Vacuum Pump Oils can basically be classified into two categories based on their composition –
1) Mineral Vacuum Oils
2) Synthetic Vacuum Oils
Synthetic Vacuum oils can further be divided into four types:
A) Esters
B) Silicones
C) Ethers
D) Fluorochemicals
1. Mineral Oils
What are mineral oils made of?
The mineral oils used in vacuum pumps are mixtures of aromatic hydrocarbons, paraffin and naphthene. Carbon analysis shows the typical mineral oil to be composed of approximately 65% paraffin, 30% naphthene, and 5% aromatic hydrocarbons.
The paraffins with high boiling point make very good lubricants. They are stable at high temperatures, are fluid at low temperatures, have fairly high viscosity index and are adherent enough to not break down under high shear loads. They tend to oxidize at high temperatures. Paraffins have many possible isomers which have differing properties. Aromatic compounds contain phenyl groups with straight- or branched-chain structures. They form sludge at high temperatures and have an undesirably low viscosity index.
Naphthenes contain rings and chains with no double bonds. Naphthenes have properties between those of paraffins and aromatics. Preparation of a vacuum fluid begins with vacuum distilled base oil that is further purified by solvent extraction and dewaxing. The oils supplied to the vacuum fluid distiller are either single cut with one peak in the molecular weight distribution, or blends made from two of the relatively few refined single cuts.
The oil is further purified by additional distillations. The distillation conditions are chosen to produce fluids of the desired viscosity and vapor pressure. Supervac make SV-68 is mineral oil that is triple distilled with molecular distillation.
Mineral oils lack many properties of the ideal fluid. When used in a diffusion pump, their ultimate pressure is unacceptably high for many applications. They are not stable in oxygen, have some tendency to sludge and foam, and do not offer adequate protection in boundary layer lubrication. Synthetics were developed to overcome the shortcomings of hydrocarbon oils. Synthetic fluids (SV-77) with low vapor pressure, high viscosity index, a high degree of lubricity, and chemical inertness have been specially designed and formulated.
1) Mineral Vacuum Oils
2) Synthetic Vacuum Oils
Synthetic Vacuum oils can further be divided into four types:
A) Esters
B) Silicones
C) Ethers
D) Fluorochemicals
Types of vacuum oils have been described in detail below so that you are able to choose the right vacuum oil for your application.
What are mineral oils made of?
 |
| Mineral Vacuum Pump Oil SV-68 |
The paraffins with high boiling point make very good lubricants. They are stable at high temperatures, are fluid at low temperatures, have fairly high viscosity index and are adherent enough to not break down under high shear loads. They tend to oxidize at high temperatures. Paraffins have many possible isomers which have differing properties. Aromatic compounds contain phenyl groups with straight- or branched-chain structures. They form sludge at high temperatures and have an undesirably low viscosity index.
Naphthenes contain rings and chains with no double bonds. Naphthenes have properties between those of paraffins and aromatics. Preparation of a vacuum fluid begins with vacuum distilled base oil that is further purified by solvent extraction and dewaxing. The oils supplied to the vacuum fluid distiller are either single cut with one peak in the molecular weight distribution, or blends made from two of the relatively few refined single cuts.
The oil is further purified by additional distillations. The distillation conditions are chosen to produce fluids of the desired viscosity and vapor pressure. Supervac make SV-68 is mineral oil that is triple distilled with molecular distillation.
Are all mineral oils the same?
No. The base oil is a natural product. Hence, it has characteristics as per its place of origin; as a result, all mineral oils are not the same. We divide mineral oils for vacuum pumps into four grades: mechanical pump, diffusion pump, fully saturated paraffin, and inhibited fluids. The “rotary pump” grade is, composed of blended fluids or single-cut fluids that have not been refined to remove light ends and tars. Vapor pressure requirements in a mechanical pump are not as severe as in a diffusion pump. This grade is typically used in rotary pumps used for rough pumping chambers and backing turbomolecular and diffusion pumps. “Diffusion pump” grade fluids may be characterized as having a single peak in the molecular weight with narrow mass dispersion. Nowadays, with the advent of silicone oils, mineral oils are rarely used for diffusion pumps.
