CARBON-FUNCTIONAL ORGANOSILICON FLUOROESTERS A S SYNTHETIC LUBRICANTS PA U L
M
.
K ERSCHN E R
,
Cities Serul'ce Research and Dei'dopment Co., Crunbury, 'V. J .
An effort to combine the best lubricant characteristics of the esters, the silicones, and the fluorinated hydrocarbons into a single molecular species resulted in the synthesis of a class of compounds known as carbonfunctional organosilicon fluoroesters. The purified products possess good viscosity-temperature characteristics over wide temperature ranges, low pour points, and good lubricity at moderate and extreme pressures.
Oxidation and corrosion stability has been demonstrated at
350' F. and the resistance to coke
formation at 590" F. would seem to indicate oxidation stability at temperatures in excess of 350" the experimental results confirmed the basic assumption motivating this research.
s
lubricants bear a relationship to mineral oils which is somewhat analogous to the relationship of alloy steels to ordinary steel. T h e synthetic oil, like a n alloy steel, is required to give superior performance under operating conditions far beyond the capacity of the more common product. I t is not unusual then that the synthetic products command a premium price. T h e major market for synthetic oils today is found in modern jet aircraft, both commercial and military, and \\-bile the demand for quantity is constantly increasing, the severity of operating conditions is also on the rise. Table I illustrates ho\v the military specifications have changed over the past 13 years. Military specification 7808, first issued in 1951, had a pour point requirement of -75" F., a maximum allo\vable evaporarion loss of 50%; at 400' F., and a minimum flash point of 385' F. T h e more recent specification, l\Iil-L-3236B, designed for aircraft turbine engine operation a t 400' F. reduccs the maximum allo\vable evaporation loss a t 400' F. to 15yc,increases the minimum flash point to 425' F.? and maintains the requirement of -75" F. as the maximum pour point. Future technological developments in supersonic aircraft and space exploration are certain to increase the severity of operating conditions on all materials of construction as \vel1 as lubricants. Since the tendency is to demand lubricants that perform satisfactorily at operaring temperatures of the order of 400' to 500' F., the ivork at Cities Service Research and Development Laboratory \vas directed toivard this target. .\ literature search revealed that six major classes of compounds dominated the synthetic lubricants field : silicon derivatives (silicones and silicates], fluorocarbons, esters, polyglb-cols, organophosphates, and polyphenyl ethers. O u r primary interest \\.as centered on the organosilicons, fluorocarbons, and esters. Table I1 compares viscosity-temperature characteristics, oxidation resistance, and lubricity. No single class is superior on all three counts, but each class has one or more deficiencies. Silicones have poor lubricity, fluorocarbons have very poor viscositytemperature characreristics, and the esters sho\c only fair to good oxidation resistance and lubricity. On the positive side, the esters sho\v good to cxcellent viscosity-temperature characteristics, silicones have cxcellent viscosity-temperature characteristics and excellent oxidation stability, and the fluorinated 11) drocarbons have excellent oxidation resistance and fair to excellent lubricity. O n the basis of this and other information, it \\-as decided to combine these three classes into a single molecular entity in a n YNTHETIC
"
Prcscnt address, Columbian Carbon C o . , Inc., Princeton,
F.
Thus,
effort to obtain a superior synthetic oil? assuming that each group \could maintain most of its good Characteristics and, in so doing, overcome the inherent lveakness of one another. Experimental
'The experimental program entailed the preparation and evaluation of a class of compounds \vhich are kno\vn as carbonfunctional organosilicon fluoroesters. or: more generally, haloesters. T h e basic structure of the compounds prepared is (1-4): ROOC(CH2) ?--Si(CH,) ?fOSi(CHa) .~-f,,~ (CII,) &OOR
R
=
HC:F2 (CF2 ) nCH ?VL
or Cl CF '1(CF ?CFCI),LCH ?-
and n = 1 to 10
T h e esters are prepared by reaction of a carbon-functional organosilicon dibasic acid \vith a fluorinated or chlorofluorinated alcohol. T h e carbon-functional organosilicon acids \yere prepared by a procedure developed by Sornrner and coxvorkers ( 6 ) . 'lhe reactions involved are : (CHa)aSiOSi(CHa)?CH,Cl f SaCHjCOOE'I')?
Table
I.
