WO2018118610A1

WO2018118610A1 - Aircraft turbine oil base stock and method of making

Info

Publication number: WO2018118610A1
Application number: PCT/US2017/066268
Authority: WO
Inventors: Michael R. Douglass; Beth A. FITCH; Susan C. Ardito; Douglas E. Johnson
Original assignee: Exxonmobil Research And Engineering Company
Priority date: 2016-12-22
Filing date: 2017-12-14
Publication date: 2018-06-28
Also published as: EP3559175A1; US20180179463A1

Abstract

Provided is a polyol ester base stock composition with reduced odor and volatility. The polyol ester back stock composition does not include C5 acid. More particularly, the polyol ester base stock includes the reaction product of: (a) a polyol represented by the formula R(OH)_n wherein R is an aliphatic or a cyclo-aliphatic hydrocarbyl group and n is at least 2, and (b) a mixture of monocarboxylic acids comprising at least one linear acid selected from the group consisting of between C6 to C10 acids and at least one branched C6 acid, wherein the amount of C6 to C10 acids is at least 55 wt.% and the amount of the at least one branched C6 acid is 45 wt.% or less, based upon the total amount of said mixture of monocarboxylic acids. The polyol ester base stock is particularly suited for aviation turbine oils.

Description

AIRCRAFT TURBINE OIL BASE STOCK AND METHOD OF MAKING

FIELD

[0001] The present disclosure relates to the field of aircraft turbine oils. It more particularly relates to aircraft turbine oil base stocks and formulations including synthetic polyol ester base stocks. Still more particularly, the present disclosure relates to aircraft turbine oil base stocks and formulations including synthetic polyol ester base stocks that yield lower odor and reduced volatility.

BACKGROUND

[0002] Lubricants in commercial use today are prepared from a variety of natural and synthetic base stocks admixed with various additive packages and solvents depending upon their intended application. The base stocks typically include mineral oils, highly refined mineral oils, poly alpha olefins (PAO), polyalkylene glycols (PAG), phosphate esters, silicone esters, diesters and polyol esters.

[0003] In end uses where higher stability is desired or required, polyol esters have been commonly used due to their high thermal and oxidative stability. One of the most demanding lubricant applications in terms of thermal and oxidative requirements is oils for aircraft turbines. In aircraft turbines, where operating temperatures and exposure to oxygen are both high, it has been the industry's practice to use polyol esters.

[0004] Generally, polyol esters used in forming aircraft turbine oils typically include a mixture of monopentaerythritol and dipentaerythritol esters. Still others have blended trimethylolpropane esters and dipentaerythritol esters, trimethylolpropane esters and monopentaerythritol/dipentaerythritol esters, or a mixture of trimethylolpropane esters and monopentaerythritol esters. More particularly, typical jet oil ester basestocks include a mixture of C5– C10 carboxylic acids, reacted with a polyol ester such as pentaerythritol or trimethylol propane. C5 acids can typically be more than half of the acid stream used to make jet oil ester base stocks. When these base stocks hydrolyze in service, pentanoic acids are the most volatile and also have an objectionable odor.

[0005] Every lubricant has a characteristic odor which is imparted to it by the compositional changes which occur when used in an engine. In particular, if there is any decomposition of the ester component, it is expected that free carboxylic acid will be generated. In general, hydrolysis of synthetic ester base stocks containing significant amounts of lower molecular weight acid give rise to decomposition products of greater odor intensity than those containing lesser amounts of lower molecular weight acids. By lower molecular weight acids is meant pentanoic acids and, to a lesser extent, hexanoic acids which have five or six carbon atoms, respectively. Further, both straight-chain and branched-chain acids are included in this definition. This is true whether the lower molecular weight acids are combined with trimethylolpropane, monopentaerythritol or dipentaerythritol.

[0006] A need exists for a synthetic ester lubricant base stock for aircraft turbine oils which provide for formulated oils having viscosity and pour point characteristics capable of meeting the military specifications for aircraft turbine oils, while minimizing the amount of pentanoic and hexanoic acids contained therein in order to improve odor, thermal stability and oxidative stability. A need also exists for the low odor and improved stability synthetic ester lubricant base stock to be combined with a standard lubricant additive package to provide an aircraft engine oil that meets military specification MIL-L-23699G and AS5780, with a viscosity at 210 degree F (99 degree C) of at least 5.0 cSt and at -40 degree C of no more than 13,000 cSt, and a pour point of less than at least -65 degree F (-54 degree C).

SUMMARY

[0007] According to the present disclosure, an advantageous polyol ester lubricant base stock comprises the reaction product of: (a) a polyol represented by the formula R(OH)n wherein R is an aliphatic or a cyclo-aliphatic hydrocarbyl group and n is at least 2, and (b) a mixture of monocarboxylic acids comprising at least one linear acid selected from the group consisting of between C6 to C10 acids and optionally at least one branched C6 acid, wherein the amount of C6 to C10 acids is at least 55 wt.% and the amount of the optional at least one branched C6 acid is 45 wt.% or less, based upon the total amount of said mixture of monocarboxylic acids, and wherein the mixture of monocarboxylic acids is substantially free of C5 acid.

