WO1993007240A1

WO1993007240A1 - Process for producing fluids of enhanced thermal stability

Info

Publication number: WO1993007240A1
Application number: PCT/US1992/008315
Authority: WO
Inventors: Richard Henry Schlosberg; James Zielinski; John Wand Chu; Gerald Dennis Dupre; Theresa Kathleen Natishan
Original assignee: Exxon Chemical Patents Inc.
Priority date: 1991-10-03
Filing date: 1992-09-28
Publication date: 1993-04-15

Abstract

The present invention provides for synthetic ester lubricants having enhanced thermal stability prepared by oxidatively treating a substrate comprising an ester lubricant at elevated temperatures in excess of 200 °C for a period of time sufficient to substantially oxidize impurities present in the lubricant, but insufficient to substantially oxidize the ester or to substantially decompose it. The process results in a predecomposition of the most reactive species of the substrate to the more stable species. The resulting ester lubricants so treated are found to exhibit enhanced thermal stability at temperatures of 375 °C and above thereby rendering them more effective as lubricants in high temperature applications and more stable over longer periods of time.

Description

PROCESS FOR PRODUCING FLUIDS OF ENHANCED THERMAL STABILITY

BACKGROUND OF THE INVENTION Field Of The Invention

The present invention relates to a process for improving the thermal stability of fluids, particularly ester-based lubricants.

Description of Related Art

The utilization of synthetic esters as lubricating oils in such applications as fiber lubricants, aviation turbo oils, automotive oils, metal working fluids, gear oils, refrigerant oils and the like is well known. These esters vary from simple esters derived from mono-, di- or polycarboxylic acids, mono alcohols or polyols to complex esters such as derived from half esters or ethers of glycols and di or polycarboxylic acids.

Performance requirements for fluids used as lubricants in machinery are becoming more and more exacting. Lubricants for use in aircraft and in gas turbines require special properties which are not possessed by conventional lubricants. Thus, they must possess a high viscosity index in order to provide adequate lubrication over a wide range of temperatures. They must have a low pour point in order that they may function at low temperatures and high flash points to avoid risk of fire at high temperature operation and minimize loss of lubricant by evaporation. In addition, as machinery is developed to run at faster operating speeds, there is also a concomitant increase in operating temperatures. Lubricating fluids, therefore, need to be both effective and stable at ever increasing temperatures. Synthetic ester lubricants such as described above are not essentially pure but contain trace to minor amounts (less than 10% by weight) of difficult to remove synthesis impurities which can be less thermally stable than the pure ester itself. These impurities can include branched or straight chain olefins, alcohols, acetals, formals, etc. and/or acids which are either residual feedstock reactants used to make the ester lubricant or the products of side reactions during manufacture. Many of these impurities are less thermally stable than the lubricant ester compounds themselves and tend to degrade more rapidly than the esters at a given temperature and at lower temperatures.

It is known in the art to include minor quantities i.e., .05 to 5% by weight, of one or more antioxidants as an additive to ester lubricants to further enhance thermal stability at higher operating temperatures. Typical of such antioxidant materials are phenolic compounds such as butylated hydroxy toluene or amines such as phenyl alpha-naphthyl amine. These materials tend to retard oxidative degradation of the lubricant, but because they offer little lubricating value on their own, there is a limit on the amount of antioxidant which can be added to the ester before lubrication properties of the fluid are seriously diminished.

Because the impurities present in the ester lubricants are frequently less thermally stable than the pure ester components, impurities tend to be more reactive with oxygen and with the antioxidant species present in the ester composition. This higher reactivity results in a more rapid consumption of the antioxidant species under operating temperatures resulting in a diminution in the overall thermal stability of the ester lubricant composition and lubricating value thereof.

