Ind. Eng. Chem. Prod. Res. Dev. 1982, 27, 515-519
Performance at much higher temperatures varies. The additives described above only partially succeeded in stopping cage wear without generating deposits. It should be pointed out that other approaches can eliminate the high-temperature cage or race wear of aromatic fluids. These techniques include changing the bearing geometry, the wear surfaces, or the oil flow. For example, early bearing tests run by SKF Industries above 260 "C gave cage bore wear. After the cage lands were increased by a factor of ca. 1.6, no further cage wear problems occurred (Wackendorfer and Sibley, 1965). (Hydrodynamic short bearing theory predicts increased load capacity with the cube of the bearing width.) Also, these bearings used black oxide race coatings to provide an easily sheared surface layer on a damage-resistant undercoat. With these design changes, a formulated C-ether gave acceptable performance in a 100-h inerted run on a 25-mm bearing with the race at 316 "C (Peacock and Rhoads, 1969). Finally, cage wear in an aircraft engine using a polyphenyl ether lubricant was eliminated by increasing the oil flow (Shevchenko, 1966). Thus, on specialized or modified bearings, lubrication at 260+ "C is possible.
cation aspects of this work, and the high-frequency rig tests were run at his laboratory. The thin-film oxidation tests were run at the laboratory of Professor Elmer Klaus of The Pennsylvania State University, University Park, PA, L i t e r a t u r e Cited Askwith, T. C.; Cameron, A.; Crouch, R. F. Proc. R . SOC.London, Ser. A 1968, 291, 500-19. Clark, F. S.;Green, R. L.; Miller, D. R. Final Report NASA '3-134643 for NASA Contract NAS 3-15333, Monsanto Research Corp. Report MRC-SL460, Dec 1974. Clark, F. S.;Miller. D. R. Final Report NASA CR-159794 for NASA Contract NAS 3-19746, Monsanto Research Corp. Report MRC-SL-1007, Dec 1980. Crltkovlc, E.; Klaus, E. E.; Lockwood, F. ASLE Trans. 1979, 22, 395-401. Grew, W.; Cameron, A. Nature (London) 1987, 274, 429-30. McHugh, K. L.; Stark, L. R. ASLE Trans. 1988, 9 , 13-23. Mills, T. N.; Cameron, A. ASLE Trans. 1882, 25, 117-124. Mlner, J. R. "Advanced Lubricating Fluids for Turbojet Engines"; 1970 Air Force Materials Symposium, Session on Fluids and Lubricants, Miami Beach. FL. Mav 19. ., 1970. . . Peackk,'L.A.; Rhoads, W. L. Final Report NASA CR-72615 for NASA Contract NAS 3-1 I 171, SKF Report AL69T069, Sept 1969. Shevchenko, R. P. Paper No. 660072 at the Automatic Engineers Congress, Detroit. MI. Jan 1966. Wackendorfer; C. J.; Sibley, L. B. Final Report NASA CR-74097 for NASA Contract W-492, SKF Report AL65T068, Aug 1965.
Received for review January 18, 1982 Accepted March 11, 1982
Acknowledgment
Mr. Lewis B. Sibley of SKF Industries, King of Prussia, PA, supervised detailed failure analyses of used custom bearings. We consulted with Professor Alastair Cameron of Imperial College, London, England, on various lubri-
515
NASA Lewis Research Center, Cleveland, OH provided financial support for the custom bearing testa. Messrs. Robert L. Johnson and William R. Loomis were the Project Managers.
Perfluoropolyether Fluids for Vacuum Technologies 0. Caporlcclo,' C. Corti, S. Soldlnl, and G. Carnlselll Montefluos, SPA. Montedison, Research Development Center, Milano, Ita@
Perfluoropolyether liquids, from hexafluoropropene photooxidation, are characterized by such a hgh level of physical, chemical, surface, and thermodynamic properties that they are rendered suitable as vacuum working fluids in hostile environments. The specific properties of a new series of fluids are overlooked, particularly the molecular weight distribution, the relationships of vapor pressure, temperature, and viscosity, with temperature, chemical, and thermooxidatiie resistance. The performances of specific perfluoropolyethergrades used as vacuum working fluids in vacuum pumps are reported, as well as the most significant case histories experienced in several fields of application.
