Technical pentaerythritol esters as lubricant base stock

Abstract: A synthetic ester base stock having reduced deposit formation which comprises the reaction product of technical pentaerythritol (a mixture of mono-and-di-pentaerythritol) and a mixture of carboxylic acids. The mixture of carboxylic acids comprises specified mole percentages of (1) at least one C 8 -C 10 carboxylic acid having 6 or less reactive hydrogens, (2) at least one C 5 -C 7 carboxylic acid having 6 or less reactive hydrogens and (3) at least one C 6 -C 10 carboxylic acid having 6 or more reactive hydrogens. The ester product has 6.0 or below gram-atoms reactive hydrogen per 100g ester; and has viscosity and pour point properties meeting the U.S. MIL-L-23699 specification.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to synthetic ester lubricant base stocks, more particularly to carboxylic acid esters of technical pentaerythriotol. 2. Background of the Invention Synthetic ester base stocks for use in lubricant formulations are well known. One important factor for synthetic ester base stocks used in jet engine lubricants is the tendency of the esters to form deposits at high temperatures. This tendency to form deposits is particularly important to modern jet engines which operate under more severe requirements, e.g., higher operating temperatures. U.S. Patent 4,826,633 is directed to synthetic ester base stocks which do not contain esters of dipentaerythritol and which provide lubricant formulations having acceptable viscosity and pour point characteristics. Esters of monopentaerythritol are stated to provide synthetic ester lubricants which exhibit reduced tendency to form deposits whereas esters of dipentaerythritol lead to increased tendency to form deposits. U.S. Patent 3,360,465 discloses technical pentaerythritol esters not containing the C 8 -C 10 acids according to the present invention. Because of the increased demands placed on synthetic lubricants by modern jet engines, there is a need for synthetic ester base stocks which have even further reduced tendencies to form deposits under operating conditions. SUMMARY OF THE INVENTION It has been discovered that a synthetic ester having reduced tendency to form deposits can be prepared from technical pentaerythritol and a mixture of C 5 -C 10 carboxylic acids. The synthetic ester base stock having reduced deposit formation comprises the reaction product of: (a) technical pentaerythritol, and (b) a mixture of C 5 -C 10 carboxylic acids, said mixture comprising (1) from 5 to 20 mole %, based on total acids, of at least one C 8 -C 10 carboxylic acid each having 6 or less reactive hydrogens, (2) from 50 to 65 mole %, based on total acids, of at least one C 5 -C 7 carboxylic acid each having 6 or less reactive hydrogens, and (3) at least 15 mole %, based on total acids, of at least one C 6 -C 10 carboxylic acid each having more than 6 reactive hydrogens; wherein the resulting mixture of esters has a total reactive hydrogen content less than or equal to 6.0 gram atoms of reactive hydrogen per 100 grams of ester and has a kinematic viscosity of at least 4.6 cSt at 99°C (210°F), a viscosity of less than 12,000 cSt at -40°C, a viscosity stability of ± 6% for 72 hours at -40°C and a pour point of -54°C or lower. In another embodiment of the invention, there is provided a method for reducing deposit formation in an aviation turbine engine which comprises operating the engine with the synthetic ester base stock described above. In contrast to the prior art, lubricants formulated with esters according to the invention produced from technical grade pentaerythritol esters exhibit lower tendencies to form deposits at temperatures between 282°C to 327°C than esters produced from monopentaerythritol esters alone. These temperatures are encountered in the lubricant systems of modern commercial gas turbine engines and the lower deposit formation tendency of technical pentaerythritol esters is important to the improved performance of the lubricant in these engines. DESCRIPTION OF THE PREFERRED EMBODIMENTS The synthetic esters according to the invention are prepared from technical pentaerythritol and C 5 -C 10 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 pentaerytritol may also contain some tri- and tetra-pentaerythritol that is normally formed as by-products during the