Polyurethane

Polyurethanes are linear polymers that have a molecular backbone containing carbamate groups (-NHCO2). These groups, called urethane, are produced through a chemical reaction between a diisocyanate and a polyol. First developed in late 1930s, polyurethanes are some of the most versatile polymers. They are used in building insulation, surface coatings, adhesives, solid plastics, and athletic apparel.

Background

Polyurethanes, also known as polycarbamates, belong to a larger class of compounds called polymers. Polymers are macromolecules made up of smaller, repeating units known as monomers. Generally, they consist of a primary long-chain backbone molecule with attached side groups. Polyurethanes are characterized by carbamate groups (-NHCO₂) in their molecular backbone.

Synthetic polymers, like polyurethane, are produced by reacting monomers in a reaction vessel. In order to produce polyurethane, a step—also known as condensation—reaction is performed. In this type of chemical reaction, the monomers that are present contain reacting end groups. Specifically, a diisocyanate (OCN-R-NCO) is reacted with a diol (HO-R-OH). The first step of this reaction results in the chemical linking of the two molecules leaving a reactive alcohol (OH) on one side and a reactive isocyanate (NCO) on the other. These groups react further with other monomers to form a larger, longer molecule. This is a rapid process which yields high molecular weight materials even at room temperature. Polyurethanes that have important commercial uses typically contain other functional groups in the molecule including esters, ethers, amides, or urea groups.

History

Polyurethane chemistry was first studied by the German chemist, Friedrich Bayer in 1937. He produced early prototypes by reacting toluene diisocyanate reacted with dihydric alcohols. From this work one of the first crystalline polyurethane fibers, Perlon U, was developed. The development of elastic polyurethanes began as a program to find a replacement for rubber during the days of World War II. In 1940, the first polyurethane elastomers were produced. These compounds gave millable gums that could be used as an adequate alternative to rubber. When scientists found that polyurethanes could be made into fine threads, they were combined with nylon to make more lightweight, stretchable garments.

In 1953, the first commercial production of a flexible polyurethane foam was begun in the United States. This material was useful for foam insulation. In 1956, more flexible, less expensive foams were introduced. During the late 1950s, moldable polyurethanes were produced. Over the years, improved polyurethane polymers have been developed including Spandex fibers, polyurethane coatings, and thermoplastic elastomers.

Raw Materials

A variety of raw materials are used to produce polyurethanes. These include monomers, prepolymers, stabilizers which protect the integrity of the polymer, and colorants.

Isocyanates

One of the key reactive materials required to produce polyurethanes are diisocyanates. These compounds are characterized by a (NCO) group, which are highly reactive alcohols. The most widely used isocyanates employed in polyurethane production are toluene diisocyanate (TDI) and polymeric isocyanate (PMDI). TDI is produced by chemically adding nitrogen groups on toluene, reacting these with hydrogen to produce a diamine, and separating the undesired isomers. PMDI is derived by a phosgenation reaction of aniline-formaldehyde polyamines. In addition to these isocyanates, higher end materials are also available. These include materials like 1,5-naphthalene diisocyanate and bitolylene diisocyanate. These more expensive materials can provide higher melting, harder segments in polyurethane elastomers.

Polyols

The other reacting species required to produce polyurethanes are compounds that contain multiple alcohol groups (OH), called polyols. Materials often used for this purpose are polyether polyols, which are polymers formed from cyclic ethers. They are typically produced through an alkylene oxide polymerization process. They are high molecular weight polymers that have a wide range of viscosity. Various polyether polyols that are used include polyethylene glycol, polypropylene glycol, and polytetramethylene glycol. These materials are generally utilized when the desired polyurethane is going to be used to make flexible foams or thermoset elastomers.

Polyester polyols may also be used as a reacting species in the production of polyurethanes. They can be obtained as a byproduct of terephthalic acid production. They are typically based on saturated aromatic carboxylic acids and diols. Branched polyester polyols are used for polyurethane foams and coatings. Polyester polyols were the most used reacting species for the production of polyurethanes. However, polyether polyols became significantly less expense and have supplanted polyester polyols.

Additives

Some polyurethane materials can be vulnerable to damage from heat, light, atmospheric contaminants, and chlorine. For this reason, stabilizers are added to protect the polymer. One type of stabilizer that protects against light degradation is a UV screener called hydroxybenzotriazole. To protect against oxidation reactions, antioxidants are used. Various antioxidants are available such as monomeric and polymeric hindered phenols. Compounds which inhibit discoloration caused by atmospheric pollutants may also be added. These are typically materials with tertiary amine functionality that can interact with the oxides of nitrogen in air pollution. For certain applications, antimildew additives are added to the polyurethane product.

After the polymers are formed and removed from the reaction vessels, they are naturally white. Therefore, colorants may be added to change their aesthetic appearance. Common covalent compounds for polyurethane fibers are dispersed and acid dyes.

Design

Polyurethanes can be produced in four different forms including elastomers, coatings, flexible foams, and cross-linked foams. Elastomers are materials that can be stretched but will eventually return to their original shape. They are useful in applications that require strength, flexibility, abrasion resistance, and shock absorbing qualities. Thermoplastic polyurethane elastomers can be molded and shaped into different parts. This makes them useful as base materials for automobile parts, ski boots, roller skate wheels, cable jackets, and other mechanical goods. When these elastomers are spun into fibers they produce a flexible material called spandex. Spandex is used to make sock tops, bras, support hose, swimsuits, and other athletic apparel.

