Polyurethane began as a way around a rival's patents and ended up in everything
Otto Bayer's team in Leverkusen first made polyurethanes in 1937, partly because they sidestepped Wallace Carothers's patents on polyesters. Today the same chemistry gives soft mattress foam, rigid fridge insulation, shoe soles and stretchy spandex, and in 2019 the world made 25 million tonnes of it.
Polyurethane is not a single plastic but a family. Each is made by reacting an isocyanate carrying two or more reactive groups with a polyol, a molecule bearing several alcohol groups, and the two building blocks alternate along the chain. Swapping ingredients changes the result dramatically. Long, flexible polyol chains with little crosslinking give something very stretchy; short chains with dense crosslinks give a hard solid; long chains with moderate crosslinking make good foam. Most versions are thermosets that will not melt when heated.
Foam, the biggest use at 67 percent of output in 2016, depends on a clever trick. Adding a little water makes it react with isocyanate to release carbon dioxide, which inflates the mixture as it sets. If the bubbles burst but their walls stay stiff, the result is open-cell foam that breathes and suits cushions; if they stay intact, it becomes closed-cell foam for rigid insulation. Frothing the polyol mechanically instead yields tough, dense material for steering wheel covers.
Industry took off after the war. Polyisocyanates went on sale in 1952, flexible foam production began in 1954, and cheaper, more water-resistant polyether polyols from DuPont, BASF and Dow followed. By 1960 output of flexible foam topped 45,000 tonnes. In 1969 Bayer showed an all-plastic car in Dusseldorf built with reaction injection moulding, where reactants are mixed and shot into a mould, and a reinforced version of that process gave the United States its first plastic-bodied car, the Pontiac Fiero, in 1983.
Blowing agents have shifted with environmental rules. The Montreal Protocol curbed chlorine-containing agents such as CFC-11 in the early 1990s because they deplete ozone, and carbon dioxide, pentane and fluorinated substitutes took over in North America and Europe. The most widely used isocyanates, TDI and MDI, are aromatic and cheap but make polyurethane darken in light, so coatings that must stay clear use aliphatic types instead.
Source: Polyurethane