manufacture of technical pentaerythritol. The C 5 -C 10 carboxylic acids which are used to prepare the synthetic ester lubricant base stocks are a blend of acids characterized by the number of reactive hydrogens. The term "reactive hydrogen" within the context of C 5 -C 10 carboxylic acids refers to hydrogens bonded to either secondary or tertiary carbon atoms contained in the carbon chain of the acid, i.e., Each C 5 -C 10 acid can be characterized by the number of reactive hydrogens. For example, straight chain c 6 , c 7 , c 8 , c 9 and c 10 carboxylic acids have 8, 10, 12, 14 and 16 reactive hydrogens, respectively. The introduction of methyl side chain branching reduces the number of reactive hydrogens. Thus n-hexanoic acid has 8 reactive hydrogens, 2-methylpentanoic acid has 5 reactive hydrogens and 2,3-dimethylbutanoic acid has 2 reactive hydrogens. The number of reactive hydrogens as a function of total carbons in the acid vs. number of branches in the alkyl chain is given in Table 1. The total reactive hydrogen content of the acid groups contained in a pentaerythritol ester base stock can be calculated from the concentration of each type of acid in the ester if the chemical structures of the acids are known. The reactive hydrogen content, in gram atoms of reactive hydrogen per 100 gm of base stock, is calculated as follows: H i = number of reactive hydrogens for each acid ester X i = concentration of each acid in acid mixture, mole fraction n = number of different acids in ester Y = concentration of monopenterythritol in technical grade, mole fraction M = average molecular weight of the pentaerythritol ester X i H i = number of reactive hydrogens contributed by each acid the number of reactive hydrogens per average acid group It has been discovered that the majority of acids reacted with technical pentaerythritol to form esters should have 6 or less reactive hydrogens in order to achieve improved cleanliness for the synthetic ester. Of the carboxylic acids having 6 or less reactive hydrogens, it is preferred that from 50 to 60 mole %, based on total amount of acids, are C 5 -C 7 carboxylic acids. Preferred C 5 to C 7 carboxylic acids having 6 or less reactive hydrogens include n-pentanoic acid, 2-methylbutanoic acid, 2,2- and 3,3-dimethylbutanoic acid and 2,2-, 3,3- and 4,4-dimethylpentanoic acid, more preferably n-pentanoic acid and 2-methylbutanoic acid, especially n-pentanoic acid. A major amount of n-pentanoic acid allows maximizing benefits with regard to seal compatibility and cleanliness and provides greater oxidation stability compared to iso-C 5 (2-methylbutanoic) acid. The amount of C 8 -C 10 carboxylic acids having 6 or less hydrogens is preferably from 6 to 12 mole % based on the total amount of acids. A preferred C 8 -C 10 acid is 3,5,5-trimethylhexanoic acid which provides excellent deposit control and balances the maximum content of C 5 -C 7 acid so that the ester meets the physical properties listed in Table 2. The third component, which is C 6 -C 10 carboxylic acids having more than 6 reactive hydrogens, is preferably present in an amount from 45 to 15 mole %, more preferably from 44 to 28 mole %, based on the total amount of acids. Preferred acids are straight chain acids including n-hexanoic, n-heptanoic, n-octanoic, n-nonanoic and n-decanoic acids. Especially preferred acids are blends of n-heptanoic, n-octanoic and n-decanoic acids. These acids impart excellent viscosity temperature characteristics to the ester base stock and help improve elastomer seal compatibility. Commercially available acids may contain small amounts of other acids. For example, a C 8 and C 10 acid mixture may contain small amounts of C 6 and C 12 acids. Synthetic ester base stocks which are used in aviation turbo oil formulations must meet certain requirements with regard to their viscosity and pour point characteristics. One such set of requirements are set forth in the U.S. Military MIL-L-23699 specifications. The target viscosity and pour point ranges for the base stock needed to meet the MIL-L-23699 specifications are in a finished oil shown in Table 2. Kinematic Viscosity at 99°C (210°F) 4.6-5.4 cSt Viscosity at -40°C < 12,000 cSt Viscosity Stability at -40°C, 72 hours ± 6% Pour Point -54°C Synthetic ester base stocks according to the invention meet these requirements while at the same time reducing deposit formation. The preparation of esters from alcohols and carboxylic acids can be accomplished using conventional methods. Technical pentaerythritol is heated with the desired carboxylic acid mixture optionally in the presence of a catalyst. Generally, a slight excess of acid is employed to force the reaction to completion. 