WHY DO PLASTICS BECOME BRITTLE WHEN EXPOSED TO THE SUN FOR A LONG PERIOD?

Plastics are made of polymers which are giant molecules having long chains of repeating units derived from short molecules. These long chains are entangled with one another and held together by weak interatomic forces such as Vander Waals force. These weak bonds can be easily broken up by sunlight and so long exposures to sunlight makes them brittle.