POLYURETHANE FOAM

IMPORTANT!
Before performing this demonstration, carefully read the MSDS (Material Safety Data Sheets) for toluene diisocyanate. This reaction must be performed in a large, well-ventilated area with qualified supervision and faculty approval. Do not attempt to modify the procedure or increase the scale of this reaction. Be sure to wear safety goggles, gloves, and a lab coat during ALL stages of this demonstration.

Description:

When two viscous liquids are mixed, a rigid foam is produced whose volume is 20-30 times that of the original mixture.

Materials:

Procedure:

Put on a lab coat and an approved pair of safety goggles. Place the 200-mL disposable cup in the center of a heat-resistant transite board. Wearing gloves, mix equal amounts of Parts A and B in the disposable cup. A few drops of a food-coloring dye can be added to the lighter-colored component. When the foam begins to expand, stop stirring. As the mixture foams, it rises out of the cup, overflows, and forms a bell-shaped solid.
Because the freshly prepared foam usually contains unreacted isocyanate, it should not be handled until it has cured several hours in a well-ventilated area and washed with water.

Hazards:

First-Aid Measures:

Disposal:

Since the individual components are soluble in acetone, they can be dissolved and the solutions flushed down the drain with water. The rigid foam, when cured, should be discarded in an appropriately labeled waste container.
The fully cured polyurethane is soluble in solvents such as dimethylformamide. Before it is fully cured, the foam can be removed from the container by using a spatula and small amounts of acetone.

Discussion:

The polyurethane foam system used in this demonstration consists of two viscous liquids. Part A, light amber in color, contains a polyether polyol, a blowing agent, a silicone surfactant, and a catalyst. Part B, which is dark, contains a polyfunctional isocyanate.
A polyurethane foam is formed by producing a polyurethane (polycarbamate) polymer in the presence of a fluorocarbon blowing agent. The polymer is formed by the reaction of a polyester polyol, HO-R-OH (whose probable molecular weight range is 400-4000), with a polyfunctional isocyanate, OCN-R'-NCO [for example, methylene bis(4-phenylisocyanate)]. Amine compounds or metal salts are used as catalysts.
The polyfunctional character of the reactants results in a high degree of cross-linking in the product, forming a rigid foam.

References:

Shakhashiri, Bassam Z. Chemical Demonstrations: A Handbook for Teachers of Chemistry,
Volume 1.. Madison, Wisconsin: University of Wisconsin Press, 1983, pp. 216-218.