Introduction
Perfluoropolyether fluids, obtained by photooxidation of hexafluoropropene (Sianesi et al., 1968, 1973) have a linear chain represented by the formula CF,O-(C,F,O),(
CF,O),-CF3
(1)
The molecular weight can be controlled by acting on photosynthesis conditions; its average value is generally of the order of a few thousands. The oxyhexafluoropropene and oxydifluoromethylene units are randomly distributed along the chain. The presence of strong chemical bonds of covalent nature ((2-0, C-F, C-C), the absence of hydrogen in the structure, and the neutral character of components impart excellent chemical and thermal stability characteristics associated with other useful physical properties peculiar to the polyether and perfluorinated chain backbone. The typical properties of polyethers I are listed in Table I. The raw perfluoropolyethers have a large molecular weight distribution, but several restricted molecular weight 0196-432 118211221-0515$01.25/0
Table I. Properties of Perfluoropolyether Fluids good viscostatic lubricity low internal cohesion energy excellent dielectric properties low pour point temperatures high thermal and thermooxidative stability inertness toward chemical reagents and solvents high compatibility with metals in the presence of air and at high temperatures high compatibility with resins, plastomers and rubbers high resistance to radiations, accelerated particles (electrons, neutrons, protons) Table 11. General Properties of Fomblin Y-VAC Fluids narrow molecular weight distribution of the components controlled viscosity (approximately * 5%) extremely low and controlled vapor pressure high purity and neutrality
fractions with controlled properties have been obtained by sophisticated fractionating operations and successfully 0 1982 American Chemical Society
516
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 3, 1982
Table 111. Fomblin Y-VAC Fluid Phisical Properties type Y-VAC series
kin visc, cSt (20 "C), av range
0616
60-66
1818
180-200
2515 2519 40111
250-270 270-290 400-500
140113
1400-1600
kin visc, cSt (100 "C)
vap press., torr (max value) b
mol wt (vpo)"
20 "C
pour
100 "C point, "C
4
1950
2x
lo',
5 x 10.'
-50
8.5
2700
2
10.'
2
-42
11 11 19
2850 2950 3700
2X 2 X 10.' 2 X lo-"
2 X lo-' 2 X lo-' 5 X 10.'
-37 -35 -32
35-40
6500
5x
5X
-23
X
X
vac pump appl fields turbomolecular, booster; lubrication under vacuum (UF,) turbomolecular, mechanical; ultraclean vacuum; diffusion; lubrication under vacuum (UF,) mechanical rotary pumps diffusion pumps lubrication under high vacuum (UFd lubrication under high vacuum (UF6)
a Vapor pressure osmometer method, solvent FC-75: number average value. metric analyzer DuPont 951, vacuum torr.
Table IV. Viscostatic Properties of Fomblin Y-VAC Fluids
fraction Y VAC 0616 1818 2515 2519 40111 14011 3
Table V. Evaporation Latent Heat of Fomblin Y-VAC Fluids
viscosity index, ASTM ASTM slope 2270164 0.80 0.71 0.70 0.65 0.64 0.58
Knudsen effusion method, thermogravi-
70 80 110 110 140 140
developed as working fluids for vacuum pumps and vacuum technologies. Table I1 summarizes the qualifying properties of perfluoropolyether vacuum fluids (Fomblin Y-VAC series). Properties of Perfluoropolyether Fractions for Vacuum Applications The main properties of the perfluoropolyether vacuum fluids are given in Table 111. Figure 1 shows the size of the average value and distribution of the molecular weight of the grade Y-VAC 06/6,18/8,25/9,40/11 compared to the molecular weight distribution of the crude perfluoropolyether mixture from which the aforementioned Y-VAC grades have been obtained. The molecular weight distribution has been determined using molecular weight measurements by vapor pressure osmometry on cuts obtained from Y-VAC series after an overcontrolled rectifymg distillation or a fractionated precipitation from solution in Algofrene 113 with dichloromethane;the data have been elaborated according to the Schultz method. The viscosity relationship with molecular weight and temperature are shown in Figures 2 and 3. The correlation between the kinematic viscosity (cSt, 20 "C)and the molecular weight determined by vapor pressure osmometry is reported in Figure 2. The thermal viscosity coefficient values can be deduced from Figure 3; the data correspond to good viscostatic properties (Table IV). The vapor pressures of VAC fractions have been determined by the Knudsen effusiometric method based on the weight loss of a sample contained in a cell provided with a calibrated bore and undergoing a thermal program under a controlled vacuum of torr; extrapolating the experimental curves, the vapor pressure values are obtained for the cuts at temperatures of 20 and 100 "C (Figure 4). The evaporation latent heat values of the Y-VAC fractions range from 22 to 30 kcal/mol, i.e., from 11to 4.5 cal/g (Table V). Table VI shows compatibility tests of perfluoropolyethers with several aggressive chemical agents
evaporation latent heat Y-VAC type 0616 1818 2515 2519 40111 140/13
kcal/mol 22 23 23 23 26 30
calk 11 9 8 7.5 7 4.5
Table VI. Compatibility Test on Fomblin Y-VAC 2515 and 1818 conditions reagent HF, HC1, HBr Cl,, Br,, BF,, BCI, UF6
