SUBJECT : The super rubber compounds 10-6

When you are selecting an o-ring, or any other elastomer shape for your mechanical seal application remember that with the exception of solvents, most chemicals and chemical compounds can be successfully sealed with either ethylene propylene or a good grade of Viton® as the dynamic (moving) elastomer.

Most mechanical seal designs incorporate both dynamic and static elastomers. Dynamic o-rings are required to flex and roll with the shaft movement. This means that a very low shaft squeeze is important to prevent seal "hang up" or hysteresis. They must also be free to flex and roll to compensate for mechanical seal face wear. Static o-rings do not have to move. They are used as a gasket and are a lot more forgiving than dynamic o-rings because a small amount of swell can be tolerated that might even improve their sealing.

There are many elastomer shapes available to you. Individual seal companies use wedges, V-rings, U-cups, Quad rings etc, but o-rings have a lot of advantages over these other elastomer shapes in mechanical seal design.

As an example:

In recent years the elastomer industry has produced a variety of newer compounds that appear to be getting closer to the universal rubber that we are all seeking. Unfortunately we are not there yet, so this paper is an attempt to put these "super compounds" into a proper perspective. There are several of these compounds that you should know about.

KALREZ®, a Dupont product that is not a true elastomer so you will experience some "compression set" depending upon the compound you select. You have a few choices:

The following compounds are also exhibited on the Special Elastomers chart .

To be classified as a true elastomer you should be able to compress the O-ring and have it return to 90% of its original shape in less than five seconds after the compression force is removed. It is this elasticity that gives the compound its memory and eliminates the need for spring loading the elastomer to the seal shaft or sleeve. If the compound does not return to 90% of its original shape in five seconds or less it is called a "plastic" and becomes less desirable as a dynamic seal in mechanical seal design. Many of these "super compounds" are plastics and present sealing problems in some seal configurations. You are going to have to depend upon your experience to select individual seal designs that work well with these materials.

Some distributors of these compounds recommend the use of mechanical seals with spring loaded dynamic O-rings. They do this to booster their sales of the compound. They forget to mention that when you spring load one of these compounds you will experience shaft fretting under the O-ring. This shaft fretting increases the probability of seal failure, and dictates the use of shaft sleeves that raise the L3/D4 rating of the shaft, contributing to excessive shaft deflection.

There are many selection charts available to help you pick the correct elastomer compound for your application. Unfortunately your fluid may not be shown on some of these charts and the temptation is to go to one of the super compounds for the solution. At other times you will tempted to standardize on a super compound to avoid the selection process altogether. The attached chart will help you to avoid a mistake in both of these instances.

The Special Elastomers chart link at the bottom of this page is unique in that it shows you where these "super compounds" cannot be used. This does not imply that if the chemical is not listed, or if no notation is made, that the compound is suitable for your service. It means nothing more than what it says. These are the chemicals that each manufacturer has designated as not suitable for a dynamic O-ring application.

If there is any question about the use of one of these compounds in a given service you can test the compound by immersing the O-ring in the fluid to be tested for about ten days to two weeks. If the fluid is going to attack the compound, the O-ring it will change weight, shape, or appearance. If the application is going to be at a hot temperature, you might want to put the test container in an oven to duplicate the seal operating conditions.

You can look to see if your chemical is listed in the charts. Just click on the appropriate box:


®DuPont Dow elastomer

For information about my CD with over 600 Seal & Pump Subjects explained, click here   

 Link to Mc Nally home page