 wt % sulfur and a
 viscosity index of 85.
 100 N High VI Group II--A basestock containing &lt;0.001 wt % sulfur and a
 viscosity index of 110. This represents C of the lubricant composition.
 150 N Group I--A basestock containing 0.33 wt % sulfur and a viscosity
 index of 94.
 All the formulated oils in Table I were evaluated in the Rotary Bomb
 Oxidation Test ASTM D 2272. The Rotary Bomb Oxidation Test (RBOT) is a
 turbine oil oxidation test used as a quality control tool for new and used
 turbine oils of known composition, as well as a research tool for
 estimating the oxidative stability of experimental oils. The test
 evaluates the oxidative stability of a turbine oil at elevated
 temperatures and oxygen pressures and in the presence of a copper coil
 oxidation catalyst and water. A rotating glass bomb provides maximum
 oil-oxygen contact. Results are reported as the time to a 25 psi drop in
 oxygen pressure. The RBOT results for all 32 oils are shown in Table I.
 The synergism between the alkylated diphenylamine (DPA) and sulfurized
 olefins and/or ashless dithiocarbamates (Ashless DTC) is shown in the
 results for oils 1 through 16 in Table I and in FIG. 1. Note that the
 sulfurized additives only (Oils 1 through 5), or the DPA only (Oil 6), are
 inferior at providing oxidation protection in the low sulfur, hydrotreated
 Group II oil, i.e., the induction times are low. However, when the
 sulfurized additives are combined with the DPA (Oils 12 through 16), a
 very high level of oxidation protection is seen, i.e. the induction times
 are very high. A very high level of oxidation protection is also seen when
 the sulfurized additives and the DPA are combined in the presence of a
 corrosion inhibitor and a neutral rust inhibitor (Oils 7 through 11).
 The superior oxidative stability that this sulfurized additive/DPA
 combination provides to hydrotreated Group II oil s is shown when
 comparing oils 7, 20, 21 and 22 in Table I and in FIG. 2. The hydrotreated
 low sulfur Group II oils (7 and 21) are significantly more oxidatively
 stable than the conventional sulfur containing Group I oils (20 and 22).
 In FIG. 2, the basestocks tested were as follows: A was the 100 N Group I
 basestock described above, B was the 150 N Group I basestock, described
 above, C was the 100 N Group II basestock, described above, and D was the
 100 N High VI Group II basestock, described above.
 A comparison between oil 7 and oil 19 shows that both acidic rust
 inhibitors (19) and neutral rust inhibitors (7) may be used in combination
 with the sulfurized additives and DPA of this invention. Neutral rust
 inhibitors, however, are often preferred because of their effectiveness at
 controlling filterability in the finished turbine oils.
 Oils 17 and 18 show that the corrosion and rust inhibitors alone (17) or
 the combination of corrosion and rust inhibitors with the sulfurized
 additive Ashless DTC (18) are ineffective at stabilizing the low sulfur
 hydrotreated group II oil.
 Oils 23 and 24 show that other combinations of corrosion and rust
 inhibitors are effective at stabilizing the low sulfur hydrotreated Group
 II oil. In oil 23 the ashless DTC and DPA are used in combination with a
 neutral rust inhibitor only. In oil 24 the ashless DTC and DPA are used in
 combination with a corrosion inhibitor only.
 Oils 25 through 29 show the effectiveness of this invention at potential
 ranges of practical treat levels that might be used. The ashless DTC
 varies from 0.05 to 0.15 wt %. The DPA varies from 0.2 to 0.4 wt %. Of
 course, lower ashless DTC and DPA levels in the finished oil will produce
 a less oxidatively stable oil. However, the combination of Ashless DTC and
 DPA provided much better oxidation protection in Group II basestocks as
 compared to Group I basestocks. In the case where oxidation performance
 equivalent to that obtained in a Group I basestock is required, lower
 levels of Ashless DTC and DPA can be used in Group II (or higher)
 basestocks (Compare oil 29 with oil 20 and oil 25 with oil 22). Further,
 the improved oxidation performance without sludging in Group II (or
 higher) basestocks, is beneficial for turbine applications.