[0008] A further aspect of the present disclosure relates to an advantageous aircraft turbine oil comprising from 70 to 95 wt% of a polyol ester base stock and from 1 to 15 wt.% of a lubricant additive package, wherein the polyol ester base stock comprises the reaction product of: (a) a polyol represented by the formula R(OH)_n wherein R is an aliphatic or a cyclo-aliphatic hydrocarbyl group and n is at least 2, and (b) a mixture of monocarboxylic acids comprising at least one linear acid selected from the group consisting of between C6 to C10 acids and at least one branched C6 acid, wherein the amount of C6 to C10 acids is at least 55 wt.% and the amount of the at least one branched C6 acid is 45 wt.% or less, based upon the total amount of said mixture of monocarboxylic acids, and wherein the mixture of monocarboxylic acids is substantially free of C5 acid. [0009] Another aspect of the present disclosure relates to an advantageous method of making a polyol ester lubricant base stock comprising esterifying reaction mixture of a polyol represented by the formula R(OH)n wherein R is an aliphatic or a cyclo-aliphatic hydrocarbyl group and n is at least 2 and an excess mixture of monocarboxylic acids comprising at least one linear acid selected from the group consisting of between C6 to C10 acids and optionally at least one branched C6 acid, wherein the amount of C6 to C10 acids is at least 55 wt.% and the amount of the optional at least one branched C6 acid is 45 wt.% or less, based upon the total amount of said mixture of monocarboxylic acids, and wherein the mixture of monocarboxylic acids is substantially free of C5 acid, wherein the esterification occurs with or without a sulfonic acid, phosphorus acid, sulfonic acid, para-toluene sulfuric acid or titanium, zirconium or tin-based catalyst, at a temperature in the range between about 140 to 250 degree C and a pressure in the range between about 30 mm Hg to 760 mm Hg.

[0010] Still yet another aspect of the present disclosure relates to an advantageous of method of reducing volatility and odor of an aircraft turbine oil in actual used conditions comprising providing to an aircraft turbine an aircraft turbine oil comprising from 70 to 95 wt% of a polyol ester base stock and from 1 to 15 wt.% of a lubricant additive package, wherein the polyol ester base stock comprises the reaction product of: (a) a polyol represented by the formula R(OH)_n wherein R is an aliphatic or a cyclo-aliphatic hydrocarbyl group and n is at least 2, and (b) a mixture of monocarboxylic acids comprising at least one linear acid selected from the group consisting of between C6 to C10 acids and optionally at least one branched C6 acid, wherein the amount of C6 to C10 acids is at least 55 wt.% and the amount of the optional at least one branched C6 acid is 45 wt.% or less, based upon the total amount of said mixture of monocarboxylic acids, and wherein the mixture of monocarboxylic acids is substantially free of C5 acid, wherein the turbine oil has an evaporative weight loss at 204^oC for 6.5 hours per ASTM D972 of less than 3.2 wt.%, a TGA- simulated Noack volatility of less than 3.1 wt. % and a GC-simulated distillation at 10% weight loss temperature of greater than 800 deg. F, and reduced odor during use in an aircraft turbine relative to a comparable aircraft turbine oil including a C5 acid in the reaction mixture.

[0011] These and other features and attributes of the disclosed polyol ester lubricant base stock for aircraft turbine oils of the present disclosure and their advantageous applications and/or uses and methods of making will be apparent from the detailed description which follows, particularly when read in conjunction with the figures appended hereto. DETAILED DESCRIPTION

[0012] All numerical values within the detailed description and the claims herein are modified by“about” or“approximately” the indicated value, and take into account experimental error and variations that would be expected by a person having ordinary skill in the art.

[0013] Prior art jet engine polyol ester base stocks are made with a significant amount of n- and iso-pentanoic acid (C5 acid is about 35– 45 wt% of the total acids). When these prior art jet engine polyol ester base stocks are formulated into c o m m e r c i a l jet engine lubricants, the C5 acids result in a very unpleasant odor, which can be detected at even at parts per billion (ppb) levels. These C5 acids can be released in-service when the esters are hydrolyzed or broken down by thermal or oxidative mechanisms. Hexanoic acid (C6) can also be detected at low levels, but its odor is far less offensive than C5 acid.

[0014] The Applicants have discovered that polyol ester base stocks that do not include C5 acid in esterfying the polyol results in a polyol ester base stock with surprisingly improved volatility and odor relative to a comparable polyol ester base stock that does include a significant amount of C5 acid. A significant amount of C5 acid means at least 5 wt.%, or at least 10 wt.%, or at least 15 wt.%, or at least 20 wt.%, or at least 25 wt.%, or at least 30 wt.%, or at least 35 wt.% , or at least 40 wt.% of the total amount of the mixture of monocarboxylic acids

[0015] More particularly, the present disclosure provides novel jet engine polyol ester base stocks that provide decreased volatility and odor relative to prior art jet engine polyol ester base stocks. The novel neopentyl polyol esters base stocks of the instant disclosure are made using C6 – C10 carboxylic acids and are substantially free of C5 acids. A polyol ester base stock that is “substantially free of C5 acids” means that the base stock includes less than 1 wt.%, or less than 0.5 wt. %, or less than 0.2 wt.%, or less than 0.1 wt.%, or less than 500 ppm, or less than 200 ppm, or less than 100 ppm. The Applicants have discovered jet engine polyol ester base stock compositions that are substantially free of C5 acids, but that still yield the balance of properties to meet the military and civil standards for jet turbine oils.

[0016] The novel neopentyl polyol esters base stocks of the instant disclosure when used in jet engine turbine oils provide less odiferous breakdown products due to hydrolysis, thermal or oxidative breakdown in aviation service. These novel neopentyl polyol esters base stocks may also find application in other areas where odor can be a concern, such as in compressor oils or thermal oils.

[0017] In one form of the present disclosure, the inventive polyol ester base stock includes the reaction product of: (a) a polyol represented by the formula R(OH)n wherein R is an aliphatic or a cyclo-aliphatic hydrocarbyl group and n is at least 2, and

(b) a mixture of monocarboxylic acids comprising at least one linear acid selected from the group consisting of between C6 to C10 acids and optionally at least one branched C6 acid, wherein the amount of C6 to C10 acids is at least 55 wt.% and the amount of the optional at least one branched C6 acid is 45 wt.% or less, based upon the total amount of said mixture of monocarboxylic acids, and wherein the mixture of monocarboxylic acids is substantially free of C5 acid.

[0018] In one advantageous form, the polyol may include technical pentaerythritol in an amount between about 50 to 100 weight %, or between 60 to 90 weight %, or between 70 to 80 weight %,, based on the total polyol, and di-pentaerythritol in an amount between about 0 to 50 weight %, or between 10 to 40 weight %, or between 20 to 30 weight %, based on the total polyol.

