Every material outgasses to some degree, and vacuum grease is no exception. The question that matters is how much and how fast. A conventional grease floods a chamber with vapor within minutes; a properly formulated high vacuum grease releases so little that a well-designed system still reaches its target base pressure.

The Science of Outgassing

Outgassing is the release of gases that were adsorbed on, absorbed into, or dissolved within a material. When chamber pressure drops, the equilibrium shifts and those molecules leave the surface. For a grease, the dominant source is the vapor pressure of its own base fluid plus any volatile additives.

A standard grease is engineered for atmospheric pressure and contains light fractions and package additives that have appreciable vapor pressure at room temperature. Under vacuum these evaporate rapidly. A high vacuum grease is built from stable, high-molecular-weight base fluids, usually silicone or perfluorinated polyether, and is refined to remove the volatile tail, so its equilibrium vapor pressure is orders of magnitude lower.

Why Outgassing Matters

• System contamination: outgassed molecules condense on cooler surfaces, leaving films on optics, detectors, and electrical contacts that degrade performance or cause failure.
• Base pressure: the vapor adds gas load the pumps must remove, which can make a deep or ultra-high vacuum unreachable.
• Process yield: in deposition and coating equipment, a hydrocarbon film on a substrate ruins adhesion and electrical properties.
• Seal life: as volatile fractions leave a conventional grease, it stiffens and loses sealing and lubricating function, shortening O-ring and stopcock service life.

How Low-Outgassing Grease Is Made

Formulators select a base fluid with inherently low vapor pressure, then vacuum-strip it to remove residual light ends. Fillers such as fumed silica provide the thixotropic body that keeps the grease in the joint without introducing volatiles. The finished product is often characterized by total mass loss and by collected volatile condensable material under standardized vacuum-thermal test conditions, and the published figures let engineers predict behavior in their own systems.

Temperature drives the rate. Outgassing roughly follows an exponential relationship with temperature, which is why a bake-out, running the empty system hot under pump, accelerates the release of the remaining volatiles so they can be removed before real work begins.

Email Us with your base pressure target and bake-out capability for help selecting a grade.

Managing Outgassing in Practice

Use the minimum quantity of grease. Outgassing scales with exposed surface area and volume, so a thin translucent film outgasses far less than a thick layer, and it also seals better.

Bake the system after assembly where components allow it. Confirm the grease is rated for the bake temperature, since exceeding it can decompose the base fluid and create new volatile species.

Keep grease off surfaces that do not need it. Wipe away everything squeezed out of a joint, and avoid smearing grease across chamber walls or internal fixtures.

Where a joint sees wide temperature swings, differential expansion between dissimilar materials continuously works the film, a stress mechanism related to how a CTE mismatch causes sealed and bonded joints to fail. For equipment where the exterior also runs hot, a high-emissivity ceramic coating manages surface temperature without affecting the seal.

Silicone Versus Perfluorinated Base Fluids

The two dominant chemistries for low-outgassing grease behave differently in service. Silicone greases offer a wide temperature range, low cost, and very low vapor pressure, and they suit general high-vacuum work. Their weakness is that silicone migrates as a thin film and, if it reaches an electrical contact or an optical surface elsewhere in the system, can leave an insulating or light-scattering residue that is hard to remove.

Perfluorinated polyether greases have even lower vapor pressure, resist oxygen and aggressive solvents, and do not carry the silicone-contamination risk, which makes them the choice for ultra-high vacuum, oxygen service, and systems with sensitive optics. They cost more and have a narrower useful temperature band at the cold end.

Choosing between them is a question of vacuum level, chemical exposure, and how damaging a stray film would be to the rest of the equipment. When in doubt for a contamination-critical system, the perfluorinated grade is the conservative option.

Whichever chemistry is used, keep a single grease type per system. Successive services with different products leave a mixed film whose outgassing behavior is unpredictable and whose migration path is hard to trace. Standardize on one grade, document it on the equipment, and clean the old film off completely at each service rather than layering a new product over it.

Key Takeaways

Vacuum grease does outgas, but a high vacuum grade built on a refined silicone or perfluorinated base fluid outgasses so little that it is compatible with high and ultra-high vacuum service. The rate depends on the base fluid’s vapor pressure, the amount of grease exposed, and temperature.

Control it by using a thin film, baking out the assembled system, keeping grease confined to the joints, and selecting a grade whose published mass-loss data suits your base-pressure target.

To match a low-outgassing grease to your system, Contact Our Team.

Visit www.incurelab.com for more information.