 Comparison of oil 25 with oil 30 shows that a supplemental antioxidant may
 be used as part of this invention to further improve the oxidative
 stability of the low sulfur, hydrotreated, Group II oil. The supplemental
 antioxidant in oil 30 is 2,6-di-t-butylphenol and this antioxidant does
 improve the oxidative stability of oil 30 relative to oil 25.
 Oil 31 utilizes phenyl-alpha-naphthylamine (PANA) in combination with DPA
 as part of this invention while oil 32 utilizes phenyl-alpha-naphthylamine
 in combination with DPA and a phenolic antioxidant in place of the
 sulfur-containing additives. Note that when PANA is used in preparing the
 finished oils of this invention, an oil with less additives (0.55 wt %
 versus 0.7 wt %) and greater oxidative stability (1554 min versus 1300
 min) is produced.

Sulfur- Acidic Neutral
 100 N 100 N 100 N 150 N
 Corrosion Ashless ized Rust 2,6- Rust
 Group Group High VI Group
 Sample Inhibitor DTC Olefin Inhibitor PANA DTBP DPA Inhibitor
 II I Group II I Total RBOT
 ID (g) (g) (g) (g) (g) (g) (g) (g)
 (g) (g) (g) (g) (g) (min)
 1* 0.200
 99.800 100 34
 2* 0.150 0.125
 99.725 100 86
 3* 0.100 0.250
 99.650 100 106
 4* 0.050 0.375
 99.575 100 121
 5* 0.500
 99.500 100 125
 6* 0.500
 99.500 100 207
 7 0.050 0.200 0.250 0.200
 99.300 100 2205
 8 0.050 0.150 0.125 0.250 0.200
 99.225 100 1706
 9 0.050 0.100 0.250 0.250 0.200
 99.150 100 1847
 10 0.050 0.050 0.375 0.250 0.200
 99.075 100 1569
 11 0.050 0.500 0.250 0.200
 99.000 100 1219
 12 0.200 0.250
 99.550 100 1671
 13 0.150 0.125 0.250
 99.475 100 2043
 14 0.100 0.250 0.250
 99.400 100 1826
 15 0.050 0.375 0.250
 99.325 100 1542
 16 0.500 0.250
 99.250 100 1360
 17* 0.050 0.200
 99.750 100 33
 18* 0.050 0.200 0.200
 99.550 100 31
 19 0.050 0.200 0.200 0.250
 99.300 100 2486
 20* 0.050 0.200 0.250 0.200
 99.300 100 1052
 21 0.050 0.200 0.250 0.200
 99.300 100 2638
 22* 0.050 0.200 0.250 0.200
 99.300 100 731
 23 0.200 0.250 0.200
 99.350 100 1720
 24 0.050 0.200 0.250
 99.500 100 2990
 25 0.050 0.050 0.200 0.200
 99.500 100 715
 26 0.050 0.050 0.400 0.200
 99.300 100 709
 27 0.050 0.150 0.200 0.200
 99.400 100 1542
 28 0.050 0.150 0.400 0.200
 99.200 100 1758
 29 0.050 0.100 0.300 0.200
 99.350 100 1050
 30 0.050 0.050 0.200 0.200 0.200
 99.300 100 929
 31 0.050 0.050 0.050 0.200 0.200
 99.450 100 1554
 32* 0.050 0.100 0.250 0.100 0.200
 99.300 100 1300
 *Comparative Example
 Example II
 A series of oils were blended using the components, concentrations, and
 basestocks indicated in Table II. The oils were blended by combining all
 components with the oils and heating the oils at 50.degree. C., with
 adequate mixing, for 1 hour. The components used were those identified in
 example I and the following:
 SBHHC--Thioethylenebis(3,5-di-t-butyl-4-hydroxyhydrocinnamate), contains
 approximately 5 wt % sulfur
 Octyl BHHC--Isooctyl 3,5-di-t-butyl-4-hydroxyhydrocinnamate
 Oils 33 through 44 represent antioxidant combinations that are commonly
 used in turbine oil applications while oils 12 and 16 represent the
 antioxidant combinations for turbine oils of this invention. These oils
 were evaluated in the RBOT ASTM D 2272 as defined in Example I. The RBOT
 results are reported in Table II.