Processes have been proposed in the prior art to modify the properties of ester fluids. For example, Cheng et al, J. Chinese Chem. Soc. 11, 79-85 (1944) disclose that viscosity modifications of certain cracked vegetable oils may be achieved by heating the oil at a temperature of up to 200°C for a period of hours while bubbling heated air through the oil. The product is characterized by an increased acid value and increased viscosity. In addition, U.S. Patent 2,847,383 discloses that the melting point of certain synthetic ester lubricants can be lowered by heating a mixture of esters in an oxygen-free atmosphere at a temperature of from about 100 to 200°C and in the presence of a small quantity of an alkali metal. However, neither of these references teaches a process for producing lubricant fluids having enhanced thermal stability.

SUMMARY OF THE INVENTION

The present invention provides for synthetic ester lubricants having enhanced thermal stability prepared by oxidatively treating a substrate comprising an ester lubricant with air or oxygen at elevated temperatures in excess of 200°C for a period of time sufficient to substantially oxidize impurities present in the lubricant, but insufficient to substantially oxidize the ester or to substantially decompose it. The process results in a predecomposition of the most reactive species of the substrate to more stable species, species which will react less readily with antioxidant additives.

The resulting ester lubricants so treated are found to exhibit enhanced thermal stability at temperatures of 375°C and above, thereby rendering them more effective as lubricants in high temperature applications and more stable over longer periods of time.

DETAILED DESCRIPTION OF THE INVENTION

As indicated above, it has now been found that the thermal stability of synthetic ester lubricants as described herein may be improved by subjecting the ester to a preoxidation step prior to the incorporation of any antioxidant material which is to be added to the ester. Preoxidation conditions are such that reactive impurities present in the ester composition are substantially oxidized while leaving the less reactive ester component of the lubricant essentially unoxidized. This preoxidation step renders the impurities more stable and less reactive with antioxidant materials added to the lubricant thereby enhancing thermal stability, extending the life of the lubricant and permitting the use of less antioxidant in the composition than would otherwise be required.

Ester lubricants which may be treated in accordance with the process of this invention may be generally characterized by the formula I: A3

I . (A -E) - A - (E-A (A ) 2 m 5 p

^{1 n} I ε

I A4

wherein E is an ester linkage selected from COO or OOC, A is the residuum of a substituted or unsubstituted aliphatic, cycloaliphatic or aromatic divalent, trivalent or tetravalent organic radical having from 1 to 32 carbon atoms, A^, A2, A₃, A and A5 are the same or different organic radicals containing from 2 to 32 carbon atoms, n, m and p are independently 0 or 1, and p is 1 when is 0.

Suitable esters wherein E is COO include di-, tri- and tetraesters prepared by reacting saturated or unsaturated di, tri or tetra carboxylic acids with monohydric alcohols containing 2 to 32 carbon atoms. Suitable esters wherein E is OOC are di-, tri- and tetra- esters prepared by reacting a diol, triol or tetraol with a saturated or unsaturated monocarboxylic acid having from 2 to 32 carbon atoms.

One preferred class of ester lubricants are tri and tetraesters prepared by forming the esterification product of at least one polyol and at least one monocarboxylic acid. Suitable polyols include those polyhydric alcohols having from 2 to 8 hydroxy groups per molecule and 2 to about 25 carbon atoms. Preferred polyols include neopentyl glycol. glycerol, trimethylolpropane, pentaerythritol, di- pentaerythritol and the like as well as mixtures thereof. Suitable monocarboxylic acids are those having from about 4 to 32 carbon atoms. Preferred monocarboxylic acids include butyric, valeric, enanthic and eicosanoic acids.

Another preferred class of ester lubricants are diesters prepared by reacting a saturated or unsaturated dicarboxylic acid and a monohydric alcohol and which satisfy the general formula II.

II. R₁OOC-R-COOR₂

wherein R is a substituted or unsubstituted hydrocarbon radical having from 1 to 12 carbon atoms and R^ and R₂ are the same or different and are each selected from the group consisting of substituted or unsubstituted hydrocarbon radicals containing from 4 to 32 carbon atoms.

