Reduced odor and high stability aircraft turbine oil base stock

Details Reduced odor and high stability aircraft turbine oil base stock Reduced odor and high stability aircraft turbine oil base stock

1999-02-19

2001-01-23

McAvoy, Ellen M. (Department: 1764)

Solid anti-friction devices, materials therefor, lubricant or se

Lubricants or separants for moving solid surfaces and...

Organic -co- compound

Reexamination Certificate

active

06177387

ABSTRACT:

FIELD OF THE INVENTION
The present invention generally relates to a high thermal and oxidative stability ester base stock for use in aircraft turbine oils and the like, wherein the incorporation of odorous C
5
acids is substantially reduced versus conventional polyol ester base stocks, while the base stock of the present invention is still capable of meeting the viscometric and low temperature properties required under Military Specification 23699C or 23699D, i.e., a lubricant having a viscosity at 210° F. (99° C.) of a least about 5.0 centistokes, a viscosity at −40° F. (−40° C.) of less than 13,000 centistokes, and a pour point of at least as low as about −65° F. (−54° C.). The synthetic ester composition can also be a complex alcohol ester or a blend of esters, so long as the complex alcohol ester or blend of esters have a total linear and branched C
5
acid concentration of45 wt. % or less, based on the total acid concentration in the base stock.
BACKGROUND OF THE INVENTION
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.
Stability requirements and the accompanying need for lubricating oils with greater stability have been increasing. As engines become smaller and tighter, and engine operating temperatures go higher, the need for higher stability lubricants has increased. In addition, higher stability lubricants which retain this feature are also desired when longer drain intervals and decreased maintenance are desired, both of which result in savings.
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.
Most lubricating oil formulations require the addition of antioxidants (also known as oxidation inhibitors). Antioxidants retard the rate at which ester base stocks (or any base stocks) deteriorate in service, which deterioration can be evidenced by the products of oxidation such as sludge and varnish-like deposits on the metal surfaces, and by viscosity and acidity growth. Such antioxidants include arylamines (e.g., dioctyl phenylamine and phenyl alphanaphthylamine), phosphosulfurized or sulfurized hydrocarbons, and hindered phenols (e.g., butylated hydroxy toluene) and the like.
Frequently replacing the lubricating oil or adding an antioxidant to suppress oxidation increases the total cost of maintaining an engine or other mechanical device. It would be most desirable to have an ester base stock which exhibits substantially enhanced thermal/oxidative stability compared to conventional ester base stocks, thus requiring less frequent replacement due to decomposition (i.e., oxidation degradation).
High Pressure Differential Scanning Calorimetry (HPDSC) has been used to evaluate the thermal/oxidative stabilities of formulated automotive lubricating oils (see J. A. Walker, W. Tsang, SAE 801383), of synthetic lubricating oils (see M. Wakakura, T. Sato, Journal of Japanese Petroleum Institute, 24 (6), pp. 383-392 (1981)) and of polyol ester derived lubricating oils (see A. Zeeman, Thermochim, Acta, 80(1984)1). In these evaluations, the time for the bulk oil to oxidize under a fixed temperature and atmosphere, which is the induction time, was measured. Longer induction times have been shown to correspond to more stable oils, to oils having higher concentrations of antioxidants, to oils having more effective antioxidants, or to oils having more stable base stocks.
The use of HPDSC as described herein provides a measure of stability through oxidative induction times. An ester base stock can be blended with a constant amount of dioctyl diphenylamine which is an antioxidant. This fixed amount of antioxidant provides a constant level of protection for any ester base stock against bulk oxidation. Thus, 0118 tested in this manner with longer induction times have greater intrinsic resistance to oxidation.
The present inventors have developed a unique ester composition and method for preparing these esters such that they have enhanced thermal/oxidative stability, higher volume resistivity, low metals, low ash and low total acid number, when compared to many conventional ester compositions.
The thermal and oxidative stability which is designed into the novel ester compositions of the present invention can permit the formulation to eliminate or reduce the level of antioxidant which must be added to a particular lubricant, thereby providing a substantial cost savings to lubricant manufacturers.
Additionally, the synthetic polyol ester base stocks according to the present invention incorporate a lesser amount of straight-chain and branched-chain monocarboxylic acids having five and six carbon atoms than conventional polyol esters used in the formulation of aircraft turbine oils, thereby markedly reducing the odor. While normal C
5
acids and branched C
5
acids including trimethylacetic acid are judged to provide thermal/oxidative stability, they are a marketing issue in that C
5
acids are highly malodorous and any time there is saponification of the polyol ester and free C
5
acid is generated, customers are acutely aware of its presence. Thus, it would be highly desirable to formulate a polyol ester base stock for use in aircraft turbine oils which has reduced amounts of C
5
and C
6
acids, while exhibiting satisfactory thermal and oxidative stability and still meeting the requirements of Military Specification MIL-L-23699C or MIL-L-23699D.
The chemical and physical requirements for Military Specification MIL-L-23699C and 23699D are set forth below in Table 1.
TABLE 1
(MIL-L-23699C)
Requirement
Limits
Test Method
Viscosity @ −40° C. (−40° F.)
13,000 cSt
ASTM D 2532
Viscosity % change after 72 hr @ −40° C. (−40° F.)
±6 cSt
ASTM D 2532
Viscosity @ 98.9° C. (210° F.)
5.0-5.5 cSt
ASTM D 445
Viscosity @ 37.8° C. (100° F.)
25.0 cSt
ASTM D 445
Flash point, minimum
246° C. (475° F.)
ASTM D 92
Pour point, maximum
−54° C. (−65° F.)
ASTM D 97
Total Acid Number (TAN), maximum
0.5
ASTM D 664
Thermal stability and corrosivity @ 274° C. (525° F.)
3411 of FED-STD-791
Viscosity change, % maximum
5.0
Total acid number change, maximum
6.0
Weight of metal change, maximum
4.0 mg/cm
2
Corrosion and Oxidation stability
5308 of FED-STD-791
after 72 hours at test temperature
175° C.
204° C.
218° C.
Viscosity % change
−5 to 15
−5 to 25
Total acid number, change maximum
2.0
3.0
Metal weight change mg/cm
2
steel
±0.2
±0.2
±0.2
silver (Ag)
±0.2
±0.2
±0.2
aluminum (Al)
±0.2
±0.2
±0.2
magnesium (Mg)
±0.2
±0.2
—
copper (Cu)
±0.4
±0.4
—
titanium (Ti)
—
—
±0.2
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.
For example, U.S. Pat. No. 4,826,633 (Carr et al.), which issued on May 2, 1989, is directed to a synthetic ester base stock for use in lubricants for gas turbine engines which meet the specifications of the bearing rig test referred to in military specification ML-23699C. The synthetic ester base stock disclosed in Carr et al. is formed by reacting at least one of monopentaerythritol a

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