Polyurethane Hearts

Introduction

Engage in the experimental design process by creating a polyurethane heart using a controlled chemical reaction. This experiment blends art with chemistry in a way that introduces the importance of the scientific method, the ability to make an informed prediction and test it by generating evidence, or data. Discover how seemingly minor changes to a chemical process can lead to different results.

Concepts

• Polymers
• Decomposition reaction
• Catalysis
• indicators

Materials

Safety Precautions

This activity should only be performed in a fume hood or well ventilated area. Avoid breathing any vapors produced and avoid skin contact, as both Part A and Part B may contain skin and tissue irritants. Wear chemical splash goggles, chemical resistant gloves and chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information. Wash hands thoroughly with soap and water before leaving the laboratory.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The disposable cups may be thrown in the trash. Any leftover liquids should be mixed together, allowed to react, and then the solidified polymer may be disposed of in the trash according to Flinn Suggested Disposal Method #26a.

Procedure

1. Using a polypropylene cup measure approximately 5-mL of polyurethane Part A.
2. Using a second polypropylene cup measure approximately 5-mL of polyurethane Part B.
3. Add a drop of red food dye to medicine cup containing Part A.
4. With a wood splint mix part A and food dye.
5. Apply carefully some petroleum jelly to the inside walls of the heart shaped cookie cutter.
6. Place the cookie cutter on a recloseable bag, in the center.
7. Pour part A into the cup containing Part B and mix using a wood splint.
8. Pour the mixed solution into the center of the cookie cutter.
9. Wait approximately 10 to 15 minutes for the polyurethane foam to harden.
10. Once the polyurethane heart has solidified and hardened carefully press the edges with your fingers to remove from the cookie cutter.

Teacher Tips

• This lab can be run a number of different ways. One approach is to make the experiment an open-inquiry lab. Do this by withholding the procedure from students. Instead, give them pre-measured amounts of the two reactants and challenge them to make a foam heart of a specified texture (i.e., hardness). They will discover that they can control the texture of the heart by varying the reaction composition, or the ratio of the two reactants.
• This procedure can be used to make holiday ornaments or Halloween sculptures if you have access to the proper molds or cookie cutters. Students may even choose to make a small sculpture of the school mascot.
• Acetone may be used to remove any hardened polymer on the table.
• Do not touch the foam. It will take about 15 minutes for the surface to firmly set and may contain unreacted material for up to 24 hours. Some people will have allergic reactions to unreacted monomers.

Discussion

The heart is constructed with polyurethane foam. There are many forms of polyurethane, such as fibers, coatings, elastomers, flexible foams and rigid foams. The foam in this system is rigid foam that is used in furniture, insulation, flotation devices and many other items. The rigid polyurethane foam is produced by mixing equal parts of two liquids, called Part A and Part B. This lightweight foam expands to about thirty times its original liquid volume and will become rigid in about five minutes.

Part A is a viscous, cream-colored liquid containing a polyether polyol, a silicone surfactant, and a catalyst. The polyether polyol contains reactive hydroxyl (–OH) end groups. The silicone surfactant reduces the surface tension between the liquids. The catalyst is a tertiary amine which aids in speeding up the reaction without being chemically changed itself. Part B is a dark brown viscous liquid containing diisocyanate and higher oligomers (dimers, trimers or tetramers). When the polyether polyol (Part A) is mixed with the diisocyanate (Part B), an exothermic polymerization reaction occurs, producing polyurethane (Equation 1).

During the course of the polymerization reaction, a small amount of water reacts with some of the diisocyanate. A decomposition reaction occurs and produces carbon dioxide gas, thus causing the solution to foam and expand in volume. Pores in the mixture are created from the gas; these pores are visible when looking at the rigid substance. The multifunctionality of both reactants leads to cross-linking in the polymer, causing it to become rigid within minutes (Equation 2).

References

Rosato, D. V. Rosato’s Plastics Encyclopedia and Dictionary; Hanser: New York, 1993; pp 318–320, 572.

Shakhashiri, B. Z. Chemical Demonstrations: A Handbook for Teachers in Chemistry; University of Wisconsin: Madison, WI; 1983; Vol. 1, p 77.