 Note that the low sulfur hydrotreated Group II oils containing the commonly
 used antioxidant systems (Oils 34, 36, 38, 41, and 44) are not
 substantially different in oxidative stability from the sulfur containing
 Group I oils containing the same antioxidant systems. In some cases the
 low sulfur hydrotreated Group II oils are slightly less oxidatively stable
 than the sulfur containing Group I oils (34 versus 33, 38 versus 37, 41
 versus 39, and 44 versus 42) while in other cases they are slightly more
 oxidatively stable (36 versus 35, 41 versus 40, and 44 versus 43). In the
 cases where the low sulfur hydrotreated Group II oils are more oxidatively
 stable than the sulfur containing Group I oils, the differences are small.
 Note that oils 33 through 38 contain a sulfurized antioxidant used in
 combination with the DPA. The antioxidant SBHHC contains approximately 5
 wt % sulfur. In oils 35 and 36, 0.019 wt % sulfur is being delivered to
 the oil from antioxidant SBHHC. This sulfur content falls within the range
 specified for component B of the invention. However, SBHHC is not an
 effective sulfurized additive for improving the oxidative stability of the
 low sulfur hydrotreated Group II oil, i.e. the RBOT induction times using
 SBHHC are not substantially different between the Group I and Group II
 oils. Furthermore, SBHHC is considerably more costly than the sulfurized
 olefins and ashless DTC's in component B of the invention. It is not
 practical to increase the sulfur content of the oil by adding higher treat
 levels of SBHHC because its sulfur content is relatively low, requiring
 substantial treat levels.
 Oils 12 and 16 represent compositions for this invention. Note the superior
 oxidative stability of these oils relative to oils 33 through 44.
 TABLE II
 Antiox- RBOT
 idant DPA Average of
 Sample Antioxidant Level Level Basestock Duplicates
 ID ID (wt %) (wt %) ID (min)
 33* SBHHC 0.25 0.25 100N Group I 448
 34* SBHHC 0.25 0.25 100N Group II 392
 35* SBHHC 0.375 0.125 100N Group I 324
 36* SBHHC 0.375 0.125 100N Group II 426
 37* SBHHC 0.125 0.375 100N Group I 434
 38* SBHHC 0.125 0.375 100N Group II 366
 39* 2,6-DTBP 0.25 0.25 100N Group I 670
 40* 2,6-DTBP 0.25 0.25 150N Group I 360
 41* 2,6-DTBP 0.25 0.25 100N Group II 549
 42* Octyl BHHC 0.25 0.25 100N Group I 461
 43* Octyl BHHC 0.25 0.25 150N Group I 255
 44* Octyl BHHC 0.25 0.25 100N Group II 286
 12 Ashless DTC 0.2 0.25 100N Group II 1671
 16 Sulfurized 0.5 0.25 100N Group II 1360
 Olefin
 * Comparative Examples
 Example III
 A variety of tests have been developed to screen a finished turbine oils
 ability to control corrosion and sludge. One very useful test is the
 Nippon Oil Color Test (NOC). The NOC method is as follows: Four 50 ml
 beakers are filled with 45 g of the oil to be tested. Iron and copper coil
 catalysts (used for ASTM D 943) are added to each of the four beakers. The
 beakers are stored at 140.degree. C. and after 4, 6, 8 and 10 days a
 beaker is removed from the oven and analyzed for color (ASTM D 1500) and
 sludge content. The copper coil is rated according to ASTM D 130 rating
 chart.
 Oils 25 through 31 were evaluated in the Nippon Oil Color Test for color
 formation by ASTM D 1500, and sludge formation by the weight of sludge
 produced in milligrams. Acceptable color and sludge results were obtained
 for all the oils, i.e. less than 8.0 for color and less than 10 milligrams
 of sludge after 10 days of oil aging.
 This invention is susceptible to considerable variation in its practice.
 Accordingly, this invention is not limited to the specific
 exemplifications set forth hereinabove. Rather, this invention is within
 the spirit and scope of the appended claims, including the equivalents
 thereof available as a matter of law.
 The patentees do not intend to dedicate any disclosed embodiments to the
 public, and to the extent any disclosed modifications or alterations may
 not literally fall within the scope of the claims, they are considered to
 be part of the invention under the doctrine of equivalents.