Have you ever taken a component off the shelf, only to discover that the O-ring on it is full of cracks? Have you ever wrapped some pencils in a rubber band, only to have the rubber band develop cracks and eventually break? You may have witnessed ozone cracking, or “ozonolysis”.
We use O-rings every single day, and most of us don’t even know it, until one of them starts to leak. One common cause of leakage in O-rings is ozone cracking. On a manufacturing floor, ozone cracking can be a source of aggravation. In the field, it can cause leaks, which may lead to serious injury or death. Fortunately, this is a very preventable phenomenon. In this blog, you will learn about the causes of ozone cracking and how you can prevent it.
What causes ozone crackingOzone cracking occurs mostly with O-rings made from nitrile rubber. If you have previous experience with O-rings, you may recognize this material as nitrile, buna, or buna-N. This material is called a polymer, which is Greek for “many units”. Each molecule consists of long chains, of individual units, which are bonded together into a long chain. In the case of nitrile rubber, the repeating unit, or link, is shown in Figure 1.
Figure 1. Repeating unit for nitrile rubber
The C, H, and N represent carbon, hydrogen, and nitrogen, while the dashes represent single, double, and triple bonds. Of particular interest is the double bond between the second and third carbon atom. This double bond is a "weak spot" within the molecule. Ozone can "donate" an oxygen atom and break this chain, as shown in Figure 2.
Figure 2. Ozone cracking at the molecular level
The two oxygen atoms are connected to the carbon atoms on either side, but they are not connected to each other. Because of this, the polymer chain is literally cut, forming a tiny crack in the O-ring. As more of these polymer chains are cut, the cracks get bigger and bigger, until they can be seen with the naked eye.
Figure 3. Ozone cracking in nitrile O-ring
How do I prevent ozone cracking in O-rings?
Oxygen is present in the air that we breathe, and is necessary for life on Earth. Oxygen atoms typically join up in pairs, forming dioxygen. This makes up the vast majority of oxygen in the atmosphere. Occasionally, oxygen atoms join in groups of three. This creates a substance called ozone.
As a general rule, ozone is good up high, but bad nearby. In our stratosphere, ozone concentration is between 2 and 8 parts per million. This forms the fantastic blue ozone layer that protects us from the sun’s harmful rays. However, in the troposphere, or the air that we breathe, concentrations above 75 parts per billion can cause health problems, according to the EPA (even though their own scientists recommended 60 parts per billion). Even very tiny concentrations such as these can cause ozone cracking in nitrile O-rings.
In general industrial applications, the primary drivers of ozone concentration are ultraviolet light, electrical arcing, and electromagnetic fields (which are the main reasons for higher ozone concentrations in the stratosphere).
Recommendations for storing O-rings- Keep O-rings away from ultraviolet light. The most common sources are direct sunlight, and fluorescent light bulbs.
- Do not store O-rings within 6 feet of an electric motor, or other potential sources of electrical arcs.
- Do not store O-rings in a stretched state. O-rings typically need to be stretched for ozone cracking to occur.
- When using nitrile O-rings on fittings, we recommend installing them into the mating part within 24 hours of installing the O-ring on the fitting. If O-rings must be stored in a stretched state, store them in an airtight bag until ready to use.
- Assemble nitrile O-rings wet with a grease to protect from ozone.
In applications where long-term environmental exposure is inevitable, we recommend using an ozone-resistant material, such as HNBR, EPDM, or fluorocarbon.
If you have any questions, please feel free to contact Parker applications engineering or check out our interactive O-Ring eHandbook.
This article was contributed by
David Mahlbacher, Parker O-Ring Division, Applications Engineer
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A Simple Guide to Selecting an O-Ring
How to Avoid Critical Component Failure in the Oil & Gas Industry