How Urethane Differs from Polyurethane

Polyurethane was introduced in the twentieth century in varnishes and heavy-duty, waterproof paints. Polyurethane produces a very durable polymeric film-that is, it is a product of polymerization-the prefix "poly" meaning many and the root "mer" meaning unit. A polymer is a substance made from many units. In a polyurethane, many organic units are joined together specifically by urethane linkages. These urethane linkages are not made from urethanes, but by combining multi-functional isocyanates with multi-functional alcohols.

Chemical Structure of Urethane

Urethanes are "carbamate esters." An ester is the product of the reaction of an alcohol with an acid. A carbamate is a derivative of carbamic acid, also called aminocarboxylic acid, which has the chemical formula NH₂-COOH. The amino group is the NH₂- part, and the carboxylic acid is the -COOH part. The hydrogen of the carboxylic group is replaced to form the carbamate.

Carbamic Acid to Carbamate - Inorganic Replacement

Carbamic acid reacts with an alkali to form the respective carbamate. Thus, sodium hydroxide reacts with carbamic acid to form sodium carbamate and water according to the reaction sequence

NaOH + NH₂-COOH → NH₂-COONa + H₂O

Carbamic Acid to Carbamate - Ester Replacement

A carbamate ester can be made from sodium carbamate and an alkyl halide. For instance, ethyl carbamate, a urethane, can be made according to the following reaction sequence

NH₂-COONa + CH₃Cl → NH₂-COOCH₃ + NaCl

This reads, sodium carbamate plus methyl chloride produces methyl carbamate (urethane) plus sodium chloride (ordinary table salt).

Isocyanate

An isocyanate is any molecule containing the group

-N=C=O

A diisocyanate is any molecule containing two isocyanate groups. One of the best known diisocyanates is methylene diphenyl diisocyanate (MDI). Its chemical formula is

O=C=N-C₆H₄-CH₂-C₆H₄-N=C=O

In which the C₆H₄ group is a 6-member ring, each of six carbons having an attached hydrogen, except for one spot where one ring attaches to the other ring, and the other spot, which is attached to isocyanate.

Example of an Isocyanate-to-Polyurethane Reaction

As an example of the formation of a polyurethane, consider the reaction between ethylene glycol, HO-CH₂-CH₂-OH and MDI

HO-CH₂-CH₂-OH + O=C=N-C₆H₄-CH₂-C₆H₄-N=C=O + HO-CH₂-CH₂-OH

gives a chain with repeating segments of the formula

-O-CH₂-CH₂-O-(O)C-NH-C₆H₄-CH₂-C₆H₄-NH-C(O)-O-CH₂-CH₂-O-

Properties and Uses of Modern Polyurethanes

Polyurethane can be spun into spandex fibers (Lycra®). It can be produced in the form of soft or hard foams, and it can be potted to form hard, enduring parts. Not only gymnasium coatings are made of polyurethane but porch and deck paints and lacquers are manufactured. Electrically, polyurethane is an excellent resistive material. It has been manufactured into gears and automotive gas tanks. They have been used in the manufacture of tires and of synthetic leather.

References and Resources:

University of Wisconsin, Madison - Lab of Shakhashiri - Chemical of the Week - Polymers

University of Nebraska, Lincoln - D.W. Brooks Site - Types of Plastics - Polyurethanes