Fluids Technical Notes
Why Are Fluids Used in Mechanical Pumps?
In an oil-sealed rough vacuum pump, the fluid or oil:
- Lubricates bearings and wear surfaces
- Seals gas into compressible volumes
- Transports heat away from wear surfaces
- Inhibits corrosion of metallic parts
- Flushes away moisture
Matching the Fluid and Application
Matching the Fluid and Application When operating, an oil-sealed pump compresses the gas to a pressure slightly greater than atmospheric to expel it through the exhaust valve.
At this pressure and an operating temperature of 70–100° C, active gases readily attack pump surfaces, bearings, seal materials, and the oil. Choosing an appropriate oil for the application is critical to good pump maintenance.
Normal or "Clean" Vacuum
When pumping a chamber from atmosphere, the major component is air. After the chamber has been at high vacuum for some hours, H2O vapor, CO, and H2 are the major gases.
Almost any vacuum-grade, distilled hydrocarbon oil works in this application. Note that fully hydrogenated oils are usually less reactive than unsaturated oils.
Many processes (e.g., photoresist stripping) create conditions where the pumped gas is 100% oxygen. While the chamber pressure may be low, to exhaust the gas from the pump, it must be more than 1 atmosphere.
Oxygen at atmospheric pressure is exceptionally dangerous. To avoid a fire or explosion, the pump fluid MUST NOT be flammable. We recommend an inert oil such as Fomblin vacuum pump oil because it is unreactive with O2 and cannot act as a fuel for an explosion.
Plasma systems, particularly those using halogen containing gases, require pump oils that either resist radical/halogen attack or are inexpensive to replace.
An inert PFPE fluid, such as Fomblin vacuum pump oil, is the best choice, but a hydrogenated hydrocarbon oil that is frequently replaced (perhaps daily) may be a lower-cost alternative.
Lewis Acids (AlCl3, BCl3, etc.)
Compounds of this type attack most pump oils and deposit sludges in the pump. To counteract these effects, an inert PFPE fluid such as Fomblin vacuum pump oil should be used together with continuous oil filtration for particulates and acid neutralization.
Choosing a Replacement Oil for an Oil-Sealed Pump
To avoid warranty questions, contaminated pump oil is often replaced by oil recommended by the pump manufacturer. However, this "safe" approach may not be the lowest-cost option once the pump is beyond warranty.
When selecting a replacement fluid, there are many relevant characteristics (noted below) but two, vapor pressure and viscosity, MUST be considered.
NOTE: Some fluid combinations are immiscible and mixtures may be hazardous for some applications. Please read "Changing Pump Fluids."
Oil vapor backstreaming depends on the fluid's vapor pressure at the pump's operating temperature. To reduce backstreaming, ensure that the new oil's vapor pressure is equal to, or lower than, the original oil's vapor pressure.
Since backstreaming occurs from any pump that has fluid in the vacuum volume, we strongly recommend the use of a good, well-maintained foreline trap for such pumps.
Viscosity and Lubricity
The new fluid's viscosity has two main requirements: at room temperature it must be low enough that the pump rotates when its motor starts; at working temperature it must be high enough to provide good lubricity and gas sealing.
Typically, rotary vane pumps have small clearances and need a low viscosity fluid. Rotary piston pumps have larger clearances and need higher viscosity oils to lubricate and seal.
Roots pumps are not oil-sealed but have high-speed gears that need good lubrication.
While viscosity values are readily available, lubricity values are not. Even experts debate the meaning of lubricity values for vacuum pumps.
In general, oils with hydrocarbon-like (-C-C-C-) or ether-like (-C-O-C-) backbone will have adequate lubricity (where appropriate for the intended application). Oils with siloxane-like backbone (-Si-O-Si-) do not.
Good water separation is rarely considered but is an important property. An oil that emulsifies with water or absorbs large concentrations of water will give poor foreline pressures.
For pumps mounted in an outdoor "shed," the pour point is important during winter maintenance. If the pump is refilled when cold, the resulting high viscosity may prevent the pump's motor from rotating the mechanism.
Flash point can be roughly defined as the temperature at which the oil vapor will catch fire in air, given an ignition source.
Normally this temperature is much higher than a pump's working temperature, but it is never a mistake to choose an oil with a high flash point.
Traditionally associated with silicone-based products, phenomenon where grease travels away from the area of application, and then contaminating adjacent surfaces. This is a concern in surface coating applications where poor surface adhesion and incomplete coating defects result from silicone contamination.
Hydrocarbon-based greases like Apiezon® products are highly resistant to this phenomenon. Resistance to this creep phenomenon allows a benefit in the industries of scientific and semiconductor applications.