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. The synthetic ester base stocks may be used in the preparation of lubricant formulations, especially aviation turbo oils. A lubricant composition for use as an aviation turbo oil contains the synthetic ester base stock and at least one of the following additives: antioxidants, antiwear agents, extreme pressure additives, corrosion inhibitors, antifoamants, detergents, hydrolytic stabilizers and metal deactivators. In the Examples which follow: Example 1 illustrates compositions in accordance with the invention; Example 2 describes a method of measuring deposit formation; Example 3 compares the deposit-forming tendencies of certain PE ester types; and Example 4 illustrates deposit formation versus reactive hydrogen content of mono - and technical PE ester types. Example 1 An ester base stock in accordance with the invention was prepared as follows. The raw materials identified in Table 3 and a tin oxalate catalyst where charged into a stirred reactor capable of delivering 240-255°C and a vacuum of at least 0.98 bar (29 inches of mercury). The reactor was provided with a nitrogen sparge or blanket. The charge was heated to a reaction temperature between about 227°C and 232°C. The water of reaction was collected in a trap during the reaction, while the acids were returned to the reactor. Vacuum was applied as needed in order to maintain the reaction. When the hydroxyl value was reduced to a sufficiently low level (a maximum of 5.0 mg KOH/gm) the bulk of the excess acid was removed by vacuum distillation. The residual acidity was neutralized with an alkali. The resulting ester base stock was dried and filtered. The acid mixture is included in the reaction in an excess of about 10 to 15 wt% of the amount required for stoichiometric reaction with the quantity of pentaerythritol used. The excess acid is used to force the reaction to completion. The excess acid is not critical to carrying out the reaction, except that the smaller the excess, the longer the reaction time. The excess acid is present in the same proportion as that in the final product, it being assumed that the reaction rate for each of the acids is approximately equal. After the reaction is complete, the excess acid is removed by stripping and refining. Generally, the esterification reaction is carried out in the presence of a conventional catalyst. The viscosity at 99°C (210°F) was between 4.86 and 5.00 cSt and at -40°C (-40°F) was between 7510 and 8500 cSt, determined in accordance with ASTM D-445 and ASTM D-2532, respectively. Viscosity stability at -40°C for 72 hours was between ± 6%. The pour point was between -54°C to -57°C (-65°F and -70°F) determined in accordance with ASTM D-97. The acid makeup of the charges are set forth as preferred embodiments. It is to be understood that these preferred embodiments can be varied so that the makeup of the acid charge can vary over a range. For example, the range may include between about 50-60 mole % normal C 5 acid, between about 17.5 to 30 mole % normal C 7 , and between 10 to 20 mole % of the normal C 8 and C 10 acid mixture. The iso-C 9 acid can be utilized between about 6 to 12 mole % of the acid charge. The base stocks used in the following examples were blended into a finished turbo oil formulation suitable for applications covered by the MIL-L-23699 specifications by using a constant package of additives. The additive package contained an antioxidant consisting of a combination of diaryl amines, a commonly used metal passivator containing triaryl phosphates, a corrosion inhibitor consisting of an alkylated benzotriazole, an antiwear additive and a hydrolytic stabilizer. The additive package was blended with a series of base stocks containing different reactive hydrogen contents as calculated from the equations indicated above. EXAMPLE 2 This example describes a method of measuring deposit formation by the Inclined Panel Deposit Test ("IPDT"). The IPDT is a bench test consisting of a stainless steel