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It has excellent
compression set, low mass loss after extended exposure to
hard vacuum, is compatible with most O-ring lubricants and
relatively inexpensive. Other O-ring materials suitable for
vacuum work include silicone, Buna-N, Ethylene-Propylene, Kalrez and FETFE, the later two being the most expensive.
Although tempting to use because of price and hardware store
availability, elastomers such as neoprene should not be used
due to it's poor compression set recovery and loss of mass
after exposure to hard vacuum for extended periods.
STOPCOCK O-RING GREASE / LUBRICANT
SVP Neon Equipment strongly recommends not using silicone
based stopcock grease, such as Dow Corning silicone high
vacuum grease, or fluorinated grease such as Krytox® or
similar products.
Silicone based grease, when used in glass vacuum apparatus,
will polymerize to the glass as a non-conducting film that
can allow static charge build-up. This invisible film will
migrate through the vacuum system, trapping contaminants as
it does so and preventing efficient pump-downs. This
polymerization also makes it extremely difficult to clean
the silicone film off the glass when the system is
dismantled for cleaning or repair, even though the
contamination is not visible to the eye.
Further, if a glassware repair is necessary the silicone
film must be completely removed by chemical means if a
long-term successful repair is to be expected. The often
used method of attempting to burn off the silicone film in
an annealing oven prior to repair can worsen the situation
as many times the oven temperature is too high and will
literally burn the film into the structure of the glass,
thereby making any future repair questionable at best. If
not removed by chemical means prior to repair and annealing,
the silicone film will be sealed into the glass where the
repair is made. This is sometimes evident by a "cloudy" area
adjacent to the repaired section. An example of such a
repair may be seen
Here. Glass that is repaired in this
manner and fused together with a silicone film present will
always be a potential leak in the area of the repair. The
repaired area may or may not leak. If it does, the leakage
and subsequent "pin hole" may not be evident for months, or
even years after the repair is performed..
Fluorinated grease such as Krytox®, is extremely resistant
to almost all solvents and chemicals, which is exactly what
it was designed to be. This makes it very difficult to
remove from glassware as well and, therefore, poses many of
the same problems encountered with silicone when attempting
to clean or repair glassware that has been exposed to it.
SVP highly recommends using a hydrocarbon based high vacuum
grease, such as the various Apiezon blends or Lubriseal®.
These products are available from SVP Neon Equipment as well
as other suppliers.
Hydrocarbon based grease or lubricant, such as the various
Apiezon blends, Lubriseal® and similar products, is easily
removed from the glass surface with a number of different
solvents when manifold components need to be cleaned. If a
repair is required it is not necessary to chemically
pre-clean the glassware prior to repair as it is with
silicone or fluorinated based greases. Annealing the
glassware prior to repair will completely remove all grease
and residue without fear of compromising the surface of the
glass, as in the case with silicone or fluorinated grease. |
