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What Causes O-Ring Cracks?


O-ring cracks are tiny, but they can cause massive problems. A nuclear power plant in Indian Point, New York, closed down unexpectedly in November as a result of a recurring issue with o-rings which are used to secure the seal between the plant’s reactors and the reactor vessel bodies, where fission takes place. O-ring problems at the plant have caused eight water leaks since 2003, prompting Nuclear Regulatory Commission intervention.

O-ring cracks can also affect car engines, household appliances, business machinery and other equipment. Fortunately, the main cause of o-ring cracks is known, and cracks can usually be prevented by following some basic guidelines. Here’s a look at what causes o-rings to crack, what steps manufacturers take to eliminate cracks and how you can prevent cracks by proper selection, storing and use of o-rings.

Why Do O-Rings Crack?

The main culprit in o-ring cracking is a chemical reaction between synthetic rubber and ozone known as ozone cracking, or ozonolysis. Ozone cracking typically happens when o-rings constructed from nitrile rubber (Buna-N) are exposed to ozone.

Nitrile rubber is composed of long polymer chains which are held together by a double bond connecting two carbon atoms. Ozone can break this double bond. Ozone, formed by three oxygen atoms, is a volatile molecule that tends to stabilize by donating one of its oxygen atoms to become a diatomic molecule. If a pair of ozone molecules donates a pair of oxygen atoms to the carbon double bonds in nitrile rubber, the atoms attach to the carbon atoms but do not connect to each other. This effectively cracks the carbon bond, breaking the nitrile rubber polymer chain in half. It only takes a small amount of ozone to create cracks in nitrile rubber. The cracks start small but quickly grow, and eventually they become large enough to see.

Ozone cracking usually occurs when nitrile rubber gets exposed to ozone in one of three ways. Ultraviolet light with short wavelengths less than 240 nanometers can cause a reaction called photolysis, which splits up oxygen molecules and generates ozone. Oxygen molecules can get separated in a similar manner by unsuppressed electrical arcs. Oxygen molecules can also form ozone after being ionized by electromagnetic fields.

How Strategic Manufacturing Design Can Keep Ozone From Cracking

Smart material selection can prevent ozone cracks. Car tires used to have the same problem with ozone cracking that occurs in nitrile rubber o-rings, but manufacturers discovered they could reduce cracks by introducing antioxidants before the vulcanization process.

Similar measures can be used to prevent o-ring cracks. For example, manufacturers of customized o-rings offer products made of materials such as FKM, which has a lower number of double carbon bonds than nitrile rubber, reducing risk of ozone cracking. Neoprene and EPDM rubber are other specialized rubbers that have higher ozone resistance than nitrile rubber. A knowledgeable o-ring specialty provider can help you select a crack-resistant material.

Proper Storage and Handling to Prevent O-Ring Cracks

O-ring users can take a number of proactive steps to prevent cracks. In addition to selecting materials that are resistant to ozone, good storage and handling procedures can reduce crack risks. O-rings should be stored away from common causes of o-ring cracking. They should be protected from ultraviolet light by keeping them out of direct sun exposure as well as fluorescent light bulbs. O-rings should be kept at least 6 feet away from electrical arc sources such as electric motors. They should be stored in an unstretched state, since stretching can also cause cracks. O-rings that must be stored stretched should be protected in airtight bags when not in use.

Handling procedures can also help prevent cracks. During installation of nitrile o-rings, applying grease can provide ozone protection. Set rings into their mating components within 24 hours of installation for best results.

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