Scanning electron microscopes show the morphology of various PTFE thickeners. The general rule of thumb: The smaller the particle size, the more efficient the PTFE.

Photo courtesy Jonathan Kannair Photography. Pictured are William Medeiros and Nicole St. Pierre

Oil is more common as an industrial lubricant because it flows so easily into critical areas. However, oil reservoirs aren’t always reliable — or even possible. Picture an overhead conveyor system with spray-lubricated wheels, where the oil drips to the floor. Even if it’s not that obvious, oil can create untidy hazards — often inescapable, since lubes are essential to protect moving parts from wear and corrosion. Or are messes avoidable? Grease is particularly effective at improving containment while reducing contamination and replenishment schedules — and friction too. What makes a grease different from plain oil is its added gellant, or thickener, that keeps the oil where it’s needed most — between moving surfaces subject to friction or vibration.

All greases are colloids, a permanent suspension where microscopic particles of gellant are dispersed through the oil. Too large to dissolve and too small to settle out, the gellant particles create a weblike matrix that keeps oil in place. Many materials are used for this purpose and offer unique benefits. For example, aluminum complex soap has closely packed fibers that make for smooth grease with very high water resistance. Kevin D. Akin, director of Product Services and Support at Nye Lubricants Inc. based in Fairhaven, Mass. adds, “Filmforming anti-wear additives can prevent electrical continuity on a switch contact, while anti-oxidants are beneficial in protecting hydrocarbon oils.” And except for polyureas, lubricant thickeners are generally compatible with thickeners in their same family. However, as with any design, greases all have their particular limitations; damping greases are one example.

Some damping greases are so tacky, objects must be forced through them; this shear resistance minimizes free-motion problems like backlash, coasting, and wear. Though it’s more the base oil’s viscosity (often highly viscous PAOs) that gives damping greases their tackiness, they are thickened with silica and PTFE. Silica thickener has a high separation point and works with all base oils. But because PTFE is a very slippery and malleable molecule, PTFEthickened damping greases offer better shear resistance. (In contrast, silica molecules under high shear tend to break down.)

PTFE and PFPE are mixed to produce a batch of grease. While the PFPE oil is the primary lubricating medium, the PTFE polymer is a very slippery molecule, which enhances the grease’s boundary lubricating capabilities.

Photo courtesy Nye Lubricants. Pictured is David Rinko

In other greases designed for very high temperatures, PTFE also improves formulas — to survive prolonged exposure to temperatures to 260°C. Another gellant that adds thermal and lubricity stability is clay; it can raise the dropping point, the temperature at which grease becomes liquid, or begins to separate oil. Other times, when temperature resistance (to 120°C) and water resistance is a priority, lithium soap is used. (If the thickener resists water like lithium or calcium soaps do, then rust and corrosion inhibitors are added to protect surfaces from rusting.)

Akin points out the caveats here: “Some additives can be detrimental. Soaps with otherwise good water resistance — like lithium and calcium — do not have good salt-water resistance. However, complex versions of these are generally better in both fresh and salt-water performance. Another example is molybdenum disulfide, which is an EP additive for metal-on-metal applications. It can accelerate wear on plastics.”

When sticky’s too sticky

Chain is a perfect candidate for grease. Its long contacting surfaces can’t often be enclosed economically, so grease offers a perfect solution. In fact, newer formulas address some of the more specific problems of these chain/grease systems. In cold environments, conventional chain oil tackifying agents — which ensure adherence of oil to chain surfaces — tend to thicken excessively and increase oil viscosity. If extreme enough, this thickening can block and prevent protective oil films from reaching internal chain components. Unprotected chain parts then seize or rust, leading to sticky linkages, erratic conveyor motion, and increased energy consumption.

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