VACUUM APPLICATIONS

Butyl rubber has long been the preferred material for vacuum applications. Among the rubber polymers used for seals, it has one of the lowest permeability rates for gases. This, together with the fact that butyl compounds have low outgassing or weight loss characteristics, good physical properties for a seal (compression set resistance), a useful temperature range (-65F to 225F) and good moisture resistance, has established this preferred position. The need for special environmental considerations in addition to low permeability will often change the recommendation. Service requirements such as high temperature, radiation resistance, long-term exposure to water or combinations of fluid media may take a careful study to ascertain the proper recommendation.

VACUUM WEIGHT LOSS. It is particularly important in many space and other vacuum applications that optical surfaces and electrical contact surfaces remain clean to serve their intended purpose. Some rubber compounds contain small quantities of oil or other ingredients that become volatile under high vacuum conditions and deposit as a thin film on all the surrounding surfaces. The table below indicates the weight loss of several Parker Seal compounds due to vacuum exposure. Where sensitive surfaces are involved, the higher weight loss compounds should be avoided.

In those compounds which show low weight loss, the small amount of volatile material that is indicated is primarily water vapor. It is not likely to deposit on nearby surfaces.

Compound Number	Polymer	Percent Weight Loss
B612-70	Butyl	.18
C873-70	Neoprene	.13
E515-80	Ethylene Propylene	.39
E529-60	Ethylene Propylene	.92
E691-75	Ethylene Propylene	.76
L499-65	Fluorosilicone	.28
L677-70	Fluorosilicone	.25
N406-60	Nitrile	3.45
N674-70	Nitrile	1.06
P648-90	Polyurethane	1.29
S455-70	Silicone	.03
S604-70	Silicone	.31
V747-75	Fluorocarbon	.09
V884-75	Fluorocarbon	.07
V894-90	Fluorocarbon	.07

VACUUM SEAL CONSIDERATIONS. The rate of flow of gases from the pressure side to the vacuum side of an elastomeric seal depends to a great extent on how the seal is designed. Compound B612-70 has been tested in face type O-ring seals, using grooves that provided 15, 30 and 50 percent squeeze. Lubricating the O-rings with a high vacuum grease also reduced the leakage of the light (15 percent) squeeze rings significantly, but the effect of the grease was considerably less at 30 percent squeeze. At 50 percent squeeze, the effect of the grease was not detectable. Several other compounds were tested in this way with similar results.

Increased O-ring squeeze reduces permeability by increasing the length of the path the gas has to travel (width of ring) and decreasing the area available to the entry of the gas (groove depth). Increasing squeeze also tends to force the rubber into any small irregularities in the mating metal surface, and thus prevents leakage around the seal. The vacuum grease aids the seal by filling these microscopic pits and grooves, thus reducing leakage around the ring, and, at the same time, it may be changing the surface tension favorably with the effect of a reduced rate of surface absorption.

It is recommended, therefore, that face type O-ring grooves be used whenever possible for static vacuum seals, using a silicone grease as a seating lubricant and surface coating in addition to a heavy squeeze of the O-ring cross section. When a radial seal is required, or when a heavy squeeze is not possible for some other reason, it becomes more important to use a vacuum grease.

As an example of the benefit of high squeeze, we have found that Gask-O-Seals and Integral Seals both make effective vacuum seals because of the generous squeeze that is built into them. Gask-O-Seals have the added advantage of a high percent fill of the groove together with a shallow depth which reduces the seal area that can be exposed to the effects of vacuum, and prevents the rubber sealing element from moving due to vibration or pressure changes. An additional benefit of high percentage confinement is the fact that increased temperatures do not increase the leak rate as much as normally expected with a lesser confinement.

Although a very heavy squeeze is necessary to reduce leakage to an absolute minimum in an O-ring seal, this kind of design may require heavy construction. When such a shallow gland is desirable, it must be wide enough to receive the full O-ring volume. In sealing gases and vacuum, it is quite feasible to use two O-ring seals in tandem, unlike reciprocating applications that seal a liquid, where pressure traps are often a problem.

Surface roughness of the gland surfaces is more critical in sealing pressurized gases or vacuum, as a gas will find its way through extremely minute passages. Therefore, surfaces against which an O-ring must seal should have a surface roughness value smoother than usual. Surface finishes of 16 RMS are quite common, but 32 RMS finishes have been used successfully too.

WATER AND STEAM RESISTANCE. Water seems like such an innocuous fluid, people are often surprised to learn that it can bring problems if it is not sealed with the proper O-ring material.

After a long period of water immersion, many compounds will swell quite drastically. In a static seal, this may be quite acceptable. Such a seal surely will not leak, and if it can be replaced with a new one after disassembly, the fact that it has become too large to put back into the gland cavity becomes only an interesting curiosity. In situations where the O-rings are routinely replaced before they have swelled more than a few percent, the user may not even be aware of their strange behavior. Used as a long term dynamic seal, however, this gradual swell of many compounds in water can cause a slow but annoying increase in friction.

Compound E540-80, ethylene propylene rubber, is a compound that is tested with virtually no swell. This is our recommended compound for water and steam to temperatures up to 300F. Where exposure to steam and hot are alternate, as in tire presses, it serves better than in either one alone. Compound E515-80 is equally good in these applications, and is often more readily available.

For even greater resistance to steam, Parker has developed compound E962-90. This ethylene propylene compound showed very little change in physical properties after 70 hours exposure to steam at 550F. (When sealing steam or water with ethylene propylene rubber, it is important to remember that it will deteriorate when exposed to petroleum lubricants. When lubrication is required, silicone oil, glycerin, or ethylene glycol are suggested.)