Table of Contents
Silicone Grease Creep
Operating Temperature Range
Pressure Operating Range
When it comes to vacuum lubricants and sealants, a number of different options are available on the market. The Apiezon range of hydrocarbon vacuum greases has a long and well-known track record in offering sealing and lubrication solutions. Although other materials have been launched in the market in more recent times, specialist hydrocarbon grease still continues to be the number one choice in many ways.
Silicone high vacuum grease is another well-known material. It has certain advantages in terms of good stability and low vapor pressure; however, some of the unique surface properties of silicone can create problems for a vacuum engineer, particularly when cleanliness is important. The following is an examination of two most important problem areas, namely creep and contamination.
Silicone Grease Creep
The reason that silicone grease is very much prone to creep is based on the chemistry of the base fluid. Silicone greases are composed of polydimethylsiloxane (PDMS) liquid integrated with some sort of filler, generally zinc oxide or PTFE. The PDMS liquid itself has a few interesting properties, one of which is that critical surface tension of wetting is very high compared to the liquid surface tension. This indicates that the polymer will spread over its own adsorbed film, in a process called creep. In some applications, this can be an advantage as it means that silicone liquid will instantly offer surface coverage for mold release or metal protection. However, this tendency to instantly creep has the potential to cause costly problems in the vacuum world.
For instance, this can mean that a silicone film can become deposited on surfaces in a vacuum chamber, causing problems with electrical contacts or cloud optics. In addition, silicone grease can break down under partial discharge or electrical arcing and form solids, which are non-conductive and if this occurs on electrical contacts, then, they will not work correctly any more.
A lot has been published concerning the potential for silicone contamination and one of the examples of this comes from the space industry, where numerous solar cells were packaged in shipping foam which was produced using a silicone mold release agent. The mold release had transferred to, and permeated through, the solar cell structure after one year in storage. This incident led to the loss of solar cells for a huge number of solar panels. It is this potential of silicone that allows it to migrate and “creep” away from where it is meant to be used that can be damaging if not managed carefully.
For instance, a key application where silicones might be less acceptable is in the painting and coating industries. When surfaces get contaminated with silicone, it can serve as a barrier to adhesion of coatings and lead to defects in painted surfaces.
Figure 1 shows a familiar example of a defect brought about by surface contamination. In certain cases, the use of silicone compounds can be limited for all process equipment in a paint plant and in this case, the hydrocarbon grease can become an extremely cost-effective alternative.
Figure 1. Defect caused by surface contamination.
Another example where the selection of vacuum grease must be carefully taken into account is in laboratories, where a huge amount of specialist glassware is used. In several instances, there is a requirement to produce custom items of equipment in order to carry out specific experiments and to cater for this, many specialist glassblowing workshops exist which are capable of building intricate bespoke glassware.
This enables users to have glass repaired or reworked in the event of accidents; however, this can be prevented if silicone contamination is present. Upon heating to the high temperature required to shape glass (in excess of 400 °C), a fine white silica powder which will fuse onto the surface of the glass can be developed. If a torch is then applied, the silica will burn into the glass, successfully destroying it. This can be very expensive if a piece of customized glassware is damaged and far outweigh the extra cost of choosing a hydrocarbon grease in place of the silicone.
The alternative to all these issues is to employ hydrocarbon-based grease, because this family of materials is far less prone to creep and does not show the migration behavior of silicones.
This is mainly because of the higher surface tension of hydrocarbon grease, when compared to silicone and a range of other alternatives such as PFPE, as shown in Table 1.
Table 1. Comparison of typical surface tension by base oil. 
|Base Oil Type
||16 - 21
||0.016 – 0.021
Hydrocarbon grease can also be removed more easily from surfaces compared to silicone products. In the case of laboratory glassware, the use of simple soap and water can be enough to remove hydrocarbon grease and if this is not ideal, solvents such as limonene can be utilized to dissolve and fully remove the product. On the other hand, silicone may leave a residue behind even after complete cleaning and can change the behavior of the surface with regard to paints and adhesives.
