Acrylic acid

Acrylic acid is a small molecule of three carbon atoms, with a double bond at one end and an acid group at the other (CH₂=CHCOOH). It's a clear liquid with a sharp, acrid smell. The double bond lets it link into polymers, and the acid end lets it be turned into esters with all sorts of properties.

It's made from propylene and oxygen from the air, in two steps. First, propylene and air pass over a hot mixed-metal oxide catalyst based on bismuth and molybdenum, which adds one oxygen and makes acrolein, a reactive aldehyde. Then the acrolein passes over a second catalyst, based on molybdenum and vanadium, which adds another oxygen and turns it into acrylic acid. The acid is separated and purified, and a stabilizer is added, because pure acrylic acid can polymerize on its own, sometimes violently.

Its biggest single use is superabsorbent polymer, the gel in disposable diapers that soaks up many times its weight in water. For a TV, it's the building block of acrylate resin, used in color filters, optical films, adhesives and solder mask.