Handling Pump Fluids
ALWAYS wear splash-proof goggles when handling pump oils. Allowing new or contaminated oils to reach one's eyes is exceptionally dangerous. If this happens get immediate medical attention.
ALWAYS wear oil-proof gloves and protective coveralls. While skin contact with fresh oil is not often hazardous, the affected area should be immediately washed multiple times using a good, hypoallergenic detergent and large volumes of water.
Skin contact MUST be avoided with "old" oil. Even if innocuous gases and materials are added to the vacuum system, the process products may be very different.
Any contact area must immediately be washed with a good detergent and large volumes of water. Get medical attention at the first sign of itching, burning, or redness.
Fresh oils have low vapor pressures. Breathing their vapors at room temperature is rarely hazardous; however, at elevated temperatures present in operating pumps, breathing any oil vapors, contaminated or not, MUST be avoided.
Breathing aerosols exhausted by a pump displacing large gas volumes MUST also be avoided. Even when using fresh hydrocarbon oil to pump air, it is never wrong to assume aerosols will cause deteriorated lung performance.
We cannot overemphasize that smoking and handling fluoro- and chlorofluorocarbon fluids are strictly incompatible.
These fluids' thermal decomposition products include HF, HCl, phosgene, and carbonyl fluoride... each an irritant, toxic, deadly, or all three!
Cigarettes are too easily contaminated with oils transferred from gloved fingers or surfaces where the cigarette has rested, and a lit cigarette may reach decomposition temperatures.
Do not throw fresh or contaminated oils into the local dump. All contaminated pump oils should be analyzed before disposal. Only then can appropriate disposal or reclamation by a licensed company be arranged.
Routine Fluid Maintenance
We strongly recommend that log books are kept for every pump, detailing its working history, oil changes, and other maintenance steps.
We suggest using the pump manufacturer's recommended oil change intervals unless some, empirically determined, shorter interval is used. The pump's service life will strongly influence the oil change schedule.
If service intervals are unclear, use an Acidity Analysis Kit to help establish them. Oil lifetime is often determined by the concentration of oxidizing components pumped.
Oxidation of the oil by O2, halogen, or Lewis acid will cause organic acids to form. Regular testing of the oil's acidity provides some measure of the service period. One thing is certain: ignoring routine oil changes quickly leads to pump damage.
Types of Vacuum Fluids
The bulk of vacuum pump oils are petroleum fractions (hydrocarbons) distilled from crude oil.
Without too much chemistry, a true hydrocarbon oil contains carbon, hydrogen (CH3-CH2-CH2-CH2-) and nothing else—a point missed by some major vacuum fluid suppliers.
Hydrocarbons from cracked crude oil inevitably contain sulfur compounds, aromatic ring compounds, plus (non-conjugated) carbon-carbon unsaturation. These components are more reactive and, therefore, undesirable for vacuum fluids.
Sulfur compounds are removed from all hydro carbon fractions no matter what their ultimate application. For vacuum fluids, the better grades are treated to remove aromatic compounds.
The best grades are also hydrogenated to remove all unsaturation and distilled once or twice to remove "light ends" and give a narrow cut in molecular weights.
The major advantages of hydrocarbon fluids are their low cost (which encourages frequent oil changes), low toxicity (when uncontaminated),and high lubricity.
The disadvantages are: reactivity to all but the simplest gases and their potential to form explosion mixtures in oxygen service.
Possible health risks from aerosols and vapors from the pump's exhaust are reduced by installing a good mist eliminator.
It is never a disadvantage to duct exhaust gases out of the building (following all local, state, and federal regulations governing emissions).
The chemical name for a silicone fluid is siloxane, which has alternating atoms (-Si-O-Si-O-Si-). A tri-siloxane has 3 Si atoms.
The three/two spare valences on the silicon atoms depending on their position in the chain) are satisfied by adding either methyl (-CH3) or phenyl (-C6H5) groups.
The fluid's basic properties—not easily oxidized by air at operating temperatures; not hydrolyzed by water vapor; unreactive with metals, elastomers, and gases such as H2 and CO; and high molecular weight—make the siloxanes excellent diffusion pump oils.
The best known polyphenyl ether, Santovac® 5, has a backbone of 5 phenyl groups, each linked through an O atom, or ether link,
(-C6H4-O-C6H4-). This structure is very resistant to oxidation.
The oil's extreme viscosity at room temperature prevents its use in an oil-sealed pump; however, its lack of chemical reactivity, its extremely low vapor pressure at RT, its reasonable boiling point under vacuum, and its high molecular weight make it an almost ideal, albeit expensive, diffusion pump oil.
Indeed, this fluid has been used to produce UHV chamber pressures with a diffusion pump.