[0019] The C6 to C10 linear acids may include, but are not limited to, hexanoic acid, heptanoic acid, caprylic acid, pelargonic acid, capric acid and combinations thereof. The C6 branched acids may include, but are not limited to, 2-methyl pentanoic acid, 4-methylpentoic acid, and combinations thereof.

[0020] The at least one linear C6 to C10 acid may alternatively constitute at least 60 wt.%, or at least 65 wt.%, or at least 70 wt.%, or at least 75 wt.%, or at least 80 wt.%, or at least 85 wt.%, or at least 90 wt.%, or at least 95 wt.%, or 100 wt.%, based upon the total amount of said mixture of monocarboxylic acids.

[0021] The at least one branched C6 acid may alternatively constitute 40 wt.% or less, 35 wt.% or less, 30 wt.% or less, 25 wt.% or less, 20 wt.% or less, 15 wt.% or less, 10 wt.% or less, 5 wt.% or less, or 0 wt.%, based upon the total amount of said mixture of monocarboxylic acids. In one preferred form, the at least one branched C6 acid constitutes 0 wt.%, based upon the total amount of said mixture of monocarboxylic acids.

[0022] In another form, the mixture of monocarboxylic acids may include linear C6 acid ranging from 20 to 70 wt.%, or 30 to 60 wt.%, or 40 to 50 wt.% of the total amount of said mixture of monocarboxylic acids.

[0023] In yet another form, the mixture of monocarboxylic acids may include linear C7 acid ranging from 16 to 40 wt.%, or 20 to 35 wt.%, or 25 to 30 wt.% of the total amount of said mixture of monocarboxylic acids. [0024] In still yet another form, the mixture of monocarboxylic acids may include C8 to C10 acids ranging from 14 to 25 wt.%, or 16 to 23 wt.%, or 18 to 21 wt.% of the total amount of said mixture of monocarboxylic acids.

[0025] Also provided are inventive aircraft turbine oils including the inventive polyol ester base stocks described above. In particular, the inventive aircraft turbine oil includes from 85 to 99 wt%, or 88 to 96 wt%, or 90 to 94 wt% of a polyol ester base stock and from 1 to 15 wt.%, or 3 to 12 wt.%, or 4 to 10 wt.% or 5 to 8% of a lubricant additive package, wherein the polyol ester base stock comprises the reaction product of:

a) a polyol represented by the formula R(OH)_n wherein R is an aliphatic or a cyclo-aliphatic hydrocarbyl group and n is at least 2, and

b) a mixture of monocarboxylic acids comprising at least one linear acid selected from the group consisting of between C6 to C10 acids and optionally at least one branched C6 acid, wherein the amount of C6 to C10 acids is at least 55 wt.% and the amount of the optional at least one branched C6 acid is 45 wt.% or less, based upon the total amount of said mixture of monocarboxylic acids, and wherein the mixture of monocarboxylic acids is substantially free of C5 acid.

[0026] Also provided are methods of making the inventive polyol base stocks described above. In particular, the method of making a polyol base stock includes: esterifying reaction mixture of a polyol represented by the formula R(OH)_n wherein R is an aliphatic or a cyclo-aliphatic hydrocarbyl group and n is at least 2 and an excess mixture of monocarboxylic acids comprising at least one linear acid selected from the group consisting of between C6 to C10 acids and optionally at least one branched C6 acid, wherein the amount of C6 to C10 acids is at least 55 wt.% and the amount of the optional at least one branched C6 acid is 45 wt.% or less, based upon the total amount of said mixture of monocarboxylic acids, and wherein the mixture of monocarboxylic acids is substantially free of C5 acid, wherein the esterification occurs with or without a sulfonic acid, phosphorus acid, sulfonic acid, para-toluene sulfuric acid or titanium, zirconium or tin-based catalyst, at a temperature in the range between about 140 to 250 degree C and a pressure in the range between about 30 mm Hg to 760 mm Hg.

[0027] The method of making the polyol base stocks of the instant disclosure may also include the step of adding an adsorbent to the reaction mixture following esterification step. Non-limiting exemplary adsorbents include alumina, silica gel, activated carbon, zeolites, clay and filter aid.

[0028] The method of making the polyol base stocks of the instant disclosure may also include the steps of adding water and base to simultaneously neutralize the residual organic and mineral acids and/or hydrolyze said catalyst; removing of the water used in the hydrolysis step by heat and vacuum in a flash step; filtering of solids from the ester mixture containing the bulk of the excess acids used in the esterification reaction; removing excess acids by steam stripping or any other distillation method; and removing any residual solids from the stripped ester in a final filtration.

[0029] Also provided are methods of reducing volatility and odor of an aircraft turbine oil that includes providing to an aircraft turbine an aircraft turbine oil including from 70 to 95 wt% of a polyol ester base stock and from 1 to 15 wt.% of a lubricant additive package, wherein the polyol ester base stock comprises the reaction product of:

[0030] The inventive turbine oils have an evaporative weight loss at 204^oC for 6.5 hours per ASTM D972 of less than 3.2 wt.%, or less than 3.0 wt.%, or less than 2.8 wt.%, or less than 2.6 wt.%, or less than 2.4 wt.%, or less than 2.2 wt.%, or less than 2.0 wt.%. The inventive turbine oils also have a TGA-simulated Noack volatility of less than 3.1 wt. %, or less than 3.0 wt.%, or less than 2.8 wt.%, or less than 2.6 wt.%, or less than 2.4 wt.%, or less than 2.2 wt.%, or less than 2.0 wt.%.

[0031] The inventive turbine oils have a GC-simulated distillation at 10% weight loss temperature of greater than 800 deg. F, or greater than 810 deg. F, or greater than 820 deg. F, or greater than 840 deg. F, or greater than 840 deg. F. The inventive turbine oils also have a GC- simulated distillation at 50% weight loss temperature of greater than 880 deg. F, or greater than 890 deg. F, or greater than 900 deg. F, or greater than 910 deg. F, or greater than 915 deg. F. The inventive turbine oils also have a GC-simulated distillation at 90% weight loss temperature of greater than 1000 deg. F, or greater than 1100 deg. F, or greater than 1020 deg. F, or greater than 1030 deg. F, or greater than 1040 deg. F, or greater than 1050 deg. F, or greater than 1060 deg. F.