The acids from which the above described esters are derived can be either saturated or unsaturated polycarboxylic acids, preferably dicarboxylic acids. Suitable acids include citric acid; alonic acid; ethyl alonic acid; succinic acid; methyl-succinic acid, 1,1-diethylsuccinic acid; 1,2- diethylsuccinic acid; glutaric acid; methylglutaric acid; ethylglutaric acid; propylglutaric acid; isopropylglutaric acid; butylglutaric acid and its isomers; amlyglutaric acid and its isomers; di ethylglutaric acid; methylethylglutaric acid and its isomers; diethylglutaric acid; ethylpropylglutaric acid; adipic acid; 2-methyladipic acid; 2-ethyladipic acid; 2-butyl adipic acid; 2,2-dimethyladipic acid; 2,3 dimethyladipic acid; 2-methyl-2-ethyladipic acid, 2- methyl-3-ethyladipic acid; 2-methyl-2-isopropyladipic acid- 2,2,4- and 2,4,4-trimethyladipic acids; pimelic acid; 2-methylpimelic acid; 3-methylpimelic acid; 2- ethylpimelic acid; 3-ethylpimelic acid; 2-and 3- propylpimelic acids; 2- and 3-isopropylpimelic acids; 2,2- and 2,3-dimethylpimelic acids; 2-methyl-2- ethylpimelic acid; 2-methyl-3-ethylpimelic acid, 3- methyl-3-ethylpimelic acid; 2-ethyl-3-methylpimelic acid; 2,2,3-trimethylpimelic acid; 2,3,4- trimethylpimelic acid; 1,2,5-trimethylpimelic acid; suberic acid; methyl and ethyl-suberic acids in which the methyl or ethyl groups may be in positions 2,3,4 or 5; and methylethylsuberic acids, wherein the methyl and ethyl groups are in positions 2,3,4 or 5; azelaic acid; methylazelaic acid; ethylazelaic acid; sebacic acid; ethylsebacic acid; ethylsebacic acid; undecanedioic acids; dodecanedioic acids; and the like. Aromatic polycarboxylic acids, such as phthalic, isophthalic, terephthalic, trimellitic and trimesic acids may also be employed. Anhydrides of acids such as phthalic and trimellitic anhydride may also be used.

The alcohols from which the above esters are derived can be either aliphatic, araliphatic or cycloaliphatic alcohols of at least 4 carbon atoms, and preferably from about 4 to about 32 carbon atoms. The saturated aliphatic alcohols are preferred. Both straight-chain and branched-chain saturated aliphatic alcohols are suitable, but branched chain alcohols are preferred. Commercial mixtures of alcohols containing substantial proportions of branched chain alcohols, such as prepared by the Oxo process, are also suitable. Guerbet di er alcohols may also be used, such as hexadecyl alcohol and eicosyl alcohol. Suitable alcohols include the alkanols, such as butanol, isobutanol; tert-butanol; 1-methylpropanol; pentanol;

2-methylbutanol; 3-methylbutanol; 1- and 2- ethylpropanols; 2,2- and 1,2-dimethylpropanols; hexanol; 2-, 3- and 4-methylpentanols; 2,2- di ethylbutanol; 2,3-dimethylbutanol; 2-ethylbutanol;

3-ethylbutanol; 2-methyl-2-ethylproponal; heptanol; 2-,

3-, 4- and 5-methylhexanols; 2-, 3- and 4- ethylpentanols; l-methyl-2-ethylbutanol; 2-methy1-3- ethylbutanol; octanol; 2-, 3- and 4-ethylhexanols; 2-,

3-, 4, and 5 methyIheptanols; 2,2-dimethylhexanol; 3,3- dimethylhexanol; 2,4-dimethylhexanol; 1- isopropyl-3- methylbutanol; nonanol; 3,3-diethylpentanol; 2-ethyl-3- methyl-hexanol; 3,5,5- trimethylhexanol; 2-isopropyl-4- butyloctanol; 3-butyldecanol; 2-butyl-5-propyl-7- ethylundecanol, and the like; aralkanols, such as benzyl alcohol and phenylethyl alcohol; and cyclo- alkanols such as cyclopentanol, cyclohexanol and methylcyclohexanol; lower alkylene glycols such as C₂ to Cio glycols; and the triols, tetraols, hexanols and octanols referred to above.

