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Slowly but surely, any rubber object, be it your car windshield wiper blades or the elastic waistband of your favorite pajama pants, will harden over time. Evidence of white or colored substances on the rubber can be one indication of a chemical reaction taking place. Natural rubber has poor resistance to both ozone and UV light, as well as to petroleum oils. Additionally, temperatures below -60F (-51C) or above 220F (104C) will degrade it quickly, while temperature extremes in general will facilitate long-term degradation and hardening. Rubber stretches and flexes because it is made up of long, tangled molecule chains that can be straightened out under tension. Degradation of these molecular chains can occur from repeated or excessive tension (think of frayed or over-stretched rubber bands), or by exposure to elements like those noted above. Cracked rubber will remain cracked even when it's softened. Once cracks form, your only options will be to try out a rubber patch kit or to simply replace the rubber object. There is no miracle cure by this point. One point worth making here is that common softening methods heat and oils actually damage the rubber in the process. So, every time you do soften up a stiff rubber gasket or shoe sole, you are also contributing to its eventual demise. There is only so much you can do to keep oxygen, temperature fluctuations, and light away from rubber, but wiping away residue can help keep deteriorating oils from damaging the object. Clean rubber objects with only warm water and a clean cloth when possible. If necessary, use a mild detergent and rinse it away completely. Solvents found in cleaners can degrade or even dissolve rubber. Avoid washing clothes with elastic waistbands more than needed, and don't use hot water. If possible, remove as much air (and thus oxygen) from the container as you can before sealing it. Placing rubber items in zip-close bags and using a straw to suck out most of the air can noticeably delay the hardening process. If you've ever compared rubber bands kept in a plastic bag in your junk drawer from those loose in the drawer, you know the difference it can make. Natural rubber is especially susceptible to oxidization because sulfur is added to the mix when making items like rubber bands. Oxygen reacts with sulfur and essentially removes it from the item, leaving it more brittle. The same place you might store apples or potatoes would actually make a decent place to also keep your bagged-up sneakers. 68 to 77F (20 to 25C) is a sensible temperature range for storing rubber items, though lower temperatures can also be good. You might even consider keeping tightly sealed rubber items in your refrigerator. The excessive cold and higher humidity of the freezer, however, makes it a less ideal choice.
Check rubber objects regularly for hardening. Examine the rubber for signs of damage. Keep rubber objects clean. Seal rubber in an airtight container to delay hardening. Place the container in a cool, dry, dark place.