panel electrically heated by means of two heaters inserted into holes in the panel body. The test temperature is held at 282°C. The panel temperature is monitored using a recording thermocouple. The panel is inclined at a 4° angle and oil is dropped onto the heated panel near the top, allowing the oil to flow the length of the panel surface, drip from the end of the heated surface and be recycled to the oil reservoir. The oil forms a thin moving film which is in contact with air flowing through the test chamber. Test duration is 24 hours. Deposits formed on the panel are rated on a scale identical to that used for deposits formed in the bearing rig test (FED. Test Method STD. No. 791C, Method 3410.1). Varnish deposits rate from 0 (clean metal) to 5 (heavy varnish). Sludge deposits rate from 6 (light) to 8 (heavy). Carbon deposits rate from 9 (light carbon) to 11 (heavy/thick carbon). Higher ratings (12 to 20) are given to carbon deposits that crinkle or flake away from the metal surface during the test. EXAMPLE 3 This example demonstrates, directionally; our discovery that technical pentaerythritol esters form less deposits than comparable monopentaerythritol esters. Deposit data in Table 4 were taken in the IPDT test described in Example 2, but at panel temperatures of 299°C and 304°C rather than 282°C. Two pairs of base stocks consisting of one mono (MONO) and one technical pentaerythritol (TECH) ester in each pair were tested. The additive package blended into the base stocks was described earlier. The first pair of base stocks contain 75 mole % normal pentanoic (n-C 5 ) and 25 mole % 3,5,5-trimethyl hexanoic (i-C 9 ) acids. Each base stock has a reactive hydrogen content of 4.4 gram atoms of hydrogen per 100 gm of base stock. These results clearly indicate that the TECH base stock produces significantly less deposits than the MONO as indicated by the lower deposit ratings. Similar results were obtained by the second pair of base stocks in Table 4. The acid compositions are 24 and 14 mole % n-C 5 and i-C 9 acids in the MONO formulation and 30 and 6 mole % n-C 5 and i-C 9 acids in the TECH formulation. Normal heptanoic (n-C 7 ) acid made up the remainder of the acid compositions. Although the MONO base stock has a lower reactive hydrogen content (5.9 vs. 6.2 for TECH), the TECH base stock exhibits lower deposit formation. Thus, technical pentaerythritol base stocks exhibit lower deposit formations. PE-Type Mole % C 5 + iC 9 Reactive Hydrogens Inclined Panel Deposit Test Rating 299°C 304°C Avg. MONO 100 4.4 2.8 3.0 2.9 TECH 100 4.4 1.1 2.1 1.6 MONO 38 5.9 2.9 4.5 3.7 TECH 36 6.2 2.3 2.4 2.4 EXAMPLE 4 A second deposit test was used to determine, directionally; the deposit formation of a series of mono and technical pentaerythritol base stocks with various reactive hydrogen contents. Each base stock was blended with an identical additive package described above. In this test, the oil is sprayed on the interior walls of an electrically heated stainless horizontal steel cylinder in the presence of flowing air. Test duration is 20 hours. About one liter of fresh oil is used for each test. Each oil is subjected to a series of tests in which the temperature of the heated cylinder is systematically increased. Test temperatures range from 282°C to 327°C. The temperature at which significant amounts of carbon deposits are formed (T i ) is noted for each base stock. The reference base stock in Figure 1 has the lowest reactive hydrogen content and exhibited the highest test temperature (T o ) at which significant amounts of carbon deposits begin to form. The temperature difference, T o -T i , is defined as the Thermal Debit in °C and is plotted on the vertical axis. The reactive hydrogen content is plotted on the horizontal axis. The thermal debits for mono (MONO PE) and technical pentaerythritol (TECH PE) are shown in Figure 1. The data clearly indicate that MONO PE esters have higher thermal debits than those for TECH PE esters for a given reactive hydrogen content. MONO PE base stocks form carbonaceous deposits at lower temperatures, confirming the higher deposition characteristics of MONO PE base stocks noted in Example 3. Base stocks prepared according to the invention, when blended with the additive package referred to in Example 1, produce finished turbo oils that meet MIL-L-23699 specifications.