The most suitable esters for use in the present invention are the esters of saturated aliphatic dicarboxylic acids in which the carboxyl radicals are separated by a chain of from about 4 to about 12 carbon atoms, and preferably from about 4 to about 8 carbon atoms, and alkyl alcohols, preferably branched-chain alcohols having from about 4 to about 20 carbon atoms, specifically from about 7 to about 18 carbon atoms.

Typical specific liquid esters which are preferred are: di(2-methylheptyl) adipate; di(3,5,5- trimethylhexyl) adipate and sebacate; di(3-ethylhexyl) adipate and sebacate; di(2-ethylhexyl) adipate and sebacate; dihexyl pimelate, diisobutyl sebacate; diisobutyl malonate; di(l-ethylpropyl) azelate; diisobutyl ethylmalonate; di(3-methylheptyl) pimelate; di(3,3-diisopropylhexyl) pimelate; dioctyl phthalate; ditridecyl adipate; diisodecyl adipate; isoeicosyl neodecanoate; isoeicosyl isoeicosanoate; ditridecyl hexahydrophthalate; and the like.

Other preferred liquid esters are neopolyol esters such as trimethylolpropane triheptanoate, trimethylolpropane tricaproate, trimethylolpropane tri- 2-methylbutyrate and similar esters based on pentaerythritol.

The ester lubricants may be oxidized by heating the material at a temperature in excess of 200^βC and contacting it with pure oxygen or air. Such contact may be carried out by passing a flow of air over the material, bubbling air through a bulk of the material or heating the material in an air-pressurized heated autoclave in a stirred tank reactor in the presence of air.

The temperature to which the ester lubricants are heated above 200°C during the oxidation process will vary depending on the identity of the ester lubricant and its physical properties, i.e., flash point and decomposition temperature. Preferably, the heating temperature should not exceed the temperature at which the ester lubricant itself decomposes as defined by an increase in the total level of impurities in the ester of more than about 1% by weight as measured by standard thermal gas chromatography techniques. For most materials, this temperature will generally range from about 200 to about 350°C, more preferably from about 225 to about 325°C.

The duration of the contact with oxygen or air will vary depending on the bulk of the material treated. Oxidation times may generally range from about 1 minute to about 10 hours or more. Generally speaking, the time should be sufficient to substantially oxidize impurities present in the ester material without oxidizing more than about 1% by weight of the ester material itself.

Subsequent to the oxidation step, the product may be utilized directly in lubricant applications or all or a portion of the oxidized impurities may be removed prior to such use. Separation techniques such as fractional distillation may be facilitated by virtue of the fact that oxidation produces a heavier and lower boiling species of impurity. The product may be formulated with conventional antioxidants used for ester lubricants such as hindered phenols or amines. Suitable antioxidants include 2,6-di-t-buty1-4-methyl phenol, N,N '-di-sec-buty1-para-phenylenediamine, butylated hydroxytoluene, phenyl alpha naphthyl araine and like materials, added at a level of from about 0.05 to about 5% by weight.

As indicated above, preoxidation of the more thermally reactive impurity species present in the ester allows for the addition of less antioxidant to achieve lubricants of improved stability.

The following Examples are illustrative of the invention. Example 1

The thermal stability of an untreated sample of ditridecyl adipate (DTDA) was determined by thermal gas chromatography. A 0.2 ml sample was injected into the injector port of a chromatograph (DB-17 Megabore column, packed column inlet, no glass liner) heated to 250^βC. The residence time of the sample in the port was approximately 3 seconds. Thermal GC analysis showed the sample to be 99.13% pure ditridecyl adipate and 0.87% impurity.

