US3074981A

US3074981A - Esters of structurally stabilized acids

Info

Publication number: US3074981A
Application number: US151430A
Authority: US
Inventors: Robert D Aylesworth; Bernard R Krabacher; Richard G Kadesch; Norman O V Sonntag
Original assignee: Emery Oleochemicals LLC
Current assignee: Emery Oleochemicals LLC
Priority date: 1961-11-10
Filing date: 1961-11-10
Publication date: 1963-01-22
Anticipated expiration: 1980-01-22

Description

This invention relates to synthetic lubricating compounds and particularly to compounds adapted to provide lubrication for metal surfaces. More particularly, the lubricants of this invention are characterized by relatively great stability at temperatures which may be as high as 4S6--600 F., by pour points which are relatively low, preferably under F, and by viscosities and ASTM slopes which compare favorably with the best synthetic lubricants now on the market. This application is a continuation-impart of our copending application, Serial No. 734,108, filed May 9, 1958, and now abandoned.

The lubricants of the present invention are ester lubricunts, that is, esters of fatty acids and alcohols. The fatty acids which are used in the practice of this invention are disclosed and claimed in United States Patent No. 2,812,342 issued November 5, 1957, entitled Hydrogenation of structurally Modified Acids and Products Produced Thereby. The alcohols which may be used for esterification with fatty acids of the type specified are those such as neopentyl glycol, 2,2-diethylpropane- 1,3-dicl and trimethylolpropane, tor example, which contain from 2 to 4 primary hydroxyl groups and a total of 5 to 7 carbon atoms, and which are sterically hindered, i.e., contain no hydrogen atoms attached to the beta carbon atom.

Although the esters of this invention may be used for a great variety of purposes, the unique characteristics of the esters render them particularly suitable for the lubrication of jet airplane engines. A suitable lubricant for a jet engine must first of all be a good lubricant but must also be physically and chemically stable when used at an elevated temperature over a substantial period of time. At the same time, the lubricant must have a pour point which is sufiiciently low to avoid solidification when subjected to temperatures of the order encountered in the arctic regions. Obviously, ester lubricants which are suitable for use under these extreme conditions are also suitable for uses under less exacting conditions; hence, the ester lubricants of this invent-ion are suitable for the lubrication of piston engines or for use as a crank case oil in an automobile.

The present invention is predicated upon the determination of the desirable properties of the esters of the relatively unique acids of Patent No. 2,812,342 and certain sterically hindered primary alcohols. One of the desirable and characteristic properties or" these esters is that they have a relatively low pour point regardless of the nature of the alcohol selected. The other desirable property is that the esters are extremely stable physically and chemically and, hence, have excellent corrosion characteristics and extreme stability even when subjected to exacting thermal conditions. In the past the synthetic lubricants which have been used in the general fields of usage for which the lubricants of the present invention are intended have been the diesters of dibasic acids such as adipic, azelaic and sebacic acids and branch chain alcohols of 8-20 carbon atoms chain length. The esters of the present invention are better than these presently used diesters because, by proper choice of alcohol, it is possible to obtain esters of equally low pour points, higher 3,@?4,8l Patented Jan. 22, 1963 2 flash points, equal or better lubrication characteristics, and greater physical and chemical stability when sub jeoted to exacting thermal conditions.

The esters of the present invention have remarkable stability as esters, that is, they do not decompose into constituent parts even when heated over prolonged periods of time. Hence, these esters do not volatilize as readily as those commonly used in synthetic lubricants. This nonvolatility results from stability which is over and above that which might be expected from the mere high resistance of the esters to oxidation. Although the structure of the fatty acids to be used in these esters is not known, it is clear from the properties exhibited that it is different from the ordinary monocarboxylic and dicarboxylic fatty acids and that it is such that the ester linkage with a primary alcohol is unusually strong and resistant to disruption. These fatty acids do not occur anywhere in nature, but rather are the products of the very specialized process which is disclosed and claimed in said Patent No. 2,812,342.

According to the disclosure of this patent, monounsaturated or polyunsaturated fatty acids are treated thermally in the presence of water with or Without a catalyst or clay to produce products which are known commercially as dimer acids. This type of process, however, inherently produces a substantial amount of by-product which is a m xture of monomeric acids. Apparently, this mixture includes saturated fatty acids which have not been affected by the polymerization treatment, probably some unsaturated fatty acids which have not been affected by the polymerization treatment and fatty acids which have been structurally modified by the polymerization treatment in such manner that they resist further polymerization. According to Patent No. 2,812,342, this fraction of monomeric acids is then hydrogenated to reduce the iodine value of the mixture below 10 or perhaps as low as 3. Obviously the hydrogenation treatment'reduces any natural unsaturated fatty acids. which may be present to saturated fatty acids. The hydrogenation also reduces the iodine value of the fatty acids which have been structurally modified to some degree by the polymerization treatment. It is difiicult to estimate the additional degree of structural modification, it any, which takes place during hydrogenation.

