Ind. Eng. Chem. Prod. Res. Dev.
1983,22,383-386
383
SYMPOSIA SECTION
I.
Symposium on Fluoropolymers K. J. L. Paciorek, Chairman 183rd National Meeting of the American Chemical Society Las Vegas, Nevada, March 1982 (Continued from June 1983 Issue)
Fluoropolymers in Fluid and Lubricant Applications Carl E. Snyder, Jr., and LOISJ. Gschwender' Air Force Wright Aeronautlcai Laboratories, Wright-Pafterson Air Force Base, Ohio 45433
The development of fluoropolymers was predicted to provide a new source of base materials for fluid and lubricant materials which were expected to find wide applications, particularly in the aerospace industry where the excellent stability at elevated temperatures would be of prime importance. Over 20 years have passed and a significant market for fluoropolymers in fluid and lubricant applications has yet to be realized. With the exception of polytetrafluoroethylene and fluorinated ethylene-propylene, which have found widespread usage as thickeners for greases, no fluoropolymers have found any significantapplication as fluids and lubricants to date. The various classes of candidate fluoropolymers(primarily liquids) are discussed, thelr advantages and disadvantages with regard to physical and chemical properties are discussed, and potential future applications are proposed. Fluid classes to be discussed shall include polyhexafluoropropylene oxides, linear perfluoropolyalkyl ethers, perfluoroalkyl ether substituted s -triazines, fluoro ethers, and oligomers of chlorotrifluoroethylene.
Introduction This paper focuses on fluoropolymers in Air Force aerospace applications as these applications are generally considered to precede commercial aircraft and non-aerospace state-of-the-art developments. This is attributed to the more severe Air Force aerospace environments imposed on fluids and lubricants to be used by the Air Force Aerospace application. Environmental extremes which may dictate the selection of a fluoropolymer include high temperatures, usually in oxidizing environments, low temperatures, and the combination of high and low temperature extremes in the same applications (e.g., outer space temperature extremes). Aerospace components also tend to be held to minimum size and weight for optimum aircraft fuel efficiency. In addition to component sizes being minimized, lubricant volumes are also minimized, which means that the lubricant has a high level of work input and is not capable of being cooled to the extent which would be best for optimum fluid life. In order to minimize hydraulic system weight, which is significantly influenced by the weight of the hydraulic tubing in the system, low viscosity fluids are desirable. In fact, low viscosity fluids generally provide optimum power transfer and flow characteristics. The viscosity of the fluid must not be too low at the maximum operating temperature to provide adequate film thickness needed to provide hydrodynamic lubrication to the hydraulic system components such as the pump, servovalves, and actuators. Inadequate hydrodynamic lubrication results in increased friction causing greater heat which is absorbed by the fluid, resulting in accelerated degradation of the fluid. There-
fore, fluid viscosity limits generally represent a compromise between many factors depending on the specific system. No matter which class of fluids is being considered for a hydraulic fluid application, minimizing the Viscosity change as a function of temperature (i-e., having a high viscosity index) is desirable. The greater density of fluoropolymers is naturally a deterrent to weight restrictions of aerospace applications, but other performance advantages, sound engineering, or the specific application itself have offset this limitation in some cases. An application such as a bearing grease is easily transitioned to a fluoropolymer since only a few microliters of grease are used in some miniature aircraft bearings. In an application such as a hydraulic fluid, it is obviously not so simple to minimize the weight penalty because of the larger fluid volume. However, fluoropolymers are seriously being considered for aerospace applications, such as hydraulic fluids, due to their excellent fire resistance. Commercial aircraft and industrial applications are expected to follow.
Discussion In the selection of a material, whether fluorinated or nonfluorinated, at least four factors are involved in material selection: cost, availability, performance, and safety. For the fluorinated materials, low cost and ready availability would certainly not be reasons for selection because they are specialty materials and as such are high cost and generally available in limited volumes. The performance and safety factors of fluoropolymers usually would be expected to dictate their use. Their improved performance
This article not subject to U S . Copyright. Published 1983 by the American Chemical Society
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 3, 1983
Table I. Classes of Fluoropolymers for Aerospace Fluid and Lubricant Applications class
application
poly te tra fluoroeth yle ne
grease thickeners solid lubricants polychlorotrifluoroethylene nonflammable hydraulic fluids gyro flotation fluids polybromotrifluoroethylene gyro flotation fluids polyhexafluoropropylene oxide gas turbine engine oils linear perfluoropolyalkyl ether greases perfluoroalkyl ether-S-triazine hydraulic fluids
is a result of their unique properties, primarily oxidative stability a t high temperatures and chemical inertness toward most system materials. For the safety aspect, related to performance, the primary advantages are in their nonflammability and their compatibility with liquid oxygen. The classes of fluoropolymers in aerospace use and/or development stages are shown in Table I. Polytetrafluoroethylene has been widely used as a grease thickener and to some extent as a solid film lubricant. Polychlorotrifluoroethylene polymers are being strongly considered as an aerospace nonflammable hydraulic fluid (Snyder and Gschwender, 1980; Snyder et al., 1982) and have been used for quite some time as gyro flotation fluids. Bromotrifluoroethylene polymers were developed as high-density gyro flotation fluids because they possess unique densities that permit them to float gyro wheels, and inertness that makes them compatible with delicate instrumentation immersed in the fluid. The perfluoroalkyl ether materials are considered prime candidates for advanced gas turbine engine oils, base fluids for