PBr3, PC1, SiCl,, SiHC1,
so,
" Flow rate,
1 Llh.
temp, "C note 200 200 200 80 150 200 200
a a
a C
b a a
5% weight.
result unchanged unchanged unchanged unchanged unchanged unchanged unchanged 15% weight.
that can be currently carried through vacuum pumps. Perfluoropolyethers are generally unaffected by acid and basic inorganic solutions, by gas, or vapors of halogens, hydrogen halides, acid halides of boron, phosphorus, arsenium, antimonium, titanium, silicium and uranium. A very stable vacuum is maintained with permanent gases such as rare gases, hydrogen, carbon dioxide, oxygen, and ozone, also with moisture present. The insolubility of perfluoropolyethers with all organic liquids (except some chlorofluoroalcane such as 1,1,2-trichlorotrifluoroethane) allows organic vapors to be pumped without vacuum fluctuations.
General Vacuum Performances of Perfluoropolyethers Much experience has been acquired by Montedison, at the Research Center of Milan and at the Metrology Institute G. Colonnetti of Torino on perfluoropolyethers as vacuum fluids (Caporiccio and Steenrod, 1978; "Colonnetti", 1978) working on several vacuum pumps of the world's major firms. At the same time, a lot of work has been produced independently and published by several vacuum technicians (Conru and Laberge, 1975; Dennis et al., 1978; Henning and Lotz, 1977; Holland et al. 1972a,b; 1973; Laurenson, 1977; Laurenson and Caporiccio, 1977; Luches and Perrone 1976; Tsai et al., 1979).
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 3, 1982
FOMBLlN Y - V A C 11 Y-VAC 'I Y-VAC )I Y-VAC 'I Y- V A C
06/6 l8/8 25/9 40/11 140/13
517
DUDU
x x x x Q 0 0 0 I
*
0 000
a
I
-
a
-
2
-
m
a
103
I
1
1
1
1
,
'
102
I I
'
1
'103
K i n e m a t i c v i s c o s i t y (cs)
Figure 1. Average numerical molecular weight distribution of Fomblin Y-VAC and crude perfluoropolyether. 5700
6000
6500
7000
7500
sow
8500
sow
9500
FrrvPd
w2
1
FOMBLIN Y - V A C FOMBLIN S T A N D A R D Y/U - - w
Figure 2. Average numerical molecular weight (vpo) vs. kinematic viscosity (20 "C).
The selected physical properties of perfluoropolyethers, their high molecular weight, and high chemical stability allow very clean vacuum and high-vacuum stability to be obtained when hostile conditions, both physical and chemical, are involved. No phenomenon of solid deposit formation has ever been formed with perfluoropolyethers even in extreme temperature conditions. The choice of the proper grade of the working fluid for the pumps can be decided on the base of the Table I11 data. Mechanical Pumps For most rotary pumps, the fluid in general use is the VAC 2515 grade that allows total ultimate pressure in the
order of 3-5 X torr (0.4-0.7 Pa) to be obtained. When very low starting temperatures are involved with directly driven pumps, the fluid VAC 0616 is a choice. An exceptionally clean vacuum (range 1-2 X torr; 0.13-0.26 Pa) with two-stage directly driven rotary pumps is obtained with VAC 1818 grade. Another grade, VAC 16 (viscosity at 20 "C: 160 f 20 cSt; at 100 "C: 75 f 5 cSt; vapor pressure at 20 "C: 5 2 X torr, i.e. I2.6 X Pa; at 100 "C: I2 X torr, i.e., 52.6 Pa) could be chosen if a moderate total (ultimate) pressure is requested (range 4-10 X torr; 0.53-1.3 Pa) (Laurenson and Caporiccio, 1977). High-speed turbomolecular pumps ran for a long time
518
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 3, 1982 1 Fomblin y - V A C 06/6
2 Fomblin y - V A C l8/8
ioqom.
