US2939874A - Preparation of polyesters - Google Patents

Description

l atented June 7, 1960 PREPARATION OF POLYESTERS No Drawing. Filed Dec. 30, 1957, Ser. No. 705,746 4 Claims. (Cl. 260-410.6)

This invention relates to the manufacture of esters of polyhydric alcohols which are substantially free of unreacted acids and also concerns a method of manufacturing synthetic ester-type lubricants having low acid numhers.

I have found that synthetic polyester lubricants produced from the esterification of mixed monobasic carboxylic acids with polyhydric alcohols have very low free acid content provided the esterification is carried out in two steps. These two steps comprise dividing the mixture of acids into a high boiling portion and a low boiling portion and then partially esterifying the polyhydric alcohol with the high boiling portion followed by completing hydric alcohols gave high molecular weight esters which i were found to be useful as lubricants for aircraft instruments, hydraulic and dampening fluids, and precision bearing lubricants where exceptionally low temperature fluidity properties were required. Of particular interest were the diester lubricants. However, these diesters were only useful as light oils since their viscosity was too low for anything but these special applications. Since the viscosity index was too low and the pour point too high for the very low temperatures required in military uses, they were found to be unsuitable for engine oils. Also, the method of their production was considered to be quite complex. This prior work has been done with essentially pure acids and the diesters produced were essentially a single compound. Interest in diester lubricants has been accelerated with the development of the gas turbine. Recent work has shown that when a mixture of monobasic carboxylic acids having greater than 6 carbon atoms per molecule is esterified with polyglycols that the diesters produced have unexpectedly superior qualities as engine lubricating oils as compared with the diesters produced from a single acid. Therefore, there is real incentive for producing diesters from mixed monobasic carboxylic acids that will meet government specifications for engine lubricating oils.

One of the big problems in producing satisfactory polyesters when using mixed monobasic carboxylic acids in the O, to C range is that the high molecular weight acids which are required for superior products are very difficult, if not impossible, to remove from the ester product when they have not reacted with the polyglycol. Government specification for lubricating oil requires that the acid number (i.e., mg. KOH/gm.) must not exceed 0.1. The normal esterification process is carried out by reacting an alcohol with an acid in the presence of an esterification catalyst, such as a strong mineral acid, and refluxing the mixture While trapping out the water of the reaction. Normally an azeotroping agent is used in the reaction to facilitate the removal of the water. When no more water is produced the reaction is complete and the product from the reaction is purified by distillation. If stoichiometric amounts have been added and the reaction has proceeded theoretically there is no excess acid. or

alcohol. However, as is true in most chemical reactions,

the reaction product in the majority of the cases contains an excess of either the acid or the alcohol. In producing synthetic diester lubricants which are suitable as engine oils, as well as special uses, it is essential that the diester product contain no free acid or monoester.

'lf excess acid is present it will cause corrosion of the 'engine when the lubricating oil is in use and if monoesters are present they will cause a reduction of viscosity and viscosity index and will increase the pour point. When esterifying mixtures of low and high boiling carboxylic acids any residual unreacted acid is very difficult, if not impossible, to remove since a portion of the residual acids will boil at a temperature above the boiling point of the lowest boiling polyesters. Experience has shown that they cannot be removed by' distillation alone. The obvious answer would appear to be the use of an excess of polyglycol in the react-ion, thereby reacting all the acids, both low boiling and high boiling. Ho; ever, the presence of an excess amount of polyglycol during the esterification will cause the formation of monoesters, sometimes called half esters. Since some of these glycol monoesters boil as high as the diesters they cannot be completely separated from the diesters and will dea grade the superior qualities of the diester synthetic lubricant. Normally an excess amount of acid is used in the reaction in order to insure that all the polyglycol will react, thereby forming the preferred diester product. Previous workers have not been faced with the difficulty of removing residual amounts of high boiling acids since their work has been done with relatively pure low boiling acids which can be removed by distillation and neutralization.

I have now discovered a new technique for producing polyesters by the reaction of mixtures of low'and high boiling monobasic carboxylic acids with polyglycols which method, after normal purification of the product, produces synthetic diester lubricants containing essentially no residual acid. The method comprises first dividing the mixture of acids into a low boiling portion and a high boiling portion; mixing the polyglycol with the high boiling acid portion in a reaction vessel along with an esteri fication catalyst; carrying out the esterification until all acidsin the high boiling portion have completely reacted, leaving unreacted hydroxyl groups in the partially esterified polyglycol; adding the low boiling acid portion to the reaction mix and resuming the esterificatio-n until all.

the hydroxyl groups of the polyglycol have reactedto form polyesters, and leaving an excess of unreacted low boiling acids in the polyester product mix. In the product work-up the low boiling acids can be easily removed by distillation. In this manner the disadvantages of the prior art are overcome and polyester synthetic lubricants of a very low acid content are produced.

