As the most commonly used seals in almost every industry you can think of, from chemical processing, fluid power, and aerospace, to life sciences, pulp and paper, as well as energy oil and gas, O-rings are used vigorously every single day. Yet, more often than not, most users don’t even realize that one might be damaged until the O-ring actually starts to leak. One of the most common causes of leakage in O-rings is ozone cracking, or “ozonolysis,” which occurs mostly with O-rings made from nitrile rubber.
On the manufacturing floor, ozone cracking can be a mere source of aggravation, but when in the field, it can cause leaks that lead to serious incidents of death or injury. Fortunately, this phenomenon of crackage and leakage is highly preventable, just as long as you understand the causes of why it happens and how to take certain measures to combat it.
Why an O-Ring Fails
Within the service field, the failure of an O-ring can usually be attributed to a combination of causes. First and foremost, it is important to maximize sealing life and reliability by reducing the probability of seal failure at the onset by using good design practices, proper compound selection, pre-production testing, and continued education and training of assembly personnel.
As mentioned, Ozone cracking occurs mostly with O-rings made from nitrile rubber (or with material most recognized as nitrile, buna, or buna-N).
This material is called a polymer, which is Greek for “many units,” with each molecule consisting of individual units that are bonded together into a long chain. Sometimes, the links in a nitrite polymer chain can develop “weak spots” and become broken due to oxygen atoms contributed by the ozone. Hence, polymer chains can literally be cut, forming a tiny crack in the O-ring that continues getting bigger and bigger until the damage is large enough to be seen with the naked eye.
Preventing Ozone Cracking in O-Rings
Since oxygen is ever present in the air that we breathe and is necessary for life on Earth, its presence among O-rings is quite inevitable.
Oxygen atoms typically join up in pairs, forming dioxygen, which go on to make up the vast majority of oxygen in the atmosphere. When oxygen atoms join in groups of three, an ozone substance is created in the air we breathe, which can not only cause health problems, according to the EPA, but can also cause ozone cracking in nitrile O-rings even at the very tiniest of concentrations levels.
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), so in order to prevent further O-ring cracking or damage to occur, it is vital to:
• Keep O-rings away from ultraviolet light when storing, as the most common sources are direct sunlight, and fluorescent light bulbs.
• To not store O-rings within six feet of an electric motor (or other potential sources of electrical arcs).
• To not store O-rings in a stretched state because O-rings typically need to be stretched for ozone cracking to actually occur.
Further, when installing O-rings, remember to assemble nitrile O-rings wet with a grease to protect from ozone and to install them into the mating part within 24 hours of installing the O-ring on the fitting. If O-rings have no choice but to be stored in a stretched state, remember to store them in an airtight bag until ready to use.
In applications where long-term environmental exposure is inevitable, it is also recommended to make use of ozone-resistant material, such as HNBR, EPDM, or fluorocarbon.
For additional information and resources regarding the causes of O-rings cracking and how to combat it, contact Real Seal today.