3 Fomblin y - V A C 25/9
e
u
4 Fomblin y - V A C 40/11
0 N
5 Fomblin y - V A C
c ,-WO. i .
140/13
u
v
%
f In
>
100
-
In
0
*
6
5c
x
10
~
50-
2.0. -100
1
0
1
100
300
200
1
400
Figure 3. Kinematic viscosity vs. temperature.
1
-a
,--
V
.n
v)
lo-*
a w
E v 3 ) v)
id
a
n
10-l
5 a P
4 >
lo-6
10
-*
10-
10-1q 0
I
20
I
'
"
50
'
I
100
'
"
1
"
"
150
1
*
"
'
200
TEMPERATURE
C'(
)
Figure 4. Vapor pressure vs. temperature. Knudsen effusion method (thermogravimetric analyzer, DuPont 951; vacuum
with Y-VAC 06/6 or 1818 forced lubrication: in particular, VAC 0616 has been tested up to 15 OOO h and several cases of operating periods longer than 25 000 h have been reported in the nuclear field without problems on bearings (Henning and Lotz, 1977). Roots pumps can run very satisfactory with Fomblin carrying aggressive gases (HF, UFe, ClFB).
torr).
Vapor Diffusion Pumps The chemical resistance, also in extreme temperature conditions (280 "C), makes Fomblii Y-VAC 18/8 and 25/9 suitable as working fluids for diffusion pumps. Partial pressures of all aggressive gases, except ammonia and their derivatives, can be carried through diffusion pumps running with Fomblin. In these cases, total Fomblin tech-
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 3, 1982 519
nology is recommended to the whole vacuum line covering backing mechanical pumps and sealing greases. Many examples of the long-life thermooxidation resistance of Fomblin VAC 18/8 can be reported it proved to support several hundred cycles of air filling from a vacuum chamber of 30 L within a running diffusion pump, recovering the ultimate pressure (2 X lo-’ torr; 2.7 X Pa) at the end of each 15-min cycle. Extensive tests with Fomblin have been reported on several diffusion pumps (Caporiccioand Steenrod,1978; “Colonnetti”,1978; Dennis et al., 1978; Holland et al. 1972a,b; Laurenson, 1977; Laurenson and Caporiccio, 1977). VAC 18/8 grade gives an ultimate pressure of about 1.5 X torr (2 X lo4 Pa, water-cooled baffle); the resulting pumping speed and vacuum stability depend on pump drawing and on dosing the input of heating power to the pump; a correct choice joins the pumping behavior near (85%) or equal to nominal values with the perfluoropolyether realiability in hostile environments. A clean vacuum is warranted by ultralow vapor backstreaming connected with a high molecular weight of Fomblin grade (with Y-VAC 25/9 grade, vapor backstreaming was unmeasurable during a 360 h test, according to the Pneurop method, and has been extrapolated to less than 2 X g/cm2 min). Examples of Vacuum Technologies The absence of oil backstreaming or of solid deposits after electron bombardment have enlarged the use of perfluoropolyethers, as pump working fluids, in vacuum systems of electron microscopes (Conru and Laberge, 1975; Holland et al., 1973; Luches and Perrone, 1976) and of mass spectrometers (Holland et al., 1972a, 1973). The resolution power of high energy electron microscopes and the absence of interference with mass spectra fragmentation of organic chemicals are assured by Fomblin vacuum fluids. The balance of Fomblin properties, such as the chemical resistance to all the acid halides of non metallic elements (Table VI), the low vapor backstreaming from pumping systems, the resistance to polymerize or to carbonize when submitted to accelerated charged particles recommend the use of Fomblin in vacuum systems of ion implanters (Tsai et al., 1979); it is well known that Fomblin has allowed significant improvement in transistor technology, due to the lack of contamination on silicon-doped wafers by ion implantation. The high inertness against oxygen (ignition temperature of 385 “C at 225 bar oxygen, as determined by Bundesanstalt fur Materialprufung) could lead to the introduction of perfluoropolyethers to the pumping assembly of plasma ashing or etching technologies (Tsai et al., 1979). The resistance to HF, Fz, and u F 6 allowed Fomblin VAC grades to enter nuclear technologies, to produce vacuum (Benetti et al., 1976), and to lubricate rotating devices under vacuum conditions, assuring ultralow levels of contamination by lubricant vapors (VAC 40/11 and 140/13 grades). Joint research by CNEN and Montedison pro-