An example of a mixture of carboxylic acids which are suitable for producing diesters for use as lubricating oils is a portion of those acids which are produced by the hydrogenation of carbon monoxide. The carboxylic acids used in making synthetic lubricants have at least 7 carbon atoms in the molecule and up to as many as 30 carbon atoms per molecule. The mixture contains many, car boxylic acids differing in the extent of unsaturation and branching in the hydrocarbon chain. This process is commonly 'known as the hydrocarbon synthesis process or synthol process.

synthol process are as follows. Synthesis gas (nan The typical conditions used in, the

covered-from the hydrocarbon phase.

aren'dtrequiree.

between 1.02.5:l is contacted at a temperature between 450 and 750 F., preferably between 600 and 700 F.,

-with1.an=ir on-type catalyst. Whilepressur'es of .between 100 and 700,p.s.=i.g. may be used, pressures higher than about.250,p.s.i-.g.,-preterablybetween 325 and 425'p.s-.i.g. "are preferred. Gaseous recycle ratios of between about lto 2 volumes of recycle gas per volume of fresh feed may be employed. Space velocities of between Sand 15 s'.c.f.- CO/hrJ-lb. of catalyst are generally satisfactory. While the operating conditions set forth above are those -which are usually consideredv preferable, it is to be strictly nnderstood that-any; of the known conditionsused in'the.

.syntholprocess forproducing hydrocarbons-and oxygen- .ated;chemicals-by'reacting carbon monoxide and'hydro- .gen at 450-7 50 .F. and between 100 and 700 .p.s.i.g.

over an .ironrtype catalyst-may be used. Catalysts suit- .able .for .use in, the synthol process are any of those-iron- .ztype catalysts known .to be useful for producing'liquid hydrocarbons andoxygenated chemicals. More detailed ,.descriptions of thesynthoLprocessmay be'found in US. 1,681,924 to -M.--B.'Kratzer and Encyclopedia ofChem- .ical Technology by Kirk-Othmer, volume 6, :pages 972-- -.about to 20% of the totalproduct will consist of oxy- ..genated compounds. The synthol acids employed in preparing hi-ghl-y useful diesters are oil-soluble acids re- Any ofanumber .of methodsimay be practicedinrecoveriug the oil-soluble earboxylic acids frointhe hydrocarbon phase. .Thealcohols, aldehydes,;and ketones may be separated-from .thehydrocarbon phase by extracting with an aqueous .solutionofa water-soluble'bisulfite, followed by extract- .ing the hydrocarbonraffinate with an aqueous solutionof a mild alkali such as sodium carbonate (note US. 2,457,257) to separate the carboxylic acid salts and then springing and recovering the acids from the salt solution. Other rnethodsof recovering the oil-soluble carboxylic acids from the hydrocarbon phase are detailed in US. 2,670,366 and US. 2,645,655.

The mixture of oil-soluble carboxylic acids which is recovered from the hydrocarbon phase consists primarily of acyclic monocarboxylic .acids. The mole percent unsaturation of the carboxylic acids may range between Qand 35%. Themixture contains both straight and branched chain acids. It may contain between 10 and 60% of straight chain -acids and between :90 and 40% of branched chain acids, the percentage of branched chain acidsincreasing-with the number of carbon atoms in the acidmolecule. Analysis of the types and structures of the various .carboxylic acids produced during the synthol process, -as well as a discussion thereof, is presented in Land Chem, volume 45, pages 359-362 (February 1953). V