In many situations, a hydrocarbon product can be chosen which provides the required vacuum performance. It is also worth noting that in the case of metal-to-metal lubrication, hydrocarbon grease provides far superior performance to silicone-based materials. It is also possible to dissolve unfilled hydrocarbon-based greases in solvents in order to aid application of very small quantities.
Operating Temperature Range
One area where silicone vacuum grease can be useful is in the available operating temperature that typically ranges from -40 to 200 °C . Hydrocarbon greases can offer a similar operating range and there are also products that increase this range down into cryogenic temperatures and up above the capabilities of silicone. The chart presented in Figure 2 displays the range of operating temperatures for different hydrocarbon greases in the Apiezon range.
Figure 2. Operating temperature range for hydrocarbon greases.
Pressure Operating Range
The other significant factor for a vacuum engineer is the operating pressure range of the vacuum grease and this is directly linked to the vapor pressure of the product. The vapor pressure of silicone grease will rely on the performance of the base oil, which is combined with a thickener in order to produce the final grease. Typically, silicone oils will have a vapor pressure that is less than 1 Torr (133 Pa) at 220 °C and silicone grease is suggested for use in systems having ultimate base pressures of 1x10-5 to 1x10-6 Torr (1.3x10-3 to 1.3x10-4 Pa). Preconditioning of the grease at operating temperature and pressure is recommended to avoid contamination of samples.
In comparison, high temperature hydrocarbon greases can contain vapor pressures less than 10x10-2 Torr (1.3 Pa) at 220 °C and can be employed at 1x10-8 Torr (1.3x10-6 Pa) for ambient temperature applications. This makes contamination of surfaces less likely and therefore, preconditioning of the grease is not required. Other hydrocarbon greases developed for ambient temperatures can operate down to the 1x10-10 Torr (1.3x10-8 Pa) range.
The vapor pressure curve, displayed in Figure 3, is an example of the behavior of high temperature hydrocarbon grease, which will be suitable as long as the vacuum pressure is above the curve for a specified temperature.
Figure 3. Vapor pressure curve for high temperature hydrocarbon grease. 
There are a wide range of vacuum greases available on the market and two frequently used ones for laboratory, production, and research work are silicone based and hydrocarbon based. Although there is stability and cost advantage to the silicone-based grease, it is worth evaluating the likely effect of creep and contamination before selecting this solution. Inadvertent contamination with silicone can lead to grave issues and might be almost impossible to clean. There may also be blanket restrictions concerning the use of silicone containing products in some paint plants and in such case, hydrocarbon can offer a possible cost-effective solution.
The extremely low vapor pressure and easy removal of hydrocarbon-based grease can make it an ideal choice in many cases. Hydrocarbon products can also provide a solution over a wider temperature and vacuum range by carefully selecting the perfect grease for the application.
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- The Removal of Silicone Contaminants from Spacecraft Hardware, K. Luey and D.J. Coleman, Space Materials Laboratory, 20 September 2002
- Paint Defects Identification and Correction, Dupont, http://www.naaa.com/standards/Identifying_and_Correcting_Paint_Defects.pdf, retrieved August 22nd 2016
- http://www.st-andrews.ac.uk/~cfms/silicon.htm, retrieved July 12th 2015
- http://www.public.asu.edu/~aomdw/GLASS/book.pdf, retrieved July 12th 2015
- Lubricants for High Vacuum Applications, M.R. Hilton and P.D. Fleischauer, April 1993
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- http://www.shinetsusilicone-global.com/products/type/oil/detail/about/index2.shtml, Retrieved 13 May 2015
- http://www.2spi.com/catalog/vac/dow.php, retrieved July 21st 2015
- DM-Fluid Technical Data, Shin-Etsu Silicone,
- Apiezon H Grease, High Temp Vacuum Grease Technical Datasheet, Nov 2012
This information has been sourced, reviewed and adapted from materials provided by APIEZON.
For more information on this source, please visit APIEZON.