Claims: A synthetic ester base stock which comprises the reaction product of: (a) technical pentaerythritol, being a mixture containing 85 to 92% monopentaerythritol and 15 to 8% dipentaerythritol and which may also contain by-product amounts of tri - and tetra-pentaerythritol; and (b) a mixture of C 5 -C 10 carboxylic acids, said mixture comprising: (1) from 5 to 20 mole %, based on total acids, of at least one C 8 -C 10 carboxylic acid each having 6 or less reactive hydrogens, said reactive hydrogens being hydrogens bonded to either secondary or tertiary carbon atoms contained in the carbon chain of said carboxylic acid(s), (2) from 50 to 65 mole %, based on total acids, of at least one C 5 -C 7 carboxylic acid each having 6 or less said reactive hydrogens, and (3) at least 15 mole %, based on total acids, of at least one C 6 -C 10 carboxylic acid each having more than 6 said reactive hydrogens; wherein the resulting mixture of esters has a total said reactive hydrogen content less than or equal to 6.0 gram atoms of reactive hydrogen per 100 grams of ester and has a kinematic viscosity of at least 4.6 x 10 -6 m 2 /sec (4.6 cSt) at 99°C, a viscosity of less than 1.2 x 10 -2 m 2 /sec (12,000 cSt) at -40°C, a viscosity stability of ± 6% for 72 hours at -40°C and a pour point of -54°C or lower. The base stock of claim 1, wherein the C 8 -C 10 carboxylic acid having 6 or less said reactive hydrogens is 3,5,5-trimethylhexanoic acid. The base stock of claim 1 or claim 2, wherein the C 5 -C 7 carboxylic acid having 6 or less said reactive hydrogens is n-pentanoic acid or 2-methylbutanoic acid. The base stock of claim 3, wherein the C 5 -C 7 carboxylic acid is n-pentanoic acid. The base stock of any preceding claim, wherein the C 6 -C 10 carboxylic acid having more than 6 said reactive hydrogen is selected from at least one of n-hexanoic, n-heptanoic, n-octanoic, n-nonanoic and n-decanoic acids. The base stock of claim 5, wherein the C 6 -C 10 carboxylic acid is selected from at least one of n-heptanoic, n-octanoic and n-decanoic acids. The base stock of claim 1, wherein the mixture of C 5 to C 10 carboxylic acids in component (b) comprises: (1) from 6 to 12 mole %, based on total acids, of at least one branched chain acid said C 8 to C 10 carboxylic acids; (2) from 50 to 65 mole %, based on total acids, of n-pentanoic acid; and (3) at least 15 mole %, based on total acids, of more than one linear said C 6 to C 10 carboxylic acid. A lubricant composition for use as an aviation turbo oil, containing a synthetic ester base stock as claimed in any preceding claim and at least one additive selected from antioxidants, antiwear agents, extreme pressure additives, corrosion inhibitors, antifoamants, detergents, hydrolytic stabilizers and metal deactivators. A method for reducing deposit formation in an aviation turbine engine which comprises operating the engine with (i) a lubricant composition based on the synthetic ester base stock claimed in any one of preceding claims 1 to 7, or (ii) the lubricant claimed in claim 8.

Inventors: ASHCRAFT THOMAS LEE, BERLOWITZ PAUL JOSEPH, CARR DALE DUANE, SCHAEFER THOMAS GEORGE, WISOTSKY MAX JAY, ASHCRAFT, THOMAS LEE, BERLOWITZ, PAUL JOSEPH, CARR, DALE DUANE, SCHAEFER, THOMAS GEORGE, WISOTSKY, MAX JAY
Applicants: BP EXPLORATION AND OIL INC, HATCO CORP, BP EXPLORATION AND OIL INC., HATCO CORPORATION
Application year: 1995
Main classification: C10M105
IPC codes: C10N20, C10M105, C10N40