We know that ultraviolet light from the Sun can be harmful to our skin, but it can also damage plastics. UV light is made up of high energy photons, and when they slam into molecules they can break the bonds that hold them together. This process is known as UV degradation. Plastics are made from long chains called polymers and, when UV light hits, these can start to break apart.
The chains are chopped into shorter fragments, and this has disastrous effects on the structure of the plastic, causing it to become weakened, brittle and discoloured. Different types of plastic are damaged by sunlight in different ways. PVC, as found on window frames, can become chalky as the top layer of plastic breaks down to reveal particles that were once trapped inside its structure. Polystyrene, on the other hand, goes yellow, and gradually loses its elasticity.