Polystyrene

Polystyrene is one of the most common plastics. Solid, it's clear, hard and a little brittle: the material of CD cases and disposable cutlery. Puffed up with gas, it becomes the white foam used for packaging and cups. Same polymer, very different results.

It's made from styrene, a benzene ring with a small vinyl group attached. The vinyl group has a double bond that can open. In a reactor, heat or a starter chemical called an initiator gets a few styrene molecules to open their double bonds, and each grabs the next, building chains thousands of units long. Plants usually run this continuously, then strip out leftover styrene and cut the polymer into pellets. The bulky benzene rings stick out at random along the chain and stop the chains packing into crystals, which is why polystyrene is clear.

For a TV, clear general-purpose polystyrene is extruded with light-scattering beads into the diffuser plate in the backlight. It's chosen for low cost and low moisture uptake. It softens at about 100 °C, well above the warmth behind a TV screen. It was first made in 1839 by the German pharmacist Eduard Simon and went into large-scale production in the 1930s.