| Polyurethane chemistry is based on the reaction of | | | | temperature, reversible reaction. If actually formed in |
| isocyanates with active hydrogen containing | | | | normal flexible foams, the allophanate linkage would |
| compounds. Isocyanates are compounds having one | | | | serve to cross-link the polymer further. The catalysts |
| or more of the highly reactive isocyanate group. This | | | | generally used in the foam formulation do not promote |
| group will readily react with hydrogen atoms more | | | | this reaction, and temperatures greater than 110 |
| electronegative than carbon. | | | | Centigrade are necessary for significant allophanate |
| Electron density is expected to be the greatest on the | | | | formation. |
| oxygen atom and least on the carbon item. This | | | | The Gas Producing Reaction |
| results in the oxygen atom having the largest net | | | | To make foam, the polyurethane polymer must be |
| negative charge, carbon a net positive charge, and the | | | | expanded or blown by the introduction of bubbles and |
| nitrogen, an intermediate net negative charge. | | | | a gas. A convenient source of gas is the carbon |
| The normal reactions essentially involve addition to the | | | | dioxide produced from the reaction of an isocyanate |
| carbon-nitrogen double bond. A nucleophilic center | | | | group with water. |
| from an active hydrogen-containing compound attacks | | | | Isocyanate + Water → Carbamic Acid |
| the electrophilic carbon. The active hydrogen atom | | | | Carbamic Acid → Amine + Carbon dioxide + Heat |
| then adds to the nitrogen atom. Electron-withdrawing | | | | The intermediate product of this reaction is a thermally |
| groups attached to the isocyanate molecule increase | | | | unstable carbamic acid, which spontaneously |
| the reactivity of the NCO group toward the nucleophilic | | | | decomposes to an amine and carbon dioxide. Diffusion |
| groups. Electron-donating groups reduce reactivity. | | | | of the carbon dioxide into bubbles previously nucleated |
| Thus, in most reactions, aromatic isocyanates are | | | | in the reacting medium causes expansion of the |
| more reactive than aliphatic isocyanates. Steric | | | | medium to make foam. Further reaction of the amine |
| hindrance effects on either the isocyanate or the | | | | with additional isocyanate gives a distributed urea. |
| active hydrogen compound will effect the reaction. | | | | Isocyanate + Amine → Distributed Urea |
| Formation of flexible polyurethane foam is a complex | | | | The approximate total heat release per mole of water |
| process involving many ingredients and at least two | | | | is 47 kcal. |
| competing reactions. | | | | Again, if the isocyanate and the amine molecules are |
| The Polymerization Reaction | | | | polyfunctional, a cross-linked polymer will result. |
| The polyurethane polymer forming reaction occurs | | | | Another conceptual method of cross-linking the |
| between an isocyanate and an alcohol as follows | | | | polymer is by reaction of a hydrogen from the |
| Isocyanate + Aocohol = Urethane | | | | distributed urea with a free isocyanate group to form |
| This is addition process for which the heat of reaction | | | | a biuret linkage. |
| has been reported to be approximately 24 kcal/mole | | | | Distributed Urea + Isocyanate → Biuret |
| of urethane. Depending on the choice of starting | | | | Since the reaction is also reversible, there is debate |
| materials, the R and R' groups may also contain | | | | about whether allophanates and biurets actually exist in |
| isocyanate or isocyanate-reactive groups respectively. | | | | the final polyurethane foam. |
| When extended to polyfunctional reactants, this | | | | Blowing can also be achieved by the physical addition |
| reaction provides a direct route to cross linked | | | | of a low-boiling nonreactive liquid to a foam formulation. |
| polymers. | | | | Historically, the most commonly used blowing agents |
| The hydrogen on the nitrogen atom of the urethane | | | | are chlorofluorocarbons, urethane grade methylene |
| group is capable of reacting with additional isocyanate | | | | chloride and trichloroethane. Vaporization of these |
| to form an allophanate group. | | | | liquids by heat from the exothermic reactions produces |
| Urethane + Isocyanate = Allophanate | | | | gas molecules which diffuse into nucleated bubbles |
| Note that the formation of allophanante is a high | | | | and contribute to foam expansion. |