We often cover our leftover food with a clear plastic wrap. It’s made from a large molecule called vinylidine chloride.
It’s ability to cling to containers depends partly on it’s elasticity. It’s elastic because vinylidine chloride consists of long chains of carbon atoms, which are coiled and kinked like fibres in a piece of wool. When the film is stretched, the molecules straighten out, but, like rubber, they try to return to their original state when released.
As well as being elastic, cling film has an unusual ability to stick to surfaces by electrostatic attraction. — the same force that encourages dust to stick to your TV screen. As cling film is peeled from the roll, it acquires a static electric charge through friction. You can sense the charge when you hold the film near your face.
The electrostatic charge on the film causes any insulator it touches, like plastic or glass, to acquire an opposite charge. The surfaces then stick together because of the attraction between positive and negative charges. When the container being covered is made of metal, which is a conductor, the plastic film won’t stick. This is because the charge on the film is dissipated through the container, thus cancelling the electrostatic attraction.
So next time you cover leftovers with cling film, remember: use a glass or plastic container if you want the cover to be airtight.