[0032] The inventive turbine oils have a reduced odor during use in an aircraft turbine relative to a comparable aircraft turbine oil including a C5 acid in the reaction mixture. The reduction in order is at least 10% lower, or at least 20% lower, or at least 30 % lower, or at least 40 % lower, or at least 50% lower than a comparable aircraft turbine oil including a C5 acid in the reaction mixture.

Polyol Ester Base Stock

[0033] Turbine oils, e.g. gas turbine oils, aviation turbine oils and jet engine turbine oils, employ synthetic esters and especially polyol esters as base oils.

[0034] The synthetic ester which can be used as the base oil is formed by the esterification of an aliphatic monohydric or polyhydric alcohol with linear or branched carboxylic acids.

[0035] The synthetic esters employed as base oils for the turbine oil have kinematic viscosities at 100ºC in the range of 2 to 12 mm²/s, preferably 3 to 8 mm²/s, more preferably 4 to 6 mm²/s, and even more preferably 5 mm²/s.

[0036] Monohydric alcohols suitable for making ester base stocks include methyl, butyl, isooctyl, didecyl and octadecyl alcohols.“Oxo” alcohols prepared by the reaction of olefins with carbon monoxide and hydrogen are suitable. Neo-alcohols, i.e., alcohols having no hydrogens on the beta carbon atom, are preferred. Examples of such alcohols are 2,2,4-trimethyl-pentanol and 2,2-dimethyl propanol.

[0037] The polyhydric alcohols which can be reacted with the linear acid are, by way of example, polyols represented by the general formula:

R^(OH) _n

wherein R is any aliphatic or cyclo-aliphatic hydrocarbyl group (preferably an alkyl) and n is at least 2. The hydrocarbyl group may contain from about 2 to about 20 or more carbon atoms, and the hydrocarbyl group may also contain substituents such as chlorine, nitrogen and/or oxygen atoms. The polyhydroxyl compounds generally may contain one or more oxyalkylene groups and, thus, the polyhydroxyl compounds include compounds such as polyetherpolyols. The number of carbon atoms (i.e., carbon number, wherein the term“carbon number” as used throughout this application refers to the total number of carbon atoms in either the acid or alcohol as the case may be) and number of hydroxyl groups contained in the polyhydroxyl compound used to form the carboxylic esters may vary over a wide range.

[0038] The following alcohols are particularly useful as polyols: 2-ethyl-1,3-hexanediol, 2-propyl-3,3-heptanediol, 2-butyl-1,3-butanediol, 2,4-dimethyl-1,3-butanediol, neopentyl glycol, 2,2-dimethylol butane, trimethylol ethane, trimethylol propane, trimethylol butane, mono-pentaerythritol, technical grade pentaerythritol, di-pentaerythritol, tri-pentaerythritol, ethylene glycol, propylene glycol and polyalkylene glycols (e.g., polyethylene glycols, polypropylene glycols, polybutylene glycols, etc., and blends thereof such as polymerized mixture of ethylene glycol and propylene glycol). Mixtures of such alcohols may also be used.

[0039] The carboxylic acid reactant used to produce the synthetic polyol ester base oil is selected from aliphatic monocarboxylic acids or a mixture of aliphatic monocarboxylic acids and aliphatic dicarboxylic acids. The carboxylic acids contain from 6 to 20 carbon atoms, or 6 to 10 carbon atoms and include the straight and branched chain aliphatic acids. The aliphatic chain may include aryl substituents. Mixtures of acids may also be used.

[0040] The carboxylic acid used is a branched or linear C6 to C20, or C6 to C10 carboxylic acid.

[0041] The branched acid is preferably a mono-carboxylic acid which has a carbon number in the range between about C6 to C20, more preferably about C6 to C10 wherein methyl or ethyl branches are preferred. The mono-carboxylic acid is preferably at least one acid selected from the group consisting of: 2,2-dimethylpropionic acid (neopentanoic acid), neoheptanoic acid, neooctanoic acid, neononanoic acid, isohexanoic acid, neodecanoic acid, 2-ethylhexanoic acid (2EH), 3,5,5-trimethylhexanoic acid (TMH), isoheptanoic acid, isooctanoic acid, isononanoic acid and isodecanoic acid. One especially preferred branched acid is 3,5,5-trimethylhexanoic acid. The term“neo” as used herein refers to a trialkyl acetic acid, i.e. an acid which is triply substituted at the alpha carbon with alkyl groups. These alkyl groups are equal to or greater than CH₃, as shown in the general structure set forth herebelow:

wherein R₁, R₂ and R₃ are greater than or equal to CH₃ and not equal to hydrogen.

[0042] 3,5,5-trimethylhexanoic acid has the structure set forth herebelow:

[0043] The mono-carboxylic linear acids are any linear saturated alkyl carboxylic acid having a carbon number in the range between about C₆ to C₂₀, preferably C₆ to C₁₀. [0044] Some examples of linear acids include sebacic, azelaic, suberic, succinic, adipic, oxalic, malonic, glutaric, pentadecanedicarboxylic, diglycolic, thiodiglycolic, acetic, propionic, lauric, palmitic, pimelic, n-hexanoic, n-heptanoic, n-octanoic, n-nonanoic, and n-decanoic acids and mixtures thereof.

[0045] Examples of suitable ester base oils are ethyl palmitate, ethyl laurate, butyl stearate, di-(2-ethylhexyl) sebacate, di(2-ethylhexyl) azealate, ethyl glycol dilaurate, di-(2-ethylhexyl) phthalate, di-(1,3-methylbutyl) adipate, di-(1-ethylpropyl) azelate, diisopropyloxylate, dicyclohexyl sebacate, glycerol tri-n-heptoate, di(undecyl) azelate, and tetraethylene glycol di-(2-ethyl caproate), and mixtures thereof.