greases, and high-temperature hydraulic fluids (Snyder et al., 1979, 1981). Whenever a new class of materials is being used for the first time in a given application, many unknown factors are discoverd, primarily for the following reasons: (1)The chemical effects of different classes of chemical fluids are generally not well known in the area of fluids and lubricants. This is because the test methods used in characterizing materials are usually inadequate because they have, for the most part, been based strictly on hydrocarbon chemistry. (2) The property requirements that are given for an application are in engineering terms rather than in chemical terms; for example, kinematic viscosity, bulk modulus, lubricity as measured by a given lubricity apparatus, and oxidation-corrosion stability as measured by a test that was developed for hydrocarbon fluids. (3) The availability of effective additives and solubility of appropriate additives in these new classes of fluids is limited or nonexistent. These factors result in a rather high risk for the design engineer selecting a class of materials for a specific application. Therefore, the trend is generally to design around material deficiencies. For example, bigger coolers or larger volumes of fluid can be used to compensate for oxidative and thermal stability deficiencies in a fluid. Various engineering methods are used to make materials of lower stability capable of providing adequate performance rather than run the high risk of designing the system around unique and new materials that may have higher stability but, because of other deficiencies that cannot be determined during the screening tests, could make the system nonfunctional. Therefore, we believe that fluoropolymers will not be used for liquid lubricant and fluid applications until designing around the deficiencies and instabilities of the nonfluorinated fluid and lubricant materials is no longer possible. The chlorotrifluoroethylene polymer structure is shown in Table I1 with some desirable properties. The use of
Table 11. Chlorotrifluoroethylene Polymers, Cl(C,F,Cl),Cl Hydraulic Fluids good stability for moderate temperatures (< 135 "C) wide liquid range (-54 to 135 "C) nonflammable low cost (-$lOO/gal) (in >lo00 000 gal/year utilization) available in large quantities (>1000 000 gal/year) good bulk modulus (190 000 psi at 3000 psi and 38 "C) Gyro Flotation Fluid high adjustable density (1.8-2.3 g/cm3) low vapor pressure high viscosity (adjustable to application) inert and compatible
chlorotrifluoroethylene polymers as gyro flotation fluids has been a reality for a number of years, but it represents only a small annual volume. In 1975, the Air Force began investigating commercially feasible materials as candidate nonflammable hydraulic fluids from which the chlorotrifluoroethylene oligomer (of a lower viscosity grade than the gyro flotation fluid) emerged as the primary candidate. The reasons for selection of the fluid and the program background have been thoroughly documented elsewhere (Snyder and Gschwender, 1980; Snyder et al., 1982). Successful development and acceptance of this fluid will mean an anticipated one million gallon a year market for the chlorotrifluoroethylene polymer by the early 1990's. In addition to the desirable properties of the appropriate oligomers selected for this application, they are relatively low cost compared to the cost of other fluorinated materials. They also have the potential for large continuous batch processing to meet anticipated volume requirements. The primary advantage of these materials over the currently used hydrocarbon materials is the safety aspect, i.e., their nonflammability over a wide range of fire hazards. They must be resistant to ignition by hot surfaces at temperatures to 900 "C. In addition, a mist of these materials must not be ignited by a torch or another ignition source. These criteria were established as the most severe anticipated fire threats for both current and future aircraft. Only highly halogenated materials met the nonflammability requirements of the program. The engineering properties of the fluid and economic feasibility of processing led to the selection of low molecular weight chlorotrifluoroethylene polymer as the primary candidate. It has been under extensive successful evaluation in both bench and aircraft component tests including hydraulic actuators, current state-of-the-art aerospace hydraulic pumps, and experimental high-pressure hydraulic pumps (55 800 kPa compared to current 20 900 kPa). As mentioned previously, changing chemical classes of fluids in an operating system often results in additional development work being required. The chlorotrifluoroethylene lubricity hydraulic fluid does not respond to conventional lubricity additives that have been found effective for hydrocarbon-based hydraulic fluids, and new ones had to be developed. In addition, the chlorotrifluoroethylene polymer does not impart to steel components the protection from rusting due to atmospheric humidity that is usually provided by hydrocarbon oils. Additive development is in progress to address this special problem that has been observed with other applications of fluoropolymer fluids. The information gained from this program will likely be useful in other fluid and lubricant fluoropolymer development efforts. Next to be discussed are a few of the fluoroalkyl ether fluids and derivatives of fluoroalkyl ethers. Because the thermal stability of hydrocarbon-typematerials is adequate for most applications, we are concerned with the severe
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Table 111. Structure and Properties of Hexafluoropropylene Oxide Oligomers F,C ,O(CFCF,O),C,F, I
CF 3 kinematic viscosity at 4 0 ° C 280 cSt shell four-ball wear scar, 1200 rpm, 75 "C, 2 h, 52100 1 kg 0.13 mm 25.3 cSt 10 kg 0.45" at 100 "C at 200 "C 3.7 cst 40 kg 0.78" at 370 "C 0.8 cSt pour point -35 "C Oxidation-Corrosion Stability Test Data: 20-mL Sample; 1 L/h; Air Flow, 24 h/FTSM 791b, 5308 metal wt change, mg/cm2 visc. change fluid temp, "C at 40 "C, % loss, 5% 4140 52100 410 M-50 440C 260 288 315
t 2.0
0.3 1.3 5.2
+ 2.7
+3.4
-0.01
-0.02 +0.06 +3.11
t 0.09
+3.11
-0.02
-0.01
t 0.04
t 0.08
+1.17
+1.80
+0.02 + 0.02 + 0.46
Table IV. Structure and Properties of Linear Perfluoroalkyl Ethers RfO-(CFzCFzO )m (CFzO kinematic viscosity at -40 "C 2875 cSt shell four-ball wear scar, 1200 rpm, 75 "C, 2 h, 52100 40 kg 1.55 mm at 40 "C 132 cSt 41.5 cSt at 100 "C pour point