, .The.-g1ycol :used in theesterification can be ethylene glycol, propylene glycol, diethyleneglycol, or triethylene glycol. .Thediestergproducedfrom these glycols can be gen in amolar ratio ofhydrogen to carbonmonoxid'e of used as -.-synthetic lubricants in aircraft or automobile engines. Other .typesofrpolyhydric alcohols, .such as dipropy lene .glycol, trimethylolethane, trimethylolpropane,,.peritaerythritol-and the like,.can be used; however, theestersproduced are less satisfactory as enginelubri- "daiifs. Theselatteresters are used in special applications w ere bothja' liigh'viscosity index anda low pourr'poin't "Ase ple of" my invention, the esterific'atio'n of thylehe ycol with a mixture of synthoba'eidsw-ill be "described. A mixt-ure at hydrogenated symbol acids {inane rangeef 'l so' -fto 500 C., which contained Kip-1C andiabout. 20% T C iC .monob'asic A acids was .fractionated to produce two portions. One contained the acids boiling below C. at 1 mm. (about 250 C. at 760 mm.) and the other contained the acids boiling above 90 C. at 1 mm. (about 250 C. at 760 mm.). Each portion contained about 50% by volume of the acids. At the 50% cut.point the C acids up to about C were taken overhead and the bottom portion contained the -IC acids up through aboutC to C The molecular weight of these higher acids is not known with precision since they cannot be segregated due to their high boiling points. This fractionation .ste'p does not need to be performed with precision since only a rough separation of .the acids is necessary. For the purposes of rnyinvention, the separation can be made in the range (if '5 to depending upon the character of the mixed acids being fractionated and the quality of the ester product desired. If high viscosity is an important factor,;it'will be desirabletto separate the acids into a largerhigh iboiling portion and a smaller low boiling portion, as for example, by preparing a 5 to 30%..portion of the low boiling acids. Thus,in the first step .of my esterification method where the higher boiling acids (-i.e., the highermolecular weight) .are reacted in the presence of an excess of hydroxyl groups, all of these higher boiling :acids will be esterified. Thus, whenthe lowerboiling :portion is added and the esterification is completed with amount of glycol) was used "as the catalyst and xylene was used as an entraining agent. The entiretmixture was refluxed while the 'waterof reaction was .removed by a trap in the overhead reflux'condenser. The reaction was allowed to proceed at reflux temperature "until no more water was taken overhead,v thus indicating that all of the high boiling acids had-been completely esterified Since the .acid to glycol molar ratio was about 1:1, there-Was an excess 'of unreacted hydroxyl groups in thereaction mix. Thesehydroxyl groups were contained in'the form of unreacted glycols'and monoeste'rs. Next, the low boiling portion ofzthesynthol acids previously separatedfwere added to the reaction mix bringingthewtotalamount of "aeid'added (both high boiling and thelow 'boilingportions) to :an acid to glycol molar :ratioof .-about.2.5 :1. The esterification was resumed andonceagaimthe water of reaction was withdrawn until no; more was produced. Since avoiding the formation of themonoester is essential to obtaining a diester with good .viscosityindexjand pour point making it suitable for syntheticalubricants, it is essential that there be an excess of .acidin'the final 'esterification step. This insures complete-.esterification of the glycol. The :product was removed fromithe reaction vessel: and distilledunder vacuum to remove the bulk of the unreacted'low boiling. syntholiacids andthe -en'- training agent. The acids are :believed .to :be in't'he C -C range. A'diester'having-an acid :number of about 50 to (mg. of KOH/gm.) was obtained. iThiSfi'aC tionated-diester was dissolved in n-pentainean'd. filtered through Attapulgus clay and'silica--gel,' and:the-solutions were then distilled to recover the ;;purified diesters.

:Evaluation'of the diestcrs pro'ducedzfromathe reaction-of synthol acids with triethylene. glycol :iniaccordance with my invention as described above, along with'estersprepared. by conventional means :and: the Government specification *for synthetic lubricants are 'showncin Table I. Run -I- shows the properties of asampl'e ;prepare'd 'in.accordanee with my. invention, that. is a two-step esterification. For, comparison,- 11m 2 shows theapropertiesf-ofa diester produced by esterifying in one step ,:the'::t6.tal

'syntholaacid mixture withatriethylene glycol followed by tires-same purificatiomm'ethod.as..used.in run 1. The

purified diester from run 2 was further purified in an attempt to remove the residual acids. About 8% of the low boiling portion of the purified diester was removed by distillation under vacuum and the remainder of the diester was treated with Attapulgus clay and silica gel in the same manner as runs 1 and 2. The properties of this product are shown in run '3. The last column of Table I gives Government specifications.

TABLE I Triethylene glycol esters of Cq+ synthol acids Run No 1 2 3 MILL-6082 A and MIL- L-7808 B 1 Wt. 01+ Acids Rescted, gm 2, 492 1, 005 Wt. Triethylene Glycol Reaeted,

gm 1, 275 450 Product Analysis:

Acid No. mg. KOH/gm 0. 06 4. 6 4. 8 0. 1 Sap. No. mg. KOH/gm 268 266 287 M01. Wt 417 10. 3 10. 4 11 2. 9 2. 9 8 ASTM Slope 0.76 o. 69 o. 71 VI 145 162 146 Pour Pt., F 70 70 50 75 Flash Pt., F., 000 450 440 460 850 1 11.8. Government military specifications.

