Ever seen old tires, the rubber in them dried and deeply cracked? They were ripped apart not by the road, but by a greenhouse gas.

Rubber gets a lot of credit for stretching, but that's not really its talent. A lot of materials stretch — plastic, clay, human skin — but the remarkable thing about rubber is that it goes back to its original shape after it has been stretched. It does this because it is a huge web of connecting polymers. A tire is made up of chains of carbon and hydrogen, crosslinked by sulfur atoms. These connections are so extensive that some people consider a tire to be one big molecule.

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Ozone breaks up that molecule. Oxygen is known to travel in atomic pairs, but when a pair is suddenly split, it can can recombine with other oxygen pairs to form three oxygen atoms, all linked together. This triple oxygen, ozone, is reactive, and will insert itself into existing molecules. When it inserts itself into a long chain of rubber, it splits the thing in two. Instead of one carbon atom connecting to the next in a near-endless series of links, two carbon chains are capped by oxygen atoms. When enough chains get broken, deep cracks start appearing in the tire. It's called ozonolysis — "lysis" being the word for "separation" or "rupture."

Modern tires don't see much ozonolysis. Antiozonants, some as simple and cheap as wax, coat the material and protect it from the gas. But adding antiozonants isn't always feasible. This poses real problems for industrial seals, valves, or tubing, because it's harder than it seems to filter out ozone. Any quantity of oxygen will contain some ozone, and even if we eliminated most human sources of ozone, a machine can make its own. Before humans came along, ozone was created at ground level whenever lightning struck. The lightning bolt would split regular oxygen pairs, which would recombine into ozone. That happens in miniature whenever static charge sparks in a machine. Too much static, or a bad connection, and machines with rubber seals or gaskets make the gas that destroys them.

[Via Introduction to Forensic Engineering, Rubber Additives, Sidewalls and Ozone, One Big Happy Molecule]

Top Image: National Archives.