George B. Mock
Nye Lubricants Inc.
Harsh chemicals, extreme temperatures, and high loads can quickly deteriorate bearings, joints, and other moving parts. The right lubrication can help decrease wear and tear, making a difference in the performance and life of components. The use of synthetic lubricants, particularly perfluoropolyether (PFPE) lubricants, is one alternative that can be customized to suit many applications.
PFPEs are a family of slippery, long-chain fluoropolymers that wet surfaces well, making them good materials for lubricants. A layer of fluorine atoms surrounds each PFPE molecule, making electron exchange difficult and keeping oxidation in check. This not only extends lubricant life, it reduces carbonaceous deposits that can increase wear and shorten component life. PFPEs are available in a variety of viscosities, and can be used as lubricating oils or combined with thickeners to make a variety of PFPE greases.
PFPEs have a reputation for working well in demanding environments. They can withstand temperatures from –90 to 250°C, and even higher spikes. Extremely inert, they don't crack, craze, discolor, or dissolve plastics, nor do they swell, shrink, or embrittle natural rubber or other elastomers. They are nonflammable, and resist harsh chemicals, fuel oil, and brake fluids. Because of their low volatility they are often the lubricants of choice for high-vacuum environments.
Important distinctions differentiate PFPE fluids. There are five different types of PFPEs: PFPE-K, PFPE-Y, PFPE-D, PFPE-M, and PFPE-Z — or, in PFPE shorthand, K, Y, D, M, and Z fluids. All of them contain carbon, oxygen, and fluorine atoms, but each type of fluid is the product of different starting materials and manufacturing processes. Molecular structure affects how the fluid works at low temperatures, its ability to prevent wear, its viscosity index, and volatility — all critical in lubricant selection.
Structurally, each PFPE molecule is classified as either a linear or pendent. Linear PFPEs are straight polymer chains which make them flexible molecules able to remain fluid at lower temperatures than pendents. Pendent molecules have side chains, sometimes called branches, which make them more rigid. While these branches decrease the molecule's flexibility, they form a series of lattices that help prevent mating parts from coming into contact. Pendents deliver more film strength than linears, which means better wear-prevention, especially under heavier loads.
D, M, and Z fluids are linear PFPEs, and Y and K fluids are pendent molecules. Depending on the viscosity, D and M oils can remain fluid and lubricious to about –75°C. The pour point of a light-viscosity Z fluid goes even lower, to –90°C. Of course, with this added cold-temperature performance comes added cost. Grease formulated with a Z fluid can cost as much as $500/lb, about four times the cost of grease formulated with D or M fluids. By contrast, the more economical Y and K fluids do not function well below –40°C, a temperature at which synthetic hydrocarbon lubricants work well.
One exception is worth noting. Compared to the usually more robust pendents, linear M fluids seem to work better as lubricants for powder-metal bearings. Experts think the low surface tension of the M fluid, combined with the sheer volume of oil in a properly impregnated sintered bearing, lets this particular linear PFPE effectively wet the nooks, crannies, and surface of sintered parts to retard friction and wear.
The viscosity of the base oil is also important. Lowtorque, low-temperature uses need a relatively light oil. High-speed, high-temperature devices that are heavily loaded call for a more viscous base oil. The viscosity of the oil should stay relatively constant throughout the temperature range of the application. Otherwise the oil may gum up at low temperatures or waft away at high temperatures. In either case, lubricant is depleted. The viscosity index (VI) is a dimensionless number that describes how viscosity varies with temperature: the higher the number, the less change. As with temperature and film strength, the molecular structure of a PFPE affects its VI.
Linear PFPEs, because of their flexibility, usually handle wide temperature excursions well. When tested at –40°C, 40°C, and 100°C, M and Z fluids yielded a VI of 340. D fluids, also linear PFPEs, had VIs ranging from 170 to 210, where lower viscosity D fluids yielded the lower VIs. In contrast, pendent Y and K oils had VIs ranging from 110 to about 140, which approximates the VI of a synthetic hydrocarbon.
All PFPEs are known for low volatility, but their molecular weight influences their tendency to vaporize. Generally, the higher the molecular weight, the lower the volatility for oils of equal viscosities. Any fluid's molecular weight is an average of the weights for each of its molecules; so volatility also depends on the molecular homogeneity in a given batch of fluid. For example, a batch of K fluid having a narrow range of molecular weights is less volatile than a batch with a wide range of molecular weights. Lighter molecules, sometimes called fractions, tend to vaporize easily, which hastens lubricant depletion and component failure. It is possible to control the range of molecular weights in a batch of oil during production or in postproduction through fractional distillation, a process that removes the lighter, more volatile fractions. Critical applications like connectors in an ABS system demand that the volatility of each batch of PFPE oil be tested.
The catalytic stability of a PFPE lubricant is equally important. Particularly at low-speed, loaded bearing and gear applications, there is a well-documented phenomenon wherein nascent metals tend to promote catalytic breakdown of the PFPE molecule. The result is a residue that looks like brown sugar. Semisolid and abrasive, it can reduce bearing life. Different PFPE molecules have different rates of catalysis. While Z fluids work the best over wide temperature ranges, they seem to be more vulnerable to catalytic degradation than D fluids. In a four-ball wear test (75°C for 1 hr at 1,200 rpm with a 40-kg load), balls lubricated with Z fluid had a wear scar of 1.22 mm. Balls lubricated with a D fluid of the same viscosity had a wear scar of only 0.39 mm. D fluids are more inert in the presence of fresh metal, so they tend to minimize the brown sugar phenomenon.
What happens when an application calls for qualities of more than one PFPE fluid?
Blend them. Recently, researchers blended a linear and a pendent PFPE to formulate a new automotive grease. The situation called for the load-carrying ability of a pendant at –40°C. While there are pendants that function at –40°C, their viscosity is too low, which jeopardizes wear reduction. A standard pendant would protect against wear, but would cause torque problems at –40°C. The solution turned out to be a 50/50 blend of K and M fluids. The K fluid was viscous enough to carry the load and the Z-fluid ensured performance at –40°C.