duced the basic test performances of perfluoropolyethers against UF6 (Ciancia et al., 1980) in the nuclear field. The absence of Fomblin vapor backstreaming from diffusion pumps permitted the obtaining of a clean high vacuum in the beam line of a Van der Graaf accelerator. The fluorine contamination measures from pumping source have been effected by bombarding suitable Ta targets with 1370-keVprotons inducing y rays through the reaction ‘9 (p, a,y) and detecting y rays with a Ge (Li) solid state detector (sensibility 0.1 pg/cm2). The protons were accelerated in the 2-MeV accelerator at the Nuclear Laboratory of Legnaro, Padova (Rossi and Bezzon, 1980). Fomblin produced undecomposed by Cu K anticathode X-rays after exposure for 12 h to a total dose of 1 Mrad, nor by accelerated proton beams of 2 MeV, which is the energy known to carbonize hydrogenated fluids (Bassi, 1978). It is interesting to point out that, after exposure at a total dose of 50-Mrad 6oCoy rays (360 h), the weight loss of Fomblin 25/5 resulted as less than 0.1% with no viscosity decay.
Conclusions Properly fractionated perfluoropolyethers, obtained from hexafluoropropene photooxidation, have been proved to be suitable as vacuum working fluids or as lubricants in vacuum environments, assuring long performances in advanced technologies, where conventional fluids could not be used because of an intrinsic lack of stability or insufficient safety. The most significant and positive experiences reported on nuclear, CVD, and surface etching, ion implantation, chemistry fields, allow us to foresee that other critical technologies could be tried with these fluids. Literature Cited Bassi, I. Montedison, Donegani Inst., Research Center, Italy, private communication, 1978. Benettl, P.; Cubeddu, R.; Sacchi, C. A.; Svelto, 0.; Zaraga. F. Chem. fhys. Lett. 1978, 40, 240. Caporiccio, G.; Steenrod, R. A. J. Vac. Sci. Technol. 1978, 15, 775. Ciancia, A.; Caporiccio, G., et ai. Paper presented at 7th European Symposium on Fluorine Chemistry, Venezia. 1980. “Colonnetti, G.” Metrology Inst., Torino, Italy, unpublished report, 1978. Conru, H. W.; Laberge, P. C.; J. fhys. E.: Sci. Inst. 1975, 8, 136. Dahlstedt, L. F. Semicond. Int. 1979, 62. Dennis, N. T. M.; Colwell, B. H.; Laurenson, L.; Newton, J. R. H. Vacuum, 1978, 2 8 , 551. Henning, J.; Lotz, H. Vacuum, 1977, 27, 171. Holland, L.; Laurenson, L.; Baker, P. N. Vacuum 1972a, 2 7 , 315. Holland, L.; Laurenson, L.; Baker, P. N.; Davis, H. J. Nature (London) 1972b, 238, 36. Holland, L.; Laurenson, L.; Fulker. M. J. Jpn. J. Appl. fhys. 1973, 12, 1468. Laurenson, L.; ResJDev. 1977, 28(11),61. Laurenson, L.; Caporiccio, G. R o c . Int. Vac. Congr. 7th 1877, 1 , 263. Luches, A.; Perrone, N. R. J. Vac. Sci. Technoi. 1978, 13, 1097. Rossi Alvarez, C.; Bezzon, G. P. Istltuto Nazionale Fisica Nucleare, LNL, INFNITC-80I8 (1980). Sianesi, D.; Pasetti, A.; Corti, C. British Patent 1 104482, 1968. Sianesi, D.; Pasetti, A.; Fontanelli, R.; Bernardi, G. C.; Caporlccio, G. Chim. Ind. (Milan) 1973, 55, 208. Tsai, M. Y.; Streetman, B. G.; Blattner, R. J.; Evans, C. A. J. Electrochem. SOC.1979, 126, 98.
Received f o r review June 29, 1981 Revised manuscript received January 11, 1982 Accepted February 5, 1982
