Rubber bands are one of our most ubiquitous fasteners. At the same time, they are perhaps the simplest in form. Unfortunately, the properties that make the rubber band flexible and useful as a fastener also contribute to its relatively short lifespan.
Rubber Band Composition
Rubber bands belong to a class of materials known as elastomers. According to the Syracuse University Department of Physics, elastomers comprise “coiled up flexible polymers that are 'crosslinked' by some chemical agents." Because of these polymers, a rubber band behaves as an “entropic spring,” meaning that it is “happiest” — or most stable — when at rest.
Heat and Strain
Unlike a paper clip or staple, a rubber band comprises material that automatically returns to its original shape after being deformed. When rubber bands are heated by sunlight or a hot lamp, the polymers contract, which makes them less flexible and more brittle. Stretching a rubber band induces "strain crystallization," which also stiffens the polymers. Therefore, simply using a rubber band eventually causes it to become brittle and break.
Light and Oxygen
In addition to being sensitive to heat, natural rubber bands are sensitive to light. According to Arizona State University's In-Visee website, natural rubber is “too soft” and tends to “degrade as it ages." Along with heat and strain, ultraviolet light and oxygen are the primary agents of this degradation.