Polyurethane coatings show a resistance to solvent degradation and have good impact resistance. These coatings are used on surfaces that require abrasion resistance, flexibility, fast curing, adhesion, and chemical resistance such as bowling alleys and dance floors. Water based polyurethane coatings are used for painting aircraft, automobiles, and other industrial equipment.

Flexible foams are the largest market for polyurethanes. These materials have high impact strength and are used for making most furniture cushioning. They also provide the material for mattresses and seat cushions in higher priced furniture. Semiflexible polyurethane foams are used to make car dashboard and door liners. Other uses include carpet underlay, packaging, sponges, squeegees, and interior padding. Rigid, or cross-linked, polyurethane foams are used to produce insulation in the form of boards or laminate. Laminates are used extensively in the commercial roofing industry. Buildings are often sprayed with a polyurethane foam.

The Manufacturing Process

While polyurethane polymers are used for a vast array of applications, their production method can be broken into three distinct phases. First, the bulk polymer product is made. Next, the polymer is exposed to various processing steps. Finally, the polymer is transformed into its final product and shipped. This production process can be illustrated by looking at the continuous production of polyurethane foams.

Polymer reactions

• 1 At the start of polyurethane foam production, the reacting raw materials are held as liquids in large, stainless steel tanks. These tanks are equipped with agitators to keep the materials fluid. A metering device is attached to the tanks so that the appropriate amount of reactive material can be pumped out. A typical ratio of polyol to diisocyanate is 1:2. Since the ratio of the component materials produces polymers with varying characteristics, it is strictly controlled.
• 2 The reacting materials are passed through a heat exchanger as they are pumped into pipes. The exchanger adjusts the temperature to the reactive level. Inside the pipes, the polymerization reaction occurs. By the time the polymerizing liquid gets to the end of the pipe, the polyurethane is already formed. On one end of the pipe is a dispensing head for the polymer.

Processing

• 3 The dispensing head is hooked up to the processing line. For the production of rigid polyurethane foam insulation, a roll of baking paper is spooled at the start of the processing line. This paper is moved along a conveyor and brought under the dispensing head.
• 4 As the paper passes under, polyurethane is blown onto it. As the polymer is dispensed, it is mixed with carbon dioxide which causes it to expand. It continues to rise as it moves along the conveyor. (The sheet of polyurethane is known as a bun because it "rises" like dough.)
• 5 After the expansion reaction begins, a second top layer of paper is rolled on. Additionally, side papers may also be rolled into the process. Each layer of paper contains the polyurethane foam giving it shape. The rigid foam is passed through a series of panels that control the width and height of the foam bun. As they travel through this section of the production line, they are typically dried.
• 6 At the end of the production line, the foam insulation is cut with an automatic saw to the desired length. The foam bun is then conveyored to the final processing steps that include packaging, stacking, and shipping.

Quality Control

To ensure the quality of the polyurethane material, producers monitor the product during all phases of production. These inspections begin with an evaluation of the incoming raw materials by quality control chemists. They test various chemical and physical characteristics using established methods. Some of characteristics that are tested include the pH, specific gravity, and viscosity or thickness. Additionally, appearance, color, and odor may also be examined. Manufacturers have found that only by strictly controlling the quality at the start of production can they ensure that a consistent finished product will be achieved.

After production, the polyurethane product is tested. Polyurethane coating products are evaluated in the same way the initial raw materials are checked. Also, characteristics like dry time, film thickness, and hardness are tested. Polyurethane fibers are tested for things such as elasticity, resilience, and absorbency. Polyurethane foams are checked to ensure they have the proper density, resistance, and flexibility.

The Future

The quality of polyurethanes has steadily improved since they were first developed. Research in a variety of areas should continue to help make superior materials. For example, scientists have found that by changing the starting prepolymers they can develop polyurethane fibers which have even better stretching characteristics. Other characteristics can be modified by incorporating different fillers, using better catalysts, and modifying the prepolymer ratios.

In addition to the polymers themselves, the future will likely bring improvements in the production process resulting in faster, less expensive, and more environmentally friendly polyurethanes. A recent trend in polyurethane production is the replacement of toluene diisocyanates with less-volatile polymeric isocyanates. Also, manufacturers have tried to eliminate chlorinated fluorocarbon blowing agents which are often used in the production of polyurethane foams.

Where to Learn More

Books

Kirk-Othmer Encyclopedia of Chemical Technology. John Wiley & Sons, 1997.

Oertel, G. Polyurethane Handbook. Second ed. Munich: Carl Hanser Publishers, 1993.

Seymour, Raymond, and Charles Carraher. Polymer Chemistry. New York: Marcel Dekker,1992.

Ulrich, H. The Chemistry and Technology of Isocyanates. New York: John Wiley & Sons, 1996.

—PerryRomanowski

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.

pol·y·u·re·thane / ˌpäliˈyoōrəˌ[unvoicedth]ān/ • n. a synthetic resin in which the polymer units are linked by urethane groups, used chiefly as constituents of paints, varnishes, adhesives, and foams.• v. [tr.] [usu. as adj.] (polyurethaned) coat or protect with paint or varnish of this kind.