Polymers used in tires are susceptible to damage from ultraviolet light and ozone. To protect tires from UV exposure, manufactures add a sacrificial element, usually carbon black, to the polymers.
A UV stabilizer, carbon black absorbs UV rays and converts them into heat, which dissipates harmlessly. As the carbon black is sacrificed over time, the tire turns gray.
You can buy UV-protection products for spares or unused trailer tires, but these products must be reapplied every three months as they are sacrificed to UV light.
Ozone in the air can also deteriorate your tires. To inhibit ozone damage, tire manufacturers mix a special wax with the polymer. As the tire flexes with use, the wax is slowly squeezed to the surface, forming a barrier between the rubber and the ozone. The sacrificial wax layer is slowly eaten away by the ozone.
When ozone reaches the polymer on an unused tire, it dries, checks and cracks the surface, eventually causing the tire’s sidewall to fail. This is why a trailer’s spare tires deteriorate while the ones moving and flexing on the ground remain in good shape. It’s also what causes an unused trailer’s tires to go bad faster.
Tire sellers sometimes coat tires to make them shiny and black, but these petrochemical- and silicone-containing products can remove the tires’ protection and hasten their demise. They may also void the tires’ warranty.
Keeping a tire inside, away from UV light, doesn’t protect it from ozone. This is why a store tire has a shelf life of about six years and shouldn’t be sold after this.
You can find out when a tire was made by looking at the tire identification number on the sidewall. For example, the number DOT MK87 FOWR 4202 tells you the tire met Department of Transportation (DOT) safety standards; MK is the plant where it was made; 87 is the tire size; and FOWR is the brand.
The last three or four numbers indicate the manufacture date. The first two numbers are the week it was made, and the next one or two indicate the year. For example, 4603 is the 46th week of 2003, and 358 is the 35th week of 1998. Don’t buy a tire with three numbers as it was made in the ’90s. Purchase tires made in 2004 or later. –Dave Brinker