Military Specifications
Physical P ~ o p tips r~ Flash point, '1:. ( i n i r i i t i i u i n ) Pour point, "I:. ( i i i a x i i n u n i ) Osidation 'corrosion stabilitytest, 'I?., Iioiir
for
Synthetic
7808 (795l)
Lubricants
!12,3/jB ( 7 9 6 d )
42 5 -75
385
7 -
-13
347 i 72 E\-aporation tcst, 'F./hour 400,"6. 5 YGMasiiiiuni loss 50 .\Iiiitary spciJkatiorl, 1iihricatinE oil. &craft Table II.
-+
4OO/6,5 15 h 1 1 6 i 1 ~ ee r 1 ~ 1 1 i e ,z/OOO
F.
Characteristics of Typical Classes of Synthetic lubricants
Class
Organic esters Silicones Fluorocarbolis
l 'iscosily Tenijeratiire Good
Excellent Very poor
Otidationl Co)?osl'on Kesisimce Good
Excellent Excellent
Iabricity Fair Poor
Excellent
K.J.
VOL.
4
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SEPTEMBER
1965
197
Briefly, the reaction involves a malonic ester synthesis with chloromethylpentamethyldisiloxane followed by decarboxylation to form a lactone 11hich on the addition of water gives the desired product. More dimethylsiloxane units !\.ere introduced into the center of the molecule by treating a n ester of the carbon-functional organosilicon dibasic acid with octamethylcyclotetrasiloxane in the presence of 83y0sulfuric acid as sho\vn (7).
R O O C (CH,) ?Si(CHS)2 +O Si(CH3) &.n (CH?)zCOOR n=ltoG The fluorinated alcohols studied were of the type produced by E. I. du Pont de Nemours & Co., to Ivhich we are indebted for our samples. The chlorofluoroalcohols \\ere prepared by the lithium aluminum hydride reduction of Kel F acid esters obtained from the hfinnesota hlining and Manufacturing Co. Three things were visualized : First, the inherently very poor viscosity-temperature characteristics of the fluorocarbons could be overcome by the siloxane center of the acid. Second, the fluorocarbons could give added lubricity to the siloxane portion of the molecule Jvhile improving the hydrolytic stability of the ester grouping. Third, the siloxane and fluoro groups should give excellent thermal and oxidation stability to the total molecule. The assumption of improved hydrolytic, thermal, and oxidation stability of esters by the addition of fluoro groups was given substance by iyork reported by hlurphy et al. (Table 111) ( 7 , 5 ) . They point out that the esters of aliphatic dibasic acids and fluorinated alcohols have vastly improved thermal and hydrolytic stability, but the pour points and the viscosity-temperature characteristics leave much to be desired. The results obtained thus far tend to prove our basic assumptions. \Ye first prepared the desired ester, di-l,1,3-trihydroperfluoro-4,4,6,6-tetramethyl-4,6-disila-5-oxanonanedioate, by reaction of the appropriate acid and alcohol in a conventional manner. The resulting ester \\.as then made to react with octamethylcyclotetrasiloxane and 8370 sulfuric acid to increase the number of dimethylsiloxane units in the center of the molecule. From this reaction, we obtained a series of ester products in which n had a value from 1 to 5 f . The chemical and physical analyses of these esters are given in Table IV. The individual cuts were analyzed and a plot
Table 111.
Dynamic Oxidation and Hydrolytic Stability of Fluoroesters Bis( $ '-hefityl) Bis( $
Diphenate
Oxidation data Test temperature, "F. Test period, hours Viscosity increase at 100' F. Neutralization number increase, nonvo!. Metal wt. changes, mg./sq. cm.
cu
Steel Ag
527 72 15 0.17 -0.40 +O.l +O.l
Difihenate
*
527 72 15 0.23 -0.38 $0.1 +O.l
Hydrolytic data Tes- temperature, O F . 200 200 Metal wt. change, mg. Culsq. cm. -0.1 -0.2 Neutralization number oil Nil Nil Neutralization number H20 0.02 0.02 Viscosity change: 210" F., 7 0 -0.1 +0.4 0.1 0.1 Insolubles, wt. yG * +'-heptyl- 1,7,7-trihydroa $'-amyl - I , 7,5-trihydroperJIuoroamyl. fierJIuoroheply1.
