Polyurethane

Polyurethane (PUR or PU) isn’t a single plastic like polyethylene or polystyrene; it’s a whole family of polymers. What ties them together is that their molecular backbones are linked by carbamate (urethane) bonds. Because manufacturers can pick from many different starting ingredients, polyurethanes come in a wide range of chemical forms. That variety leads to an equally wide range of uses: rigid and flexible foams, coatings, adhesives, electrical potting compounds, and fibers like spandex and polyurethane laminate (PUL). Foams dominate the market—in 2016, they made up 67% of all polyurethane produced.

To make a polyurethane, you typically react a diisocyanate (or a polymeric isocyanate) with a polyol. Since the polymer forms by alternating these two types of monomers, it’s classified as an alternating copolymer. Both the isocyanate and the polyol must have at least two reactive groups per molecule. In 2019, global production hit 25 million metric tons, which was about 6% of all polymers made that year.

The story starts in 1937, when Otto Bayer and his team at IG Farben in Leverkusen, Germany, first created polyurethanes. These new materials had advantages over existing plastics made from olefins or polycondensation, and they didn’t infringe on Wallace Carothers’ polyester patents. Early work focused on fibers and flexible foams, and during World War II, polyurethanes saw limited use as aircraft coatings. Commercial polyisocyanates became available in 1952, and by 1954, flexible polyurethane foam was being made from toluene diisocyanate (TDI) and polyester polyols. That same chemistry also produced rigid foams, gum rubber, and elastomers. Linear fibers came from hexamethylene diisocyanate (HDI) and 1,4-butanediol (BDO).

In 1956, DuPont introduced polyethers, specifically poly(tetramethylene ether) glycol. The next year, BASF and Dow Chemical brought polyalkylene glycols to market. These polyether polyols were cheaper, easier to handle, and more water-resistant than polyester polyols. Union Carbide and Mobay (a U.S. joint venture between Monsanto and Bayer) also started making polyurethane chemicals. By 1960, more than 45,000 metric tons of flexible polyurethane foam were produced each year. The arrival of chlorofluoroalkane blowing agents, inexpensive polyether polyols, and methylene diphenyl diisocyanate (MDI) let rigid polyurethane foams serve as high-performance insulation. In 1967, urethane-modified polyisocyanurate rigid foams hit the scene, offering better thermal stability and flammability resistance. During the 1960s, automakers began using semi-rigid foam to back-fill thermoplastic skins for interior safety parts like instrument and door panels.

In 1969, Bayer showed off an all-plastic car in Düsseldorf, Germany. Parts like the fascia and body panels were made using a new process called reaction injection molding (RIM), where reactants were mixed and then injected into a mold. Adding fillers such as milled glass, mica, or processed mineral fibers created reinforced RIM (RRIM), which improved stiffness, reduced thermal expansion, and boosted thermal stability. This technology led to the first plastic-body car in the United States—the Pontiac Fiero, launched in 1983. Even greater stiffness came from placing pre-formed glass mats into the RIM mold cavity, a method known as resin injection molding or structural RIM.

Starting in the early 1980s, water-blown microcellular flexible foams replaced PVC polymers in molded gaskets for automotive panels and air-filter seals. Polyurethane foams are now common in vehicle seating, head and arm rests, and headliners. Foam (including foam rubber) sometimes uses small amounts of blowing agents to create a less dense material with better cushioning, energy absorption, or thermal insulation. In the early 1990s, the Montreal Protocol restricted many chlorine-based blowing agents like trichlorofluoromethane (CFC-11) due to ozone depletion. By the late 1990s, blowing agents such as carbon dioxide, pentane, 1,1,1,2-tetrafluoroethane (HFC-134a), and 1,1,1,3,3-pentafluoropropane (HFC-245fa) were common in North America and the EU, though chlorinated agents remained in use in many developing countries. Later, HFC-134a was also banned because of its high ozone-depletion and global-warming potential, and HFC-141B was introduced in the early 2000s as an alternative blowing agent for developing nations.

Chemically, polyurethanes form when diisocyanates react with polyols, often with a catalyst or under ultraviolet light. Common catalysts include tertiary amines like DABCO or DMDEE, and metallic soaps like dibutyltin dilaurate. The ratio of starting materials must be carefully controlled—too much isocyanate can cause trimerization, creating rigid polyisocyanurates. The resulting polymer usually has a highly crosslinked structure, making it a thermoset that won’t melt when heated, though some thermoplastic polyurethanes are also made.

The most widespread use of polyurethane is as solid foams, which require a gas or blowing agent during polymerization. This is often done by adding a little water, which reacts with isocyanates to produce CO₂ gas and an amine via an unstable carbamic acid intermediate. That amine can then react with more isocyanates to form urea groups, so the final polymer contains both urethane and urea linkages.