Polyurethane

OVERVIEW

Polyurethanes (pol-ee-YUR-eth-anes) are a group of thermoplastic polymers formed in the reaction between a diisocyanate and a polyol, an alcohol with two or more hydoxyl (-OH) groups. Diisocyanates are compounds that contain two isocyanate (-N=C=O) groups.

KEY FACTS

OTHER NAMES:

None

FORMULA:

-[-CONH-C₆H₄-NCOO-CH₂CH₂-O-]-_n; other structures are possible

ELEMENTS:

Carbon, hydrogen, oxygen, nitrogen

COMPOUND TYPE:

Organic polymer

STATE:

Solid

MOLECULAR WEIGHT:

Varies; very large

MELTING POINT:

Variable

BOILING POINT:

Not applicable

SOLUBILITY:

Insoluble in water; soluble in aromatic hydrocarbons, such as benzene and toluene

Polyurethanes are available in a variety of forms, including fibers, foams, coatings, and elastomers, rubber-like materials. Each form of polyurethane has its own set of physical and chemical properties. For example, fibers are moisture proof, stretchable, and resistant to the flow of electric current. Foams can be either rigid or flexible, with densities as low as 32 kilograms per cubic meter (2 pounds per cubic foot) to as high as 800 kilograms per cubic meter (50 pounds per cubic foot). They are excellent thermal (heat) insulators. Polyurethane coatings are hard, glossy, flexible, readily adhesive to other surfaces, and resistant to abrasion, weathering, and most inorganic chemicals. Elastomeric polyurethanes are resistant to abrasion, weathering, and most organic solvents. With this variety of properties, polyurethanes have a very wide range of uses.

The basic process for making polyurethanes was first developed in 1937 by the German chemist Otto Bayer(1902–1982), who patented his discovery and founded a company for its commercial production. The polyurethanes were first put to widespread use during World War II as substitutes for natural rubber in the manufacture of tires for military uses. The first rigid polyurethane foam was made in 1940, again for military purposes; the first polyurethane adhesive, for the joining of rubber and glass, in 1941; and the first use of polyurethane as an insulator, in beer barrels, in 1948. Polyurethane fibers were also used during World War II for the manufacture of protective clothing to be worn by soldiers in case of attacks by poison gas. In the late 1950s, a stretchable material made of polyurethane called spandex was introduced.

HOW IT IS MADE

Polyurethanes are formed by a reaction known as rearrangement. In this reaction, a hydrogen atom from the polyol (alcohol with two or more hydroxyl, -OH, groups) leaves the polyol and attaches itself to one of the nitrogen atoms in the diisocyanate molecule. The remnant of the polyol left after the hydrogen atom leaves then attaches itself to the carbon atom next to the nitrogen that has just received the hydrogen. The end result of this reaction is that two molecules, a diisocyanate molecule and a polyol molecule, have joined to make a single molecule. But that molecule has the same general structure as the original reactants. So the reaction can be repeated with a second diisocyanate molecule and a second polyol molecule adding to the former product.

All that is needed to make this reaction happen is a suitable catalyst, a substance that will encourage the first hydrogen atom to leave the polyol and move to the diisocyanate. The catalyst most frequently used is diazobicyclo[2.2.2]octane, more commonly known as DABCO. DABCO provides an initial "tug" on the hydrogen atom that needs to be moved. Once it gets the reaction started, DABCO has no further role and can be recovered for re-use.

Various forms of polyurethane are made by adding additional steps to this fundamental process. For example, polyurethane foams are made by adding water and carbon dioxide to the molten polymer. The carbon dioxide creates bubbles, which makes the mixture rise like bread and then harden into a foam. Flexible polyurethane for fibers and foam rubber can be made by adding polyethylene glycol, a softening agent, to the mixture.

Interesting Facts

The U.S. polyurethane industry produced 2,915 million kilograms (6,393 million pounds) of polyurethane in 2002.

COMMON USES AND POTENTIAL HAZARDS

Flexible and rigid foams are the most popular types of polyurethane made in the United States. Flexible foams are used for cushioning, as in mattresses, upholstered furniture, and automobile seats. Semiflexible foams are component of carpet pads, sponges, packaging, and door panels for automobiles. Rigid polyurethane foams are used to produce insulation for roofs, refrigerators, and freezers. The construction industry accounted for more than half of all the rigid polyurethane foam made in 2002, primarily for roofing and wall insulation. The automotive industry was the second largest user of rigid foams, where polyurethane was used for floor cushions, headliners, and heating and air-conditioning systems.

Polyurethane elastomers are flexible and strong. They can be stretched to a significant amount before returning to their original length. They are also shock absorbent. These properties account for their uses in tires, shoe soles, skateboards, and inline skates. Polyurethane elastomers can also be spun into fibers which are then used to make light, flexible clothing. Spandex, one of the most popular fabrics made of polyurethane fibers, is both sturdy and flexible. It is used to make bathing suits and exercise clothing because it can be distorted without losing its original shape or tearing. Polyurethane is also one of the main ingredients in the fabric known as pleather, or plastic leather. This synthetic form of leather is less expensive to produce and easier to dye than real leather.

Polyurethane coatings are strong and durable. They can be found on surfaces that need to be flexible, abrasion resistant, and chemical resistant, such as dance floors and bowling alleys. Water-based coatings are often used on airplanes and other transportation equipment. Polyurethane powder coatings are also used on fluorescent lights, refrigerators, car wheels and trim, lawnmowers, patio furniture, and ornamental iron.

Words to Know

POLYMER

a compound consisting of very large molecules made of one or two small repeated units called monomers.

THERMOPLASTIC

capable of being repeatedly softened and hardened by alternately heating and cooling.

Dust formed by the breakdown of polyurethane products may be an irritant to skin, eyes, and the respiratory system. No serious health problems have been associated with such materials, however. Some of the raw materials used in the manufacture of the polyurethanes, such as the isocyanates, do pose health risks. These risks are of concern primarily to workers who come into contact with those raw materials.

FOR FURTHER INFORMATION

Alliance for the Polyurethane Industry. "About Polyurethane" http://www.polyurethane.org/about/ (accessed on October 26, 2005).

"Making Polyurethanes." Polymer Science Learning Center, University of Southern Mississippi. http://www.pslc.ws/mactest/uresyn.htm (accessed on October 26, 2005).

"Polyurethanes." Huntsman. http://www.huntsman.com/pu/ (accessed on October 26, 2005).

Vartan, Starre. "Pretty in Plastic: Pleather Is a Versatile, though Controversial, Alternative to Leather." E (September-October 2002): 53-54.