198
was made of the viscosity index us. the silicon-fluorine ratio (Figure 1). There is a progressive increase in the viscosity index with an increase in the ratio of silicon to fluorine and the poor viscosity-temperature characteristics of the fluorocarbons are overcome by the siloxane center of the ester molecule. Viscosity, temperature, and pour point data of simple aliphatic acid fluoroesters are compared with similar data for carbon-functional organosilicon acid fluoroesters in Table V. In rating esters of similar chain length, the carbon-functional organosilicons have superior viscosity indices, while maintaining pour points of less than -65" F. The next factor to be considered is lubricity. Table VI compares the medium wear properties of four synthetic oils, a polymeric methyl silicone, a simple aliphatic acid ester, a n aliphatic fluoroester, and a carbon-functional organosilicon fluoroester. The data were obtained on a Shell-4-ball wear machine. Tests were run a t 70" C. for 2 hours a t a rotor speed of 600 r.p.m. The four balls were made of Type 52-100 steel. A comparison of wear scar size is a measure of the effectiveness of lubricity. Our esters were superior to both the commercial polymethyl siloxane and simple aliphatic esters, but not as good as the aliphatic fluoroesters. There was no apparent relationship between lubricity and silicon-fluorine ratio, as indicated previously for viscositytemperature characteristics. Under extreme pressure conditions, the carbon-functional organosilicon fluoroesters were superior to all the other types tested. The data shown in Table VI1 were obtained on a Shell-4-ball E. P. wear machine and load seizures are recorded for comparison. The rotor speed for these tests was 1800 r.p.m. a t an oil temperature of 70" C. The load seizure values for carbon-functional organosilicon fluoroesters are from 25 to 85 kg. above the values for the other three types of esters. Oxidation-corrosion data shown in Table VI11 indicate that these compounds are stable in the 350" F. temperature region. Fair to good results were also obtained for samples evaluated on the WADC deposition test rig a t a coking temperature of 590" F. and an oil-circulating temperature of 300" to 390" F.
l&EC PRODUCT RESEARCH A N D DEVELOPMENT
I I
0
0.15
Figure 1.
I
0.30
I I
0.45
I
I
I I
I
0.60
0.75
0.90
I
I I
1.05
1.20
Si/F Viscosity index as a function of Si/F ratio
Table IV.
Carbon Functional Organosilicon Fluoroesters
Dibasic acid, HOOC(CH?)PS~(CH~)?-[OS~(CH~)~~~~(CI-I~)?COOH, where n = 1 to 4 Alcohol, HCF2(CF2),CH20H, where ??I = 1 to 6 Sarnjle 2 2 1 60-70 13 1 ,3998 194 193 15 3 14.5 25.3 26.1 0.57:l 9.68 2,58 108 - 65
1
k i d - , n value Alcohol,
value
??I
Hoilinq ranqe, "C. Pressure: microns Hg K. I . at 28.5O
c.
Sap number 'Theoretical c ; Si Theoretical c; F Theoretical Si:'F, \veiqht ratio \'is. at 100' F., cs. Vis, at 210' F., cs. 17. I. Pour Iioint, OF. Flash point, "F Fire point, O F . 1
,
1 1 55-60 13 1 ,3990 208 222 12.8 11.1 27.7 30.0 0.46:1 8.72 2.31 81 -75
... ...
..
3 3 1 7 0-8 0 13 1.4001 174 172 17 1 17.2 22.9 23.2 0.77:l 9.78 2.77 146 - 65
..
5 4+ 1 loo+ 13 1 ,4040 151
4
4 1 70-100 10-1 3 1.4014 152 154 19 7 19.2 20.4 20.8 0.90:l 10.38 3.04 169 - 65
1.1:l 11.57 3.34 180 - 65
...
...
...
...
20.9 ... 19.4
...
...
6 1 5 84 18
121 124 6 1 6.2 51.2 51.5 0.12:l 17.02 3.31 52 - 65 445 47 5
7 1 2 50-60 50-90
I
182 180 9.3 9.1 40.1 39.8 0.23:l 5.59 1.62
. ~ .
- 60
... ...
Effect of Chlorine Table V. Comparison of Sir,iple Fluorinated Alcohol Ester with Carbon Functional Organosilicon Acid Fluoroesters
ViSCO.?itj, cs.
Pour Point. Conipound~ Zb F. F. V . I. F. Bis (+'-aiii>-I)adipate 18 2.86 14.7 71 25 His ($'-hut,-l) sebncate 20 1 . 8 4 6 . 6 3 112 36 Ris ($'-octyl) adipate 24 2 . 7 3 1 6 . 0 6 99-102 Bis ($'-propyl) siloxane ester 19 2 . 5 8 9 . 6 8 108 - 65 Bis ($'-prop!-l) siloxane ester 23 3 . 0 4 1 0 . 3 8 169 -65 $'-amyl = 1 . 1 , 5 - t r i I i ~ i f r n / ~ ~ ~ ~ ~ i o r o a$n'l -) i6. ~ f ~=l 1: l-dihjdiojrij'uoroliiifyl. $ ' - 0 c t j l = I , l - d i h ~ d i o f i c ? f i i i o ~ o o c i ~$l '-profijl . = 1: 1,.3t,-ihyd,.olif?Piio,-oj,-o/~~l. Z' = niiniber of aionir in sfraighf chain.