[0046] If it is desired to form a complex alcohol ester or complex acid ester, then the synthetic ester can also include a polybasic acid selected from the group consisting of: any C₂ to C₁₂ polybasic acids, e.g. adipic, azelaic, sebacic and dodecanedioic acids.

[0047] Other preferred polyol ester base oils are those ones prepared from technical pentaerythritol and a mixture of linear and branched C6 to C20 carboxylic acids, or C6 to C10 carboxylic acids. Technical pentaerythritol is a mixture which includes about 85 to 92% monopentaerythritol and 8 to 15% dipentaerythritol. A typical commercial technical pentaerythritol contains about 88% monopentaerythritol having the formula:

and about 12% of dipentaerythritol having the formula:

The technical pentaerythritol may also contain some tri- and tetrapentaerythritol that is normally formed as by-products during the manufacture of technical pentaerythritol.

[0048] The preparation of esters from alcohols and carboxylic acids can be accomplished using conventional methods and techniques known and familiar to those skilled in the art. In general, the monohydric alcohol or polyhydric alcohol, e.g. technical pentaerythritol, is heated with the desired carboxylic acid or mixture of acids either neat or in the presence of a solvent such as an aromatic hydrocarbon and optionally in the presence of catalyst such as, e.g. titanium, zirconium and tin catalysts such as titanium, zirconium or tin alcoholates, carboxylates and chelates, HCl, HF, HBr, H2SO4, BF3, etc., see for example U.S. Patent 3,038,859 and U.S. Patent 3,121,109, herein incorporated by reference in their entirety.

[0049] Generally, a slight excess of acid is employed to force the reaction to completion to produce a fully esterified product. Water is removed during the reaction and any excess acid is then stripped from the reaction mixture. The esters of technical pentaerythritol may be used without further purification or may be further purified using conventional techniques such as distillation or other methods known to those of skill in the art.

[0050] Other polyol esters useful as turbine oil base oils are those made by synthesizing the polyol esters from a polyol and a branched or linear carboxylic acid in such a way that it has a substantial amount of unreacted hydroxyl groups; that is, the product is not fully esterified. The presence of the unreacted hydroxyl group in the ester is believed to allow this“high” hydroxyl ester to exhibit increased thermal/oxidation stability, as measured by high pressure differential scanning calorimetry (HPDSC). It is believed the presence of the unreacted hydroxyl group provides a pathway capable of scavenging alkoxide and alkyl peroxide radicals formed in the turbine oil during use, such scavenging thereby reducing the rate at which oxidation degradation can occur.

[0051] The high hydroxyl polyester is the reaction product of a linear or branched alcohol and at least one branched and/or linear carboxylic acid, the resulting synthetic ester having a hydroxyl number between 5 to 180 depending on the acid and polyol used (e.g. 1 to 25% unconverted hydroxyl groups, based on the total amount of hydroxyl groups in the branched or linear alcohol), preferably between about 5 to 100 (e.g.1 to 15% unconverted hydroxyl groups), more preferably between 10 to 80 (e.g.2 to 10% unconverted hydroxyl groups).

[0052] Hydroxyl number measures the free hydroxyl groups by determining the amount of acetic anhydride that the sample will react with under certain conditions. Anhydride is introduced in excess with the sample. Once the reaction is complete, the remaining anhydride is determined by titration with a base solution. The hydroxyl number is reported as milligrams of KOH/gram of sample. A standard method for measuring hydroxyl number is detailed by the American Oil Chemist’s Society as A.O.C.S. Cd. 13-60. For highly converted esters, e.g. 99% or more conversion to ester (almost no unreacted hydroxyl groups), the hydroxyl number is generally less than or equal to 5. [0053] In the case of both the fully esterified ester and the ester containing free hydroxyl groups, the alcohols and acids employed can be the same, the only difference in the products being, as previously indicated, that in one instance the product is fully esterified and in the other the product has free hydroxyl groups.

[0054] Mixtures of fully esterified synthetic esters and of synthetic esters containing free hydroxyl groups can also be used.

[0055] Esters suitable for use as base stocks for turbine oils are esters of monocarboxylic acids having six to twelve carbons and polyalcohols such as pentaerythritol, dipentaerythritol and trimethylolpropane. Examples of these esters are pentaerythrityl tetrabutyrate, pentaerythrityl tetravalerate, pentaerythrityl tetracaproate, pentaerythrityl dibutyratedicaproate, pentaerythrityl butyratecaproate divalerate, pentaerythrityl butyrate trivalerate, pentaerythrityl butyrate tricaproate, pentaerythrityl tributyratecaproate, mixed C6- to C10-saturated fatty acid esters of pentaerythritol, dipentaerythrityl hexavalerate, dipentaerythrityl hexacaproate, dipentaerythrityl hexaheptoate, dipentaerythrityl hexacaprylate, dipentaerythrityl tributyrate tricaproate, dipentaerythrityl trivalerate trinonylate, dipentaerythrityl mixed hexaesters of C₆ to C₁₀ fatty acids and trimethylolpropane heptylate. Pentaerythrityl esters of mixtures of C6 to C12 acids are excellent base oils.

[0056] If desired the synthetic esters, e.g. fully esterified and/or esters containing free hydroxyl groups, can be further used with other base stocks such as mineral oil, highly refined mineral oil, polyalpha olefins, polyalkylene glycols, phosphate esters, silicone oils, other polyol esters, as well as hydrocarbon oils made by hydrodewaxing/hydroisomerizing waxy feeds such as hydrodewaxed/hydroisomerized slack wax or Fischer-Tropsch synthesis waxes.

[0057] It is preferred, however, that the synthetic ester be it a fully esterified material or an ester containing free hydroxyl groups either be used individually or only in the mixture of two or more esters.