An examination of the data reported in Table I shows that the diesters produced in accordance with my invention, as reported in run 1, pass all Government specifications including the acid number specification of 0.1. The acid number in run 1 is 0.06, while run 2 shows that the diester produced by conventional methods gives an acid number of 4.5. Despite the sacrifice in yield by removing the first 8% of the purified diester from run 2 in an attempt to reduce its acid number below 4.5, run 3 shows that the acid number increased to 4.8. These results indicate that the acids are distributed throughout the diester boiling range and after considerable efiort no method was found to reduce their concentration to the desired low level by further refining of the purified diester product.

While I have shown that the use of synthol acids in preparing diesters suitable as synthetic lubricants for aircraft and automotive engines, I want to emphasize that my invention can be practiced using other mixtures of monobasic carboxylic acids to produce satisfactory lubricants. An example of other acid mixtures which are satisfactory are those produced by the oxo process which is the reaction of carbon monoxide and olefins followed by oxidation. Though I have stressed that the diesters having a low acid number are particularly suited for use as an engine lubricant where high viscosity index, low

pour point and low acid number are required, it should be understood that my method can be used in preparing diesters for any use where these requirements must be met. Neither do I want to be bound by the use of only triethylene glycol as the polyhydric alcohol. As I have pointed out previously, other polyhydric alcohols such as ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, triols, etc., can be used where producing a suitable diester requires a mixture of monobasic carboxylic acids having a low acid number. Examples of other uses of my invention are in the production of plasticizers for polyvinyl resins, such as polyvinyl chloride, copolymers of vinyl chloride with vinyl acetate, copolymers of vinyl chloride with vinylidine chloride, and the like.

Thus having described my invention, what is claimed is:

l. The method of producing synthetic polyesters having low free acid contents from a polyhydric alcohol and a mixture of acyclic monocarboxylic acids containing both straight and branched chain acids and having more than 6 carbon atoms per molecule, which method com prises (1) distilling the mixture of acids into a high boiling portion and a low boiling portion, (2) partially esterifying the polyhydric alcohol with an amount of said high boiling portion of the acids insuiiicient to eifect complete esterification of the polyhydric alcohol, (3) adding the lower boiling portion of acids to the partially esterified polyhydric alcohol in an amount more than suificient to effect complete esterification, and completing the esterification of the partially esterified polyhydric alcohol, (4) removing unreacted low boiling acids from the esterification product by distillation, (5) and recovering a purified low free acid content synthetic polyester.

2. The method of claim 1 wherein the polyhydric alcohol is a polyglycol.

3. The method of claim 1 wherein the polyhydric alcohol is triethylene glycol.

4. The method of claim 1 wherein the mixture of monobasic carboxylic acids is a mixture of synthol process acids.

References Cited in the file of this patent UNITED STATES PATENT-S 2,369,036 Fitzpatrick et al. Feb. 6, 1945 2,441,555 Barth et a1. May 18, 1948 2,666,706 Valko Jan. 19, 1954 2,681,924 Kratzer June 22, 1954 2,742,371 Albus et a1. Apr. 17, 1956

Claims (1)

1. THE METHOD OF PRODUCING SYNTHETIC POLYESTERS HAVING LOW FREE ACID CONTENTS FROM A POLYHYDRIC ALCOHOL AND A MIXTURE OF ACYCLIC MONOCARBOXYLIC ACIDS CONTAINING BOTH STRAIGHT AND BRANCHED CHAIN ACIDS AND HAVING MORE THAN 6 CARBON ATOMS PER MOLECULE, WHICH METHOD COMPRISES (U) DISTILLING THE MIXTURE OF ACIDS INTO A HIGH BOILING PORTION AND A LOW BOILING PORTION, (2) PARTIALLY ESTERIFYING THE POLYHYDRIC ALCOHOL WITH AN AMOUNT OF SAID HIGH BOILING PORTION OF THE ACIDS INSUFFICIENT TO EFFECT COMPLETE ESTERIFICATION OF THE POLYHYDRIC ALCOHOL, (3) ADDING THE LOWER BOILING PORTION OF ACIDS TO THE PARTIALLY ESTERIFIED POLYHYDRIC ALCOHOL IN AN AMOUNT MORE THAN SUFFICIENT TO EFFECT COMPLETE ESTERIFICATION, AND COMPLETING THE ESTERIFICATION OF THE PARTIALLY ESTERIFIED POLYHYDRIC ALCOLHOL, (4) REMOVING UNREACTED LOW BOILING ACIDS FROM THE ESTERIFICATION PRODUCT BY DISTILLATION, (5) AND RECOVERING A PURIFIED LOW FREE ACID CONTENT SYNTHETIC POLYESTER.