2. Synthetic Fluids
 |
| SV-77 - Synthetic Vacuum Pump Oil (Supervac make) |
 |
| Comparison of Molecules |
Synthetic Vacuum Oils can be classified in four categories -
A) Esters
Esters are chemicals formed by the reaction of an organic acid and an alcohol. The esters used in vacuum pump fluids all contain the same ester chemical bond, but have differing structures and rather widely varying properties. Sebacate esters (derived from sebacic acid) are organic esters that were originally developed as jet engine and aircraft instrument lubricants and today used to lubricate turbomolecular pumps. Environmental concerns have resulted in the disuse of many older ester lubricants.
 |
| Temperature/ Viscosity graph of different vacuum oils |
B) Silicones
What are Silicone oils/fluids?
 |
| Molecular structure of silicone oil |
Silicones, or siloxane polymers, are made up of repeated silicon oxygen groups with silicon bonds to side groups. The unique character of the silicon-oxygen bond gives similarly unique properties to silicone-based fluids.
The type of side groups (methyl, phenyl, alkyl, chloro, etc.) and the number of silicon atoms determine the properties and applications of the fluid. The large size of the silicon atom allows the phenyl and methyl side groups great mobility. The high flexibility of the siloxane chain accounts for the high viscosity index of silicones. As a class, silicones have the highest viscosity index of any fluid.
 |
| Diffusion pump oil SV-SIGMA+ (Alt.of DC-705) |
Which Silicone fluids are used in Diffusion pumps?
Trisiloxanes and polysiloxanes are two fluids used in vacuum pumps. Trisiloxanes are widely used diffusion pump fluids. They do not adhere to steel and cannot be used as lubricants in rotary/mechanical pumps. They are manufactured by controlled hydrolysis of silanes and addition of phenyl groups, followed by distillation. The first silicone diffusion pump fluids, IX-702 and DC-703, were mixtures of closely related molecular species with similar boiling points. Further separation leads to the isolation of two specific chemical compounds - Tetraphenyl tetramethyl trisiloxane (DC 704/ SV-SIGMA) and Pentaphenyl trimethyl trisiloxane (DC-705/ SV-SIGMA+). One compound of Pentaphenyl silicone is DC705/ SV-SIGMA PLUS and has one of the lowest vapor pressures of any diffusion pump fluid.
C) Ethers
An ether may be regarded as a derivative of a water molecule in which the hydrogen has been replaced by alkyl or aryl groups. Polyphenyl ethers were synthesized in an attempt to develop high temperature jet engine lubricants. Hickman was the first to use them as diffusion pump fluids. He found the five-ring phenyl to be stable and have extremely low vapor pressure. Commercially available fluids are mixed meta- and para-isomers of the pentaphenyl ether, which contain trace impurities of the four-ring compound. The four-ring compound has a high vapor pressure, while the six-ring compounds are either solids or glasses. Pentaphenyl ether is very viscous at low temperatures but is stable and has excellent high-temperature lubricating properties. Its wear, friction, and load capacity are in some cases equal to mineral oil. Its chemical stability and low vapor pressure make it an outstanding fluid for critical diffusion pump applications.
D) Fluorochemicals
Fluorochemical fluids are characterized by their inertness to a wide range of chemical compounds. Partially and fully fluorinated fluids have found use as lubricants for space applications, oxygen compressors, and liquid oxygen systems.
Fluorinated pump fluids, perfluoro alkyl polyethers (perfluoropolyethers or PFPE for short), are currently manufactured by two techniques. Fomblin fluids are prepared by the UV-stimulated photooxidation of hexafluoropropylene and oxygen. It is a random copolymer of C3F6O and COF2. ranges from 10-to-40. Krytox fluids are prepared by the polymerization of hexafluoropropylene epoxide. Krytox consists of 20-30 repeating C3F60 groups.
Raw perfluoropolyethers have a distribution of molecular weights extending as high as 10,000 AMU. They are distilled to yield cuts with average molecular weights in the range 1800-3700 that are suitable for use in mechanical, turbomolecular, and diffusion pumps.
Perfluoropolyethers are stable Lewis bases that react with few chemicals. They should not be placed in contact with ammonia, amines, liquid fluorine, liquid boron trifluoride, or sodium or potassium metal. Laboratory experiments have shown PFPE fluids to decompose when heated sufficiently (>lOO°C) in the presence of Lewis acids.
 |
| Molecular structure of PTFE vacuum pump oil |
