Rubber is probably one of humanity’s greatest inventions, but chances are something made of rubber has let you down at some point in your life. Maybe the tires on your classic muscle car were fallingapart after the vehicle sat in your garage for years, or (a more likely scenario) a rubber band you’d relied on to hold some important items together in storage dried up and broke apart.
Dry rot is the nemesis of all things rubber, but at the risk of sounding like Dr. Suess, rubber “dry rot” is rot that is not — dry rot, that is. Real dry rot is caused by a fungus that attacks organic materials containing cellulose, like wood or cotton. The cracking, flaking and decaying that can happen to old car tires, rubber bands (and other items made with either natural or synthetic rubber) may look like true dry rot, but a fungus is not to blame.
To understand what’s really happening when rubber decays, it’s important to understand what it’s made from. Natural rubber has been around for about 1,000 years and it’s made from latex harvested from a variety of plants, but mostly from — no surprise — the rubber tree. Isoprene molecules bond together to create a polymer whose molecular chains can be easily pulled apart, but also spring back together quickly and easily, which is why rubber is so elastic and useful.
Synthetic rubber is a newer invention, one we had to discover because we could probably never grow enough latex-producing plants to supply the amount we’d need to manufacture all the rubber the world makes and uses in a year. Polymerizing petroleum-based monomers like styrene, butadiene, isoprene, choloroprene and isobutylene allow us to produce synthetic rubber. It’s every bit as good as the natural stuff in terms of elasticity and is actually more heat-resistant than natural rubber. Of course, during the polymerization process, rubber-makers can incorporate a range of additives into their formulation to impart certain properties to the rubber — such as carbon black to make rubber black for use in tires or heat stabilizers to make rubber resistance to damage caused by high temperatures.
So how is it that such a marvelous substance — one that humanity has had a millennium to perfect — can still fall prey to rot?
Well, rubber, whether natural or synthetic, is a polymer and its chains of molecules are vulnerable to many of the same factors that affect all polymers — ultraviolet radiation, temperature extremes, ozone and oxidation.
Rubber that gets regular use will retain its flexibility and molecular properties longer than neglected rubber. For example, although they’re exposed to UV rays daily, the tires on the car you drive regularly are actually more resistant to rot than tires hidden away in a dark garage. During the manufacturing process, the tire-maker adds a protective compound to the rubber formulation. Regular use causes the tire to flex and compress, pushing this protective substance to the surface. This doesn’t happen in an unused tire, so the rubber is more vulnerable to damage from ozone and oxidation.
Lack of use, exposure to extremely high or low temperatures, or prolonged exposure to UV radiation can all cause rubber’s molecular chains to break down over time — and time itself is a factor, too. Rubber is one polymer that just doesn’t last forever, and its descent into decay begins the moment it’s manufactured.
So take your classic car out for a spin every now and then; its tires will thank you.