The Basic Chemistry of Polyurethanes

The basic chemistry of the isocyanate group can be tracked back to the pioneering work of Wurtz (1849) and Hofsman (1850). Dozen of reactions are known, but the ones of most commercial significance are those leading to the formation of polyurethane polymers. Polyurethanes are formed by the reaction of a polyisocyanate with a polyhydroxy, or polyol compound. The reaction of polyisocyanate with water is a convenient way to produce a gas useful for blowing the polymer to produce a foam structure.

BASIC CHEMISTRY

Polyurethane chemistry is based on the reaction of isocyanates with active hydrogen containing compounds. Isocyanates are compounds having one or more of the highly reactive isocyanate group. This group will readily react with hydrogen atoms more electronegative than carbon.

Electron density is expected to be the greatest on the oxygen atom and least on the carbon item. This results in the oxygen atom having the largest net negative charge, carbon a net positive charge, and the nitrogen, an intermediate net negative charge.

The normal reactions essentially involve addition to the carbon-nitrogen double bond. A nucleophilic center from an active hydrogen-containing compound attacks the electrophilic carbon. The active hydrogen atom then adds to the nitrogen atom. Electron-withdrawing groups attached to the isocyanate molecule increase the reactivity of the NCO group toward the nucleophilic groups. Electron-donating groups reduce reactivity. Thus, in most reactions, aromatic isocyanates are more reactive than aliphatic isocyanates. Steric hindrance effects on either the isocyanate or the active hydrogen compound will effect the reaction.

Formation of a flexible polyurethane foam is a complex process involving many ingredients and at least two competing reactions.

THE POLYMERIZATION REACTION

The polyurethane polymer forming reaction occurs between an isocyanate and an alcohol as follows

Isocyanate + Aocohol = Urethane

This is addition process for which the heat of reaction has been reported to be approximately 24 kcal/mole of urethane. Depending on the choice of starting materials, the R and R’ groups may also contain isocyanate or isocyanate-reactive groups respectively. When extended to polyfunctional reactants, this reaction provides a direct route to cross linked polymers.

The hydrogen on the nitrogen atom of the urethane group is capable of reacting with additional isocyanate to form an allophanate group.
Urethane + Isocyanate = Allophanate

Note that the formation of allophanante is a high temperature, reversible reaction. If actually formed in normal flexible foams, the allophanate linkage would serve to cross-link the polymer further. The catalysts generally used in the foam formulation do not promote this reaction, and temperatures greater than 110 Centigrade are necessary for significant allophanate formation.

THE GAS PRODUCING REACTION

To make foam, the polyurethane polymer must be expanded or blown by the introduction of bubbles and a gas. A convenient source of gas is the carbon dioxide produced from the reaction of an isocyanate group with water.

Isocyanate + Water → Carbamic Acid

Carbamic Acid → Amine + Carbon dioxide + Heat

The intermediate product of this reaction is a thermally unstable carbamic acid. Which spontaneously decomposes to an amine and carbon dioxide.. Diffusion of the carbon dioxide into bubbles previouslt nucleatedin the reacting medium causes expansion of the medium to make foam. Further reaction of the amine with additional isocyanate gives a distributed urea.

Isocyanate + Amine → Distributed Urea

The approximate total heat release per mole of water is 47 kcal.

Again, if the isocyanate and the amine molecules are polyfunctional, a cross-linked polymer will result. Another conceptual method of cross-linking the polymer is by reaction of a hydrogen from the distributed urea with a free isocyanate group to form a biuret linkage.

Distributed Urea + Isocyanate → Biuret

Since the reaction is also reversible, there is debate about whether allophanates and biurets actually exist in the final polyurethane foam.

Blowing can also be achieved by the physical addition of a low-boiling nonreactive liquid to a foam formulation. Historically, the most commonly used blowing agents are chlorofluorocarbons, urethane grade methylene chloride and trichloroethane. Vaporization of these liquids by heat from the exothermic reactions produces gas molecules which diffuse into nucleated bubbles and contribute to foam expansion.

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