These hydrogenated fatty acids are then solvent separated to remove solid fatty acids such as stearic and palrnitic acids. The remaining fatty acids are all structurally modified products which have the unique characteristics of being of 18 carbon atoms chain length, having a low titre, i.e. below 15 0., and having a low iodine value of substantially 3 to 10.

In manufacturing fatty acids for use as lubricant esters, the above described process must be practiced very carefully to effect the best possible segregation of the different classes of acids. For instance, if the hydrogenation process is practiced ineffectively, then oleic acids will show up as an impurity in the end product. The presence of oleic acid in the final product is undesirable because oleic acid is not stable in the sense of being resistant to oxidation. A lubricant containing an ester of oleic acid will inherently tend to oxidize more readily and hence tend to corrode the metal being lubricated. Also, if the solvent separation operation is not practiced effectively, then stearic or other solid acids will be present and the esters which include esters of stearic acid will have an undesirably high pour point. In summary, the presence or the esters of both oleic and stearic acids is highly undesirable in the finished product, and the process of Patent No. 2,812,342 must be practiced with sufo 3 ficient care to form the structurally modified acids without the'presence of any objectionable quantity of the undesirable impurities.

While Patent No. 2,812,342 refers to the fatty acids in questionas structurally modified acids, which they are in relation to the starting materials, from the point ofivi'ew of the present'invention these acids are more aptly described as'structurally stabilized acids. These acids have received two very severe treatments one, the polymerization treatment whichtends both to polymerize and'structurally to modify the unsaturated acids, and two, hydrogenation which saturates and perhaps otherwise modifies the structure of the acids. Hence, these structurally modified acids have become stabilized acids in thELSCIlSC that they have been subjected to such rigorous treatment asito have developed their most stable struc-- turalform. Hence, through out'thedescription of this invention,'the unique acids which are the components of the lubricant 'e'ster are referred to as structurally stabilized acids, inasmuch as there is no better name by which to..call them. V

. The alcohols which may be employedin connection with .thetstructur'allystabilized fatty acids to form the esters of this invention. are those of the so-called hindered?"variety which contain a total of from to 7 carbon atoms and from 2 to 4 methylol groups, said groups being separated by afcarbon atom (termed the beta carbon atom. of the compound) which is free of attached hydrogen atoms. structurally, said alcohols can be represented by the following general formula:

Y wherein X and Y represent members selected from the group consisting of methylol, methyl, ethyl, propyl and isopropyl groups, it being recognized that X (or Y) can be a C group only when Y (or X) is methyl or methylol in view of the overall limitation on the number of carbon atoms to 7.

For convenience of description, these alcohols will be referredjto herein and in the appended claims, as betadisubstituted propane-1,3 diols containing from 2 to 4 methylol groups and a total of 5 to 7 carbon atoms. The compounds which come withinthe scope of this language and of the structural formula givenabove are: 2,2-dimethyl-propane-1,3-diol (neopentyl glycol) 2-methyl-2-ethyl-propane-1,3-di01 2,2-jdiethyl-propane-1,3-diol 2-methyl-2-isopropyl-propane-1,3-diol 2-methyl-2-propyl-propane-l,3-diol ,2-methyl-2 methylol-propane-1,3rdiol Z-ethyI-Z-methylol-propane-1,3 diol 2-propyl-2-methylol-propane-l,3,-diol 2-isopropyl-2-methylol-propane 1,3-diol 2,2-dimethylol-propane-1,3-diol (pentaerythritol) The present esters can readily'be prepared by means of conventional techniques well known to those versed in.

the art; Ester ification of polyol esters with high molecular weight, aliphatic, monocarboxylicacids is generally carried out by reacting a suitable excess of'the acid with the polyol under such conditions of time and temperature as to'remove the water formed during the esterfication reaction. This'is generally followed by vacuum stripping to. remove the excess organic acid; A suitable catalyst 4 The following examples illustrate the preparation of various ester compounds coming within the scope of the present invention, data thereafter being given as to the viscosity, slope, pour point and stability of said esters, the stability being that obtained under the conditions specified in the MIL-L-9236 test.

EXAMPLE 1 In this operation the pentaerythritol ester of a structurally stabilized fatty acid was prepared by reacting 119 grams of the alcohol with '1281 grams (representing a 25% molal excess) of a structurally stabilized fatty acid having an iodine value of 5.4, a 3 C. titre and an 18 carbon atom chain length. To this mixture was added 6.3 gramsof 75% phosphoric acid, with the mixture then being refluxed in a suitable vessel for one hour at 170 C. under a carbon dioxide atmosphere. A Barrett type distilling receiver was installed to remove the water of esterification and the reaction continued for three hours during which the temperature increased. to 225 C. The pressure in the vessel was reduced to approximately 100 millimeters mercury absolute and the esterification resumed for six hours at 230 C. The acid catalyst was neutralized with four grams of sodium carbonate and the excess fatty acid removed under two millimeter, mercury absolute pressure to a pot temperature of 290C. The crude product had an acid value of 2.6

and a hydroxyl value of 2.1. The ester was alkali refined using 40% excess caustic soda as a 10% aqueous solution. The washing was carried out carefully to avoid emulsification and the initial separation effected by means of a centrifuge. The final ester had an acid value of 0.33 'and a hydroxyl value of 0.83.

