Clingfilm clings for two reasons: when stretched, its elasticity makes it try to -return to its original size; and it holds a ;static electrical charge, which creates a form of attraction to many other things.
The key to the film’s elasticity lies in its molecular structure. Plastics consist of molecules which are ‘long’ – hundreds of thousands of repeated units of one carbon and two hydrogen atoms in a molecule of polythene, for example. Most common substances are made up of small molecules – a molecule of water has only two hydrogen atoms and one oxygen atom.
The long molecules of clingfilm are coiled and kinked, like the fibres in wool. When the film is stretched, the molecules straighten out. But, like wool fibres or an elastic band, the molecules try to return to their original state.
The stickiness of clingfilm occurs naturally in most thin plastic films. They stick because they acquire a static electric charge. The film can obtain, for example, a negative electric charge through friction displacing electrons from the surface of an adjacent piece of film or other material. This will result in a positive electric charge on the second surface, and the two will be bonded by the electrical attraction.
Clingfilm can be made from either of two plastics: PVC (polyvinyl chloride) and polythene. The chemical PVC is normally hard, but it is softened by the addition of a substance called a plasticiser. Polythene is naturally soft and requires no plasticiser.
PVC film is clearer than polythene film, but it suffers more from ‘fatigue’. Within 24 hours of use it loses over two-thirds of its elasticity, while polythene loses only one-third.