The above process was repeated with all conditions the same, except that the sample was injected into the port heated to 375°C The resulting analysis showed the heated sample to be 98.41% pure ditridecyl adipate containing 1.59% impurity. Thus, the sample had decomposed 0.72% between 250 and 375°C.

Example 2

Eight drops of an untreated sample of DTDA were introduced into a 25 ml. pear shaped flask equipped with a thermometer, condenser, air inlet tube and stirrer. The contents of the flask were heated in an oil bath on a hot plate and air was introduced through the air tube and bubbled through the contents while stirring. The contents were held at 250-280°C for 30 minutes during which time air introduction was continued. Heating was then discontinued and the flow of air was replaced by a nitrogen flow to assist cooling of the contents. The thus treated sample was thereafter evaluated for thermal stability at 250^βc and 375°C by the same process as set forth in Example 1. Results showed that the sample had the same impurity content at 250^βc and 375°C, namely 1.53%, indicating that no thermal decomposition took place between 250 and 375^βC. Results are illustrated in Table l.

Example 3

Example 1 was repeated except that the material evaluated for thermal stability was diisodecyl adipate (DIDA) . The content of impurity of the sample increased from 1.22% at the 250°c analysis temperature to 5.54% at the 375^βC temperature, resulting in a % decomposition of 4.32% over that range.

Example 4

An untreated sample of DIDA was oxidized by the method of Example 2 and the thermal stability was evaluated as in Example 2. Results showed that the sample had an impurity content at 250°C of 3.41% and at 375°C of 3.51%, resulting in a net decomposition of only 0.1% over that range. Results are shown in Table 1.

While the invention has been particularly shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that changes in form and details may be made therein without departing from the scope and spirit of the invention.

Claims

CLAIMS :

1. A process for enhancing the thermal stability of a synthetic ester lubricant containing synthesis impurities comprising contacting said lubricant with air or oxygen at a temperature in excess of 200°c and for a period of time sufficient to substantially oxidize the impurities present in said lubricant but insufficient to substantially oxidize said ester or to substantially decompose said ester.

2. The process of claim 1 wherein air or oxygen is bubbled through said lubricant.

3. The process of claim 1 wherein said lubricant is heated to a temperature of up to about 350°C.

4. The process of claim 1 wherein said lubricant comprises an ester having the formula:

A3

I E

E

I

A4

wherein E is an ester linkage selected from COO or OOC, A is the residuum of a substituted or unsubstituted aliphatic, cycloalphatic or aromatic divalent, trivalent or tetravalent organic radical having from 1 to 32 carbon atoms, A^, A₂, A3, A , and A5 are the same or different organic radicals containing from 2 to 32 carbon atoms, n, m and p are independently 0 or 1, and p is 1 when m is 0.

5. The process of claim 1 wherein said lubricant comprises an ester derived from a polyhydric alcohol having from 2 to about 8 hydroxy groups per molecule and said polyhydric alcohol having from 2 to about 25 carbon atoms.

6. The process of claim 5 wherein said polyhydric alcohol is trimethylolpropane, pentaerythritol, dipentaerythritol or mixtures thereof.

7. The process of claim 1 wherein said lubricant comprises an ester having the formula:

RiOOC - R - COOR₂

wherein R is a substituted or unsubstituted hydrocarbon radical having from 1 to 12 carbon atoms and R^ and R₂ are the same of different and are each selected from the group consisting of substituted or unsubstituted hydrocarbon radicals containing from 4 to 32 carbon atoms.

8. The process of claim 7 wherein said ester is di-tridecyladipate.

9. The process of claim 7 wherein said ester is di-isodecyladipate.

10. The process of claim 7 wherein said ester is heated within the range of from about 225 to 325°C. - 16 -

11. The process of claim 1 wherein at least a portion of the oxidized impurities are separated from said ester.

12. The process of claim 1 wherein the product of the process is formulated with an antioxidant.

13. A synthetic ester lubricant produced by the process of claim 1.