Lubricant Additives

[0058] The lubricant compositions of the present invention may also comprise other conventional lubricant additives. Thus, a fully formulated turbine oil may contain one or more of the following classes of additives: antioxidants, antiwear agents, extreme pressure additives, antifoamants, detergents, hydrolytic stabilizers, metal deactivators, other rust inhibitors, etc. Total amounts of such other additives can be in the range 1 to 15 wt %, or 3 to 12 wt.%, or 4 to 10 wt.%, or 5 to 8 wt. %. [0059] Lubricating oil additives are described generally in "Lubricants and Related Products" by Dieter Klamann, Verlag Chemie, Deerfield, Fla., 1984, and also in "Lubricant Additives" by C. V. Smalheer and R. Kennedy Smith, 1967, pp.1-11, the contents of which are incorporated herein by reference. Lubricating oil additives are also described in U.S. Pat. Nos.6,043,199, 5,856,280, and 5,698,502, the contents of which are incorporated herein by reference.

[0060] The synthetic polyol ester base stock disclosed herein may also contain one or more of the following classes of additives: antioxidants, antiwear agents, extreme pressure additives, antifoamants, detergents, hydrolytic stabilizers and metal deactivators.

[0061] Antioxidants, which can be used, include aryl amines, e.g. phenylnaphthylamines and dialkyl diphenylamines, mixtures thereof and reaction products thereof which are described in U.S. Pat. No.6,426,324 the contents of which are incorporated herein by reference; hindered phenols, phenothiazines, and their derivatives. The antioxidants are typically used in an amount in the range 1 to 5 wt % of the lubricant composition.

[0062] Antiwear/extreme pressure additives include hydrocarbyl phosphate esters, particularly trihydrocarbyl phosphate esters in which the hydrocarbyl radical is an aryl or alkaryl radical or mixture thereof. Particular antiwear/extreme pressure additives include tricresyl phosphate, triaryl phosphate and mixtures thereof. Other or additional antiwear/extreme pressure additives may also be used. The antiwear/extreme pressure additives are typically used in an amount in the range 0 to 4 wt. %, preferably 1 to 3 wt % of the lubricant composition.

[0063] Corrosion inhibitors may also be included into the turbine oil. Exemplary corrosion inhibitors include the various triazoles. For example, tolyltriazol, 1,2,4 benzene triazole, 1,2,3 benzene triazole, carboxy benzotriazole, alkylated benzotriazole. The corrosion inhibitor additive can be used in an amount in the range 0.02 to 0.5 wt %, preferably 0.05 to 0.25 wt % of the lubricant composition. Other rust inhibitors common to the industry include the various hydrocarbyl amine phosphates and/or amine phosphates.

[0064] Other additives can also be employed including hydrolytic stabilizers pour point depressants, anti foaming agents, viscosity and viscosity index improver, etc.

[0065] Foam control can be provided by many compounds including an antifoamant of the polysiloxane type, e.g., silicone oil or polydimethyl siloxane.

[0066] Another additive that can be used is an anti-deposition and oxidative additive. A typical anti-deposition and oxidation additive is a sulfur containing carboxylic acid (SCCA) as described in U.S. Pat. No.5,856,280, herein incorporated by reference in its entirety.. The SCCA derivative is used in an amount in the range 100 to 2000 ppm, preferably 200 to 1000 ppm, most preferably 300 to 600 ppm.

[0067] The lubricant composition according to the present disclosure preferably comprises about 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99.9 wt % by weight of the mixed polyol ester composition of the present invention and about 0.1, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, 9.5, 10.0, 10.5, 11.0, 11.5, 12.0, 12.5, 13.0, 13.5, 14.0, 14.5 to 15 wt %, preferably 2 to 10 wt %, most preferably 3 to 8 wt % by weight of a lubricant additive package.

[0068] The individual additives may be incorporated into the present lubricant composition in any convenient way. Thus, each of the components can be added directly to the base stock by dispersing or dissolving it in the base stock at the desired level of concentration. Such blending may occur at ambient temperature or at an elevated temperature. Preferably, all the additives are blended into a concentrate or additive package, which is subsequently blended into base stock to make finished lubricant. Use of such concentrates in this manner is conventional. The concentrate will typically be formulated to contain the additive(s) in proper amounts to provide the desired concentration in the final formulation when the concentrate is combined with a predetermined amount of base lubricant. The pre-mix is cooled to at least 85 degree C and the additional components are added.

[0069] To a partially formulated polyol ester base stock of the present disclosure, with additives that include antioxidants, corrosion inhibitors and hydrolytic stabilizers, may be optionally added a minor portion of 3-(di-isobutoxy-thiophosphonylsulfanyl)-2-methyl-propionic acid (DITMPA), TCP and yellow metal passivator such that the DITMPA generally comprises from about 0.01 to about 0.40 weight percent, and the yellow metal passivator comprises from about 0.01 to about 0.40 weight percent, of the fully formulated lubricating oil composition.

[0070] The structure of the DITMPA additive is as shown below.

3-(di-isobutoxy-thiophosphonylsulfanyl)-2-methyl-propionic acid (DITMPA).

[0071] More particularly, the DITMPA may include from about 0.02 to about 0.20 weight percent of the fully formulated lubricating oil composition, for example from about 0.03 to about 0.10 weight percent of the fully formulated lubricating oil composition. The DITMPA may be mixed or blended with the polyol ester base stock by any convenient and known means. If desirable, concentrates may be prepared for subsequent dilution with additional polyol ester base prior to deployment.

[0072] The yellow metal passivator can be selected from the general class of such additives which includes, but is not limited to, benzotriazole, quinizarin and tolutriazole also known as methyl benzotriazole. For example, the yellow metal passivator can be tolutriazole and comprises from about 0.05 to about 0.1 weight percent of the fully formulated lubricating oil composition. With the addition of DITMPA, the weight percent of other load carrying additives such as TCP can be reduced while still retaining enhanced load-carrying capacity and enhanced copper passivation.