In a manner generally similar to that described above, corresponding esters were prepared from the following alcohols, the acid value and hydroxyl value for the particular ester product obtained being given after each alcohol:

Acid

Hydroxyl values values Neopentyl glycol 2-methyl-2-methylol-propane-l,3-diol .1 2-ethyl-2-methylol-propane-1,3-diol 2-methyl-2-ethyl-propaned ,B-dioL 2-methyl-Z-propyhpropane-l,3-diol 2,2-diethyl-propane-1,3-diol Hexane-1,6-diol 1 00 Sip-w be present in the final products, and the data given above indicate'that the ester compounds prepared contained essentially no such groups.

These ester lubricants may be used in various ways. :They maybe used as straight lubricants, or they may be blended with other ester lubricants or with petroleum oils. The" esters may be thickened and used as greases if desired. On the other hand, higher molecular weight esters may themselves be used as thickening agents or viscosity improvers for other ester lubricants or for petroleum lubricants or for mixtures of them.

Obviously, these esters may be used for practically all of the purposes for which petroleum oils and other esters are now used, and the alcohols which are incorporated in the esters may be chosen to provide the desired characteristics for the finished product. However, for a lubricant adapted to be used in a jet engine, the following alcohols are particularly recommended: neopentyl glycol, 2-methyl-2-methylol-propane-1,3-diol, 2-ethyl-2- methylol-propane-l,3-diol and pentaerythritol.

The following table provides viscosity, ASTM, slope and pour point of various esters of structurally stabilized fatty acids.

1 Refers to relationship between viscosity and temperature, a low value being most desirable. Dioctyl scbacate has an ASTM slope of 0.700, while diisotlecyl adipatc has an ASTM slope of 0.727.

2 This ester is not that of a hindered alcohol.

The data presented below in Table 11 show that the esters of the present invention exhibit outstanding stability characteristics under the conditions of heating specified in MIL-L-9236 designed for the testing of lubricants to be employed in aircraft engines. In making these tests samples of the various esters were heated at 500 F. for 48 hours in the presence of 0.5% of recrystallized phenothiazine. The following results were obtained showing the increase in acid number and the change in viscosity obtained during the test.

These esters do not exemplify the invention.

The data of the foregoing table show that the esters of the present invention have oxidative stability characteristics which are many times better than those of the sebacate and adipate esters now in common use as lubricants. At the same time, the data of Table I show that their viscosity and pour point qualities are such as to make them well adapted to be used under the severest service conditions.

We claim:

1. A new composition of matter adapted to be used as a lubricant, said composition comprising esters of structurally stabilized fatty acids of 18 carbon atom chain length having an iodine value of substantially 3 to 10 and a titre below substantially 5,, a beta-disubstituted propane-1,3-diol containing from 2 to 4 methylol groups and a total of from 5 to 7 carbon atoms, substantially all of the carboxylic acid and hydroxyl groups present in the ester components being reacted with each other to provide a composition containing substantially no free bydroxyl and free carboxylic groups.

2. The composition of claim 1 wherein the substituents on the beta carbon atom are selected from the group consisting of methylol, methyl, ethyl, propyl and isopropy]. radicals.

3. The composition of claim 1 wherein the beta-disubstituted propane-1,3-diol is pentaerythritol.

4. The composition of claim 1 wherein the beta-disubstituted propane-1,3-diol is neopentyl glycol.

5. The composition of claim 1 wherein the beta-disubstituted1 propane-1,3-dio1 is 2-methyl-2-methylol-propane- 1,3-dio 6. The composition of claim 1 wherein the beta-disubsltituged1 propane-1,3-di0l is 2-ethyl-Z-methylol-propane- References Cited in the file of this patent UNITED STATES PATENTS 2,783,270 Polly Feb. 26, 1957 2,798,083 Bell et a1. July 2, 1957 2,812,342 Peters Nov. 5, 1957

Claims

1. A NEW COMPOSITION OF MATTER ADAPTED TO BE USED AS A LUBRICANT, SAID COMPOSITION COMPRISING ESTERS OF STRUCTURALLY STABILIZED FATTY ACIDS OF 18 CARBON ATOM CHAIN LENGTH HAVING AN IODINE VALUE OF SUBSTANTIALLY 3 TO 10 AND A TIRE BELOW SUBSTANTIALLY 5, A BETA-DISUBSTITUTED PROPANE-1, 3-DIOL CONTAINING FROM 2 4 METHYLOL GROUPS AND A TOTAL OF FROM 5 TO 7 CARBON ATOMS, SUBSTANTIALLY ALL OF THE CARBOXYLIC ACID AND HYDROXYYL GROUPS PRESENT IN THE ESTER COMPONENTS BEING REACTED WITH EACH OTHER TO PROVIDE A COMPOSITION CONTAINING SUBSTANTIALLY NO FREE HYDROXYL AND FREE CARBOXYLIC GROUPS.