An o-ring is a small component, but an o-ring crack can have major consequences. Unfortunately, you often won’t notice one has a problem until it starts to leak. Discovering an o-ring problem on a manufacturing floor can be aggravating and expensive, but experiencing an o-ring problem in the field when a car or plane engine or medical seal doesn’t work properly can be potentially deadly.
Fortunately, the most common cause of o-ring cracks can be avoided by taking some simple preventive measures. Here’s what you need to know about what causes cracks, how they’re designed to prevent cracks and what you can do when selecting, storing and using o-rings to keep them from cracking.
What Causes O-Ring Cracks
O-ring cracks usually occur when o-rings made out of nitrile rubber (Buna-N), one of the most common forms of synthetic rubber, are exposed to ozone. This produces a chemical reaction called ozonolysis, or ozone cracking.
Nitrile rubber is made out of long polymer chains that include a double bond between two carbon atoms. This bond forms a weak link in the chain when it is exposed to ozone. Ozone contains three oxygen atoms, and can donate one of its oxygen atoms to become a more stable diatomic oxygen molecule. When two ozone molecules donate two oxygen atoms to nitrile rubber’s double carbon bond, the oxygen atoms bond to the carbon atoms but not to each other. This breaks the double carbon bond, splitting the polymer chain that forms the nitrile rubber in half.
Just a trace amount of ozone can initiate ozone cracking in nitrile rubber. Initially, this creates small, microscopic cracks perpendicular to the direction of stress. Eventually, the cracks grow big enough to become visible.
There are three main ways that nitrile rubber gets exposed to ozone during industrial applications. Ultraviolet light with wavelengths of 100 to 240 nano meters will break down oxygen molecules through photolysis, creating ozone. Unsuppressed electrical arcs can similarly separate oxygen molecules, producing ozone. Electromagnetic fields can also create ozone by ionizing oxygen molecules.
How Manufacturers Prevent O-Ring Cracks
Ozone cracking can often be prevented by strategic material selection. For instance, ozone cracking used to occur in tires as well as o-rings, but tire manufacturers began adding antioxidants to rubber prior to vulcanization, which vastly reduced tire cracking.
Similarly, o-ring manufacturers have begun using ozone-resistant rubbers for applications where there is a risk of ozone exposure. For instance, o-rings made of fluorocarbon such as Viton have fewer double carbon bonds than nitrile rubber, making them less susceptible to ozone cracking. EPDM rubber and polychloroprene rubbers such as neoprene are also ozone resistant. Talking to your o-ring supplier about which material would be best for your application can help prevent o-ring cracks.
How You Can Prevent O-Ring Cracks
In addition to choosing ozone-resistant materials, users can help prevent ozone cracks through proper storage and handling of o-rings. O-rings should be kept away from ultraviolet light by storing them out of direct sunlight and away from fluorescent light bulbs. O-rings should be stored at least 6 feet away from electric motors and other possible sources of electrical arcs. O-rings should also not be stored in a stretched state, as stretching can promote cracking. If you cannot avoid storing o-rings in a stretched state, keep them in airtight bags until ready for use.
How you handle o-rings during installation can also prevent cracking. When assembling nitrile o-rings, use grease to protect them from ozone. Install o-rings into their mating parts within 24 hours of installing them onto their fittings.