[0073] The aircraft engine oils of the present disclosure meet or exceed the requirements set out by the United States Navy in MIL-L-23699G and AS5780 for standard performance category or high performance category 5 cSt turbo oils.

TEST METHODS

[0074] Evaporative weight loss at 204^oC for 6.5 hours was measured per ASTM D972.

[0075] TGA-simulated Noack volatility was measured according to modified ASTM D6375.

[0076] GC-simulated distillation was measured according to ASTM D7169.

[0077] The Oxidation & Corrosion Test was measured according to FTM5308, as per specifications MIL-PRF-23699G and AS5780.

[0078] The following are examples of the present disclosure and are not to be construed as limiting.

EXAMPLES

[0079] Polyol ester base stocks for jet engine oils were formulated with C6– C10 carboxylic acids (see Table 1 below). The C5 acid for the inventive base stocks was replaced with n-hexanoic acid (n-C6) or 2-methylpentanoic acid (i-C6). The ratios of the remaining acids were adjusted so that the total carbon number was similar to the carbon number in the comparative examples. The comparative and inventive oils tested were as follows:

1. Comparative Example 1: Mixed ester, including n-C5 and iso-C5 acids.

2. Comparative Example 2: Mixed ester, including n-C5 and iso-C5 acids.

3. Comparative Example 3: Mixed ester, including n-C5 and iso-C5 acids.

4. Inventive Example 1: Mixed ester produced using iso-C6 acid, lower C8/C10 acid level than Comparative Example 1. 5. Inventive Example 2: Mixed ester produced using n-C6 acid, otherwise has similar composition as Comparative Example 1.

6. Inventive Example 3: Mixed ester produced using n-C6 acids, higher level of C8/C10 acid than Comparative Example 1.

Inventive Examples 1, 2 and 3 had monopentaerythritol, dipentaerythritol, and tripentaerythritol (monoPE, diPE, triPE) at the same ratio as Comparative Example 1.

7. Inventive Example 4: Mixed ester produced using n-C6 acids, less dipentaerythritol than the comparative examples.

8. Inventive Example 5– Mixed ester produced using high amounts of C6 acid; same monopentaerythritol, dipentaerythritol, and tripentaerythritol at the same ratio as Comparative Example 1.

9. Inventive Example 6– Mixed ester produced using low amount of C6 acid; same monopentaerythritol, dipentaerythritol, and tripentaerythritol at the same ratio as Comparative Example 1.

10. Inventive Example 7– Mixed ester produced using a relatively higher amount of iso- C6 acid and relatively lower amount of n-C6 acid.

11. Inventive Example 8– Mixed ester produced using a similar amounts of iso-C6 acid and n-C6 acid.

[0080] The acid content and properties of each of the above designated comparative examples and inventive examples are shown in Tables 1 and 2 below.

[0082] It was surprising and unexpected that the viscosities (-40, 40, and 100C) and pour points of each inventive example was very similar to the comparative examples even though C5 acid was excluded from the inventive examples. Some of the inventive examples had improved l o w t e m p e r a t u r e kV (-40^oC) viscosities (Examples 2, 3, 4, 5, 6) relative to the comparative examples. The Viscosity Indices, the pour points, and the measured RPVOT values for the comparative and inventive examples were all typical.

[0083] Replacing the C6 acid with C5 acid showed no harm in jet oil formulation testing. Each ester was formulated using a standard oil antioxidant / antiwear / metal passsivator system. For the formulated oils using the comparative and inventive base stocks shown in Table 1, typical standard jet oil lubricant additives were used. This includes monomeric amine antioxidants, antiwear additives, metal passivators, and defoamants in a total combined level of 4-6 weight percent.

[0084] The fully formulated oils had similar viscometric properties, at 40° C and 100° C and also at lower temperatures. Each oil was also tested in the Oxidation & Corrosion Test at 204° C for 72 hours. The metal corrosion was low in all cases, and the changes in viscosity and acid number were similar to those for the comparative examples.

[0086] Volatility-related properties of the fully formulated oils are shown in Table 3. It was also surprising and unexpected that the inventive examples exhibited lower volatility than the comparative examples using the Evaporation Loss (D972) test, the TGA-simulated Noack test, and the GC method for a Simulated Distillation. In all three tests, for the fully formulated fresh oils, Inventive Example 1 was similar in performance to the comparative examples, showing that an ester with no C5 acid is not necessarily less volatile than an ester including C5 acid.

[0087] For Inventive Examples 2–8 evaporation loss was lower, showing reduced volatility. TGA-simulated Noack was also lower, showing reduced volatility. The simulated distillation 10% loss, 50% loss, and 90% loss points were higher for the inventive examples versus the comparative examples, again showing reduced volatility. Comparative Examples 1 and 2 and Inventive Example 1, 7, and 8 were produced with some branched acid, and show that Inventive Example 1, 7 and 8 have similar to significantly lower evaporative weight loss than Comparative Examples 1 and 2. Comparative Example 3 and Inventive Examples 2 - 6 were produced with all linear acids, and show that Inventive Examples 2 - 6 have significantly lower evaporative weight loss than Comparative Example 3.

[0089] In addition, the volatility of the inventive compositions was tested on the oils after the 72 hour Oxidation & Corrosion Test at 204° C, see Table 4. The evaporation rates for the comparative example after oxidation are around 3 minutes. The inventive examples are similar or lower than 3 minutes in all cases, showing that the inventive jet oils maintain a lower volatility versus the comparative examples, even after being oxidized. Similarly, the simulated Noack is over 4 minutes for the comparative examples, whereas for the 8 inventive examples, the simulated Noack is about 4 minutes, or less than 4 minutes, or even less than 3 minutes. This also indicates that the inventive jet oils maintain a lower volatility versus the comparative examples, even after being oxidized. The same trend is seen in the Simulated Distillation, where for the inventive examples, the 10%, 50%, and 90% off loss points are similar to or higher than than the same points in the comparative examples.