~da-l~O"- H-S
Siloxane ester = ROOCC:I-I.ClH?Si(CI-I3)?fO(SiCI 1 3 ) ~ + ~ (CH?)?COOR R =
Table VI.
1 to 3
Wear Properties of Synthetic lubricants
Scar _ Iliarntte7~ _ _ _ .ifin, ~__ 20 kg. hlethyl silicone 2.70 ... Di-2-ethylhcsyl selxicate 0.53 0.72 Di-$'-propyl siloxane ester 0.45 0.62 Di-$ "-lieptyl 3-nietli)-l glutarate 0.42 0,52 Halo-siloxane ester 0.51 0.75 = FICF?CF?CH$''' = HCF2(CF.)sCi-I--eHalo = C:l( CF2CFC1)3CF2CH2 Siloxane acid = I-IOOC(C>I-I?)?Si( Cl-13)?f0 Si(Cl-13)+?, (CI-I?)zCOOH ~
Cornjounil
Table VII.
10 kg.
Extreme Pressure of Synthetic lubricants
Load Seizure, Corn,&o!inii
Kg .
Methyl siiiconc I>i-2-eth);lhesylsebacate Di-$'-propyl siloxane ester Di-# "-hcptyl 3-methyl glutarate
0 55 85 60 210
I ialo-siloxane ester $'-Propyl = I-ICF*CF*CIHz
$"-IJeptyl = I-ICFs(CF2)jCHI.= CI(CF2CFCl)aCFyCH2Siloxane acid = HOOC( CI-I?)?Si(CH3)2f0 Si(CHa)ef, (CH?)?COOH Halo
I t was decided to introduce a small amount of chlorine as well as fluorine into the molecule to improve lubricity and a t the same time determine whether desirable viscosity-temperature characteristics could be maintained. Tivo acids, Kel-F 683 and Kel-F 8114, \yere obtained from the hlinnesota Mining and Manufacturing Co. Their ethyl esters were prepared and finally the products \yere reduced Jvith lithium aluminum hydride to obtain the desired alcohols, The procedure folloived to obtain the desired carbon-functional organosilicon esters for evaluation was as previously described-namely, preparing the ester of the disiloxane dibasic acid and then expanding the siloxane center by reaction \vith octamethylcyclotetrasiloxane. T h e resulting esters were of moderate viscosity and density. T h e chemical and physical analyses of the esters produced are shown in Table IX. Even over a limited range of increasing silicon-halogen ratio there is a steady increase in the viscosity index, which signifies a n improvement in viscosity-temperature characteristics. Pour points remain rather lo\\., even though viscosities run as high as 85 centistokes a t 100' F. The high densities of these esters, as well as of the fluoroesters previously discussed, mean a higher absolute viscosity than is possible for simple aliphatic esters of equal kinematic viscosity. Lubricity characteristics of these esters under moderate wear conditions are not quite equal to the simple fluoroester or the carbon-functional organosilicon fluoroesters (Table VI). A \year scar of 0.75 mm. a t a load pressure of 20 kg. is superior to the straight polymethylsiloxane fluid, equal to the aliphatic esters, but some 0.1 to 0.2 mm. greater than the other fluoroesters. Holyever, under extreme wear conditions, the chlorofluoroesters are far superior to the other classes mentioned (Table V I I I ) . A seizure load of 210 kg. is more than twice the value for the carbon-functional organosilicon fluoroesters and points to the benefit of the presence of chlorine. Carbon-Functional Organosilicon Polyesters
In an effort to obtain lubricants of higher molecular weights and viscosities, a number of polyesters were prepared using the carbon-functional organosilicon acid \vith different combinations of fluorinated and unfluorinated aliphatic di- and monoalcohols. T h e mole ratio of diacid to dialcohol to monoalcohol \vas VOL.
4 NO. 3
SEPTEMBER
1965
199
-
Table VIII.