The ester grouping is formed when an organic alcohol (>C-OH) reacts with an organic acid (-COOH) to eliminate water (>C-O-C(O)-). di-alcohols and/or di-acids are reacted, multiple ester groups form in one molecule.
For example, phthalic acid reacted with a C8 length alcohol gives the well-known dioctyl phthalate used as a vacuum oil and as a PVC plasticizer.
Esters, in general, are unreactive and, depending on molecular weight, used in both mechanical and diffusion pumps.
If a long chain alcohol is reacted with an inorganic acid (phosphoric), the resulting fluid (erroneously called a phosphate ester) has good lubricity and oxidation resistance. Both organic esters and inorganic "esters" are used in oil-sealed pumps.
Low molecular weight, highly volatile chlorofluorocarbons (CFCs) were employed as refrigerator fluids and aerosol propellants until their ozone layer destruction properties were recognized.
High molecular weight (low volatility) CFCs are used as vacuum pump oils because they do not react with aggressive gases used in many semiconductor processes.
The perfluoropolyethers (PFPE) fluids are completely inert and do not react with aggressive chemical agents. The only known (rare) reaction is the cracking of a single molecule into two shorter chain molecules.
In the polyether structure, a single molecule has many (-C-O-C-) groups throughout the molecular chain. The chain length is adjusted to achieve an appropriate viscosity.
The degree of branching (-C-O-C-(C)) and the chain length of the branch controls lubricity.
Unresolved valances on all carbon are satisfied with F atoms. The key to PFPE's stability is the high energy of C-F and C-O bonds in a molecule.
PFPEs of appropriate viscosity and vapor pressure are used in both oil-sealed and diffusion pumps. NOTE: Any application warranting inert pump fluid makes hazardous products.
All exhaust gases must be directed to an exhaust gas abatement system or, if appropriate, vented to outside air (following all local, state, and federal regulations governing emissions).
The advantage of expensive synthetic fluids (Fomblin®, Santovac 5, silicone) is that, if correctly chosen, their performance in harsh operating conditions is substantially better than hydrocarbon fluids. However, any fluid may be contaminated by the gases, vapors, and particulate materials from the vacuum process.
Because the synthetic fluids are so expensive, many companies have sought local, state, and federal licensing to reclaim contaminated fluids.
By combining chemistry (active filtering, neutralization, distillation, etc.) with administration (solid waste disposal, tracking documentation, etc.) the fluids are reclaimed and restored to "as new" condition.
Reclaiming is less costly than purchasing new oil and causes less environmental damage because the contaminants are concentrated to a small, solid bulk before disposal.
NOTE: There is no limit to the number of times a particular fluid volume can be reclaimed. Contact us for fluid reclaim information.
Changing Pump FluidsChanging fluid can mean two things:
- Replacing a used oil with more of the same (equivalent to a car's oil change)
- Changing from one oil type to another because the new oil is less expensive or the pump is entering a different application
Different procedures and precautions must be observed in each case.
Changing Used Oil
If the oil is discolored, dirty, acidic, or has reached the end of its recommended life, shut down the pump, vent the inlet, and drain out the oil while hot.
Replace the oil with a new charge. Remember, if the pump has an integrated oil filter, it, too, must be drained and the elements changed.
If process gases cause sludge buildup, there are two recommendations:
- Where hydrocarbon (or hydrocarbonmiscible) oils are used, drain the hot oil, fill the pump with a vacuum-grade flushing fluid, and run it for a few minutes. Drain the pump again while still hot and refill with the correct grade of operating oil.
- The safest course is to strip the pump and thoroughly clean/degrease, and rebuild it. If this is beyond local capabilities, call our Vacuum Services group
Changing Oil Type (same application)
Here, the only concern is miscibility. If the old and new oils are miscible, then simply drain and refill. If they are not miscible, then just draining may be unacceptable because even a few ccs of old oil will remain. When mixed with immiscible new oil, the bulk turns milky. Often this is only a cosmetic problem and the milkiness disappears after the next oil change.
If it is unacceptable, however, drain, strip the pump, clean with a suitable solvent, rebuild, (lubricating the moving parts with new oil) and then recharge with new oil.
Changing Oil Type(different application)
If the pump's service will change and the oils are immiscible, then the new service's details are critical. For example, if a hydrocarbon-filled pump is being transferred to oxygen or corrosive gas service, then any residual hydrocarbon oil is a serious hazard.
The pump MUST be completely stripped, degreased, cleaned, re-lubricated with PFPE fluid, reassembled, and charged with PFPE fluid. Contact us with any special application questions.
We have been formulating, testing, selling, and reclaiming pump fluids for over 45 years. Please call for advice on more complicated applications (or email email@example.com).