The Embedded Penny:  Making Glyptal Resin Plastic

Phthalic anhydride and glycerol are heated in a test tube.  The clear plastic that is formed is poured into an aluminum cap from a plastic cola bottle.  A penny is placed in the cap and permanently embedded in the plastic.

Procedure

Wear safety goggles and disposable gloves.  Do this demonstration in a hood.

• Place 10 g of phthalic anhydride in a large test tube.  Use an old test tube because it will be discarded at the end of the demonstration.  Avoid breatihing the vapors and avoid skin contact.
• Add 0.5 g of sodium acetate to the test tube.
• Add 4.0 mL of glycerol.
• Heat the test tube slowly to dissolve any remaining solids.
• Continue heating until the mixture boils for about 5 minute.
• Prepare a mold by cleaning and drying the aluminum cap from a large plastic cola container (the 3-L size works best).
• Pour about half of the hot liquid into the aluminum cap.
• Carefully place a penny (or some other object) in the liquid in the cap and then add the remaining liquid on top of the penny.
• Let the plastic cool for about 10 minutes at room temperature.  Then place it on top of a container of crushed ice for further cooling.
• Using pliers, strip away the aluminum cap to free the plastic with its embedded penny.

Reactions

This is essentially an esterification reaction between the glycerol and phthalic anhydride.  Water is produced during esterification and is removed by boiling the mixture.  The plastic, glyptal resin, is a thermosetting plastic formed by phthalic anhydride acting as a cross-linker to hold strands of glycerol molecules together in a polymer.

Questions for Students

1. When the reaction appears to boil, what substance is actually boiling off?

2. Write a generic equation for an esterification reaction.

3. How does the structure of the phthalic anhydirde molecule change in the esterification reaction?

4. How does the structure of glycerol change?

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