[0091] Applicants have attempted to disclose all embodiments and applications of the disclosed subject matter that could be reasonably foreseen. However, there may be unforeseeable, insubstantial modifications that remain as equivalents. While the present invention has been described in conjunction with specific, exemplary embodiments thereof, it is evident that many alterations, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description without departing from the spirit or scope of the present disclosure. Accordingly, the present disclosure is intended to embrace all such alterations, modifications, and variations of the above detailed description.

[0092] All patents, test procedures, and other documents cited herein, including priority documents, are fully incorporated by reference to the extent such disclosure is not inconsistent with this invention and for all jurisdictions in which such incorporation is permitted.

[0093] When numerical lower limits and numerical upper limits are listed herein, ranges from any lower limit to any upper limit are contemplated.

Claims

1. A polyol ester base stock comprising the reaction product of:

(a) a polyol represented by the formula R(OH)n wherein R is an aliphatic or a cyclo-aliphatic hydrocarbyl group and n is at least 2, and

2. The base stock of claim 1, wherein the hydrocarbyl group of the polyol includes from 2 to 20 carbon atoms.

3. The base stock of claim 2, wherein the hydrocarbyl group includes a substituent selected from the group consisting of chlorine, nitrogen, oxygen and combinations thereof.

4. The base stock of claims 1-3, wherein the polyol is selected from the group consisting of 2-ethyl-1,3-hexanediol, 2-propyl-3,3-heptanediol, 2-butyl-1,3-butanediol, 2,4-dimethyl-1,3-butanediol, neopentyl glycol, 2,2-dimethylol butane, trimethylol ethane, trimethylol propane, trimethylol butane, mono-pentaerythritol, technical grade pentaerythritol, di-pentaerythritol, tri-pentaerythritol, ethylene glycol, propylene glycol, polyalkylene glycols and combinations thereof.

5. The base stock of claim 4, wherein the polyalkylene glycol is selected from the group consisting of polyethylene glycols, polypropylene glycols, polybutylene glycols and combinations thereof.

6. The base stock of claims 1-5, wherein said at least one C6 to C10 linear acid is selected from the group consisting of hexanoic acid, heptanoic acid, caprylic acid, pelargonic acid and capric acid.

7. The base stock of claims 1-6, wherein said at least one C6 branched acid is selected from the group consisting of: 2-methylpentanoic, 4-methylpentoic acid, and combinations thereof.

8. The base stock of claims 1-7, wherein said polyol comprises technical pentaerythritol in an amount between about 50 to 100 weight %, based on the total polyol, and di-pentaerythritol in an amount between about 0 to 50 weight %, based on the total polyol.

9. The base stock of claims 1-8 including a linear C6 acid ranging from 20 to 70 wt.% of the total amount of said mixture of monocarboxylic acids.

10. The base stock of claims 1-9 including a linear C7 acid ranging from 16 to 40 wt.% of the total amount of said mixture of monocarboxylic acids.

11. The base stock of claims 1-10 including a linear C8-C10 acid ranging from 14 to 25 wt.% of the total amount of said mixture of monocarboxylic acids.

12. The base stock of claims 1-11, wherein the base stock has a viscosity of at least 4 cSt at 100 degree C, a viscosity of less than 11,000 cSt at -40 degree C, a viscosity index of at least 120, and a pour point of at least as low as -54 degree C.

13. An aircraft turbine oil including the polyol ester base stock of claim 1, wherein the oil comprises from 70 to 95 wt% of a polyol ester base stock and from 1 to 15 wt.% of a lubricant additive package.

14. The turbine oil of claim 13, wherein the lubricant additive package comprises at least one additive selected from the group consisting of viscosity index improvers, corrosion inhibitors, antioxidants, dispersants, anti-emulsifying agents, color stabilizers, detergents and rust inhibitors, and pour point depressants.

15. The turbine oil of claims 13-14, wherein turbine oil comprises a blend of the polyol ester base stock and at least one additional base stock selected from the group consisting of: mineral oils, highly refined mineral oils, poly alpha olefins, polyalkylene glycols, phosphate ester, silicone oils, diesters and polyol ester.

16. The turbine oil of claims 13-15, wherein the turbine oil has an evaporative weight loss at 204^oC for 6.5 hours per ASTM D972 of less than 3.2 wt.%.

17. The turbine oil of claims 13-16, wherein the turbine oil has a TGA-simulated Noack volatility of less than 3.1 wt. %.

18. The turbine oil of claims 13-17, wherein the turbine oil has a GC-simulated distillation at 10% weight loss temperature of greater than 800 deg. F.

19. The turbine oil of claims 13-18, wherein the turbine oil has a reduced odor during use in an aircraft turbine relative to a comparable aircraft turbine oil including a C5 acid in the mixture of monocarboxylic acids.

20. The turbine oil of claims 13-19, wherein the turbine oil meets military specification MIL-L- 23699G with a viscosity at 210 degree F (99 degree C) of at least 5.0 cSt and at -40 degree C of no more than 13,000 cSt, and a pour point of less than at least -65 degree F (-54 degree C).

21. A method of making a polyol base stock comprising: esterifying reaction mixture of a polyol represented by the formula R(OH)_n wherein R is an aliphatic or a cyclo-aliphatic hydrocarbyl group and n is at least 2 and an excess mixture of monocarboxylic acids comprising at least one linear acid selected from the group consisting of between C6 to C10 acids and optionally at least one branched C6 acid, wherein the amount of C6 to C10 acids is at least 55 wt.% and the amount of the optional at least one branched C6 acid is 45 wt.% or less, based upon the total amount of said mixture of monocarboxylic acids, and wherein the mixture of monocarboxylic acids is substantially free of C5 acid, wherein the esterification occurs with or without a sulfonic acid, phosphorus acid, sulfonic acid, para-toluene sulfuric acid or titanium, zirconium or tin-based catalyst, at a temperature in the range between about 140 to 250 degree C and a pressure in the range between about 30 mm Hg to 760 mm Hg.