Oxidation Corrosion Stability of Carbon Functional Organosilicon Fluoroesters
Temperatstre, 347" F. Time, 72 hours Air rate, 5 f 0.1 liters/hour YO Vis. Change in Weight of Panels, Mg./Sq. Cm. at 730" Cu A1 Fe
4
0 06 0.06 0.03
0.09 0.07 0.09
, i Neut. Sample No. Siloxane A 5.0 3 0.04 Siloxane B 2.7 4 + O . 06 Commercial ester 2.5 3 0.05 Siloxane acid = HOOC( CHz)gSi(CH3)*+0 Si( CH3)2+J CH*)2COOH Siloxane A, n = 1 Siloxane B, n = 2 Esterifying alcohol = H C F ~ C F ~ C H Z O H
0.06 0.04 0.05
0.06 0.06 0.04
Table IX. Carbon-Functional Organosilicon Haloesters Dibasic acid, HOOC(CH2)2-Si(CH3)2-[O-Si(CH~)2+n(CH2)~-COOH, where n = 1 to 5 Alcohol, C1(C F ~ C F C ~ ) S C F ~ C H ~ O H Sample ~-
Boiling range, "C. Pressure, microns Hg Acid number 7 0 Silicon yo Fluorine Viscosity, cs. at 100' F. Viscosity, cs. at 210' F. v. I . Pour point, OF.
1 80-128 16-1 8 3.7 5.09 37.4 73.40 7.24 47 - 60
Table X.
Sample 1 2 3
Acid la la la
2 128-138 13-14 0.49 6.05 36.2 85.01 8.34 66 - 55
3 138-141 13-1 4 0.36 6.54 35.3 84.09 9.20 92 - 60
4
5
141-160 10-14 0.96 7.14 33.4 75.69 8.95 100 - 60
160+ 10-14
...
9.2 31.4 73.58 9.73 118 50
-
Properties of Carbon-Functional Organosilicon Fluoropolyerter
Dialcohol HO( CH2)dOH HOCHz( CF2)3CH*OH HOCHz( CF*)aCH20H
Monoalcohol HCF2CF2CH20H CH3(CH2)40H HCF2( CF*),CH2OH l a = [HOOC(CHp)2 Si(CH3)&0
Viscosity, Cs. 7 0 0 ' F T21OOF. 22.80 4.65 26.41 5.90 44.96 7.95 ~
v. I. 137 163 141
Pour Point, F. - 45 - 65 70
-
Wear Properties Scar Diameter, M m . 5 kg. 70 kg. Sample 1 Load seizure 0.79 0.90 120 kg.
2 : 1 : 2 in all cases. These ester oils were mixtures and no attempt was made to separate a n individual component. Rather, excess reactants were removed by distilling a t reduced pressures and the desired residue oils were further purified by washing with caustic and clay treatment. Viscosity, pour point, and some wear data for the polyesters in Table X show that esters of reasonably high viscosity can be prepared while high viscosity indices and low pour points are maintained. T h e most stable esters were those prepared from fluorinated mono- and dialcohols. Wear data for sample 1 indicate that the polyesters offer less protection a t medium wear conditions. However, under extreme pressure wear, a load seizure of 120 kg. was obtained as compared to 85 kg. for a diester and 50 kg. for a polymethylsiloxane oil. Conclusions
Carbon-functional organosilicon haloesters possess good viscosity-temperature characteristics over wide temperature ranges and exhibit low pour points and good lubricity under moderate and extreme pressure conditions. Good oxidation and corrosion stability has been demonstrated a t 350' F. and the resistance to coking a t 590' F. on the WADC deposition 200
I&EC PRODUCT RESEARCH A N D DEVELOPMENT
test rig would seem to indicate oxidation stability a t temperatures in excess of 350' F. Acknowledgment
The author owes much to B. W. Greenwald, E. McAdams, W. Burke, and many other persons of the Cities Service Research and Development Co. literature Cited
(1) Faurote, P. D., Henderson, C. M., Murphy, C. M., O'Rear, J. G., Ravner, H., Ind. Eng. Chem. 48, 445 (1956). (2) Kerschner, P. M., Greenwald, B. W. (to Cities Service Research and Development Co.), U. S. Patent 2,966,508 (Dec. 27, 1960). (3) Ibid., 3,014,056 (Dec. 19, 1961). (4) Ibid., 3,074,988 (Jan. 22, 1963). (5) Murphy, C. M., O'Rear, J. G., Ravner, H., Sniegoski, P. J., Timmons, C. O., J . Chem. Eng. Data 4, 344 (1959). (6) Sornmer, L. H., Masterson, J. M., Steward, 0. IV., Leitheiser, R. H., J . Am. Chem. Soc. 78, 2010 (1956). (7) Sommer, L. H., Pioch, R. P., Ibid., 75, 6337 (1953).
RECEIVED for review December 29, 1964 ACCEPTEDJuly 19, 1965
Division of Petroleum Chemistry, 148th Meeting, ACS, Chicago, Ill., September 1964.
