Question

Explain why the addition of a small amount of glycerol to the polymerization mixture gives a stiffer urethane foam.

Short answer

Stiffer urethane foam is formed due to the formation of cross-linking polymer of urethane. Cross linking between chains is possible since glycerol (most basic triol) is used. Cross linking helps to obtain a thermosetting polymer.

Step-by-step solution

Urethanes (or carbamate ester)

A urethane is an ester of a carbamic acid (R-NH-COOH) , a half amide of carbonic acid. They are commonly made by treating an isocyanate with an alcohol or a phenol.

Thermosetting polymers

They are semifluid substances with low molecular weights which on heating on a mould undergo a change in chemical composition to yield a hard, infusible and insoluble mass. This hardening on heating is due to excessive cross linking between different polymer chains.

The reaction

Stiffer urethane foam is formed due to the formation of cross-linking polymer of urethane. Cross linking between chains is possible since glycerol (most basic triol) is used. Cross linking helps to obtain a thermosetting polymer.

Formation of stiffer urethane foam upon addition of glycerol