CHEMISTRY 308
PREPARATION OF POLYSTYRENE FOAM
Work in groups of two for this experiment. Turn in one report for each group.
First period
Place 30 mL of styrene monomer, 8 mL of pentane and 0.25 g of benzoyl peroxide into a 150-mL screw cap plastic
bottle (notes 1 and 2). Pentane is very volatile so it needs to be measured rapidly. Squeeze the bottle while
replacing the cap in order to remove most of the air from the container. Make sure that the top is screwed on
securely! Shake the mixture. Write your names on a piece of labeling tape and secure it to the plastic bottle. Heat
the mixture in an oven at 48-51 oC for 7 days after which time the mixture should have solidified.
Second period
After the heating period, cut off the upper part of the plastic bottle, and leave the solid chunk of polystyrene in the
bottle. Fold over the remaining part of the plastic bottle, and crush the polystyrene with a hammer. The sample is
extremely hard! It may be preferable to wrap the solid in a piece of shop-quality toweling and hit it with a hammer.
The cloth helps to keep the material from flying all over the place and makes it easier to get more consistently sized
pieces of polystyrene. Don't strike the sample directly on the bench top! The polystyrene is very hard and you may
break the desktop! Break it up on the floor of the lab. Crush the solid polystyrene into small pieces about the size of
B.B's or smaller. The material must not be ground into a powder because the pentane will be lost from the sample,
so don't use a mortar and pestle. Foam the polystyrene on the same day it is crushed so that the pentane isn't lost
from the sample. If the polystyrene must be stored, place the solid in a tightly sealed screw cap bottle but this
shouldn't be necessary since we will foam the sample during this second lab period.
I have available a number of molds made out of a muffin tin and one spherical mold for your use (notes 3 and 4).
Check to see that the holes in the mold are open. They allow air to escape from the molds as they are being heated.
Lubricate the spherical mold with silicone stopcock grease (tube), but the others do not need to be lubricated since
they have been made out of a muffin tin that has a Teflon coating.
After you have broken up the polystyrene, partly fill the mold (1/3 to 1/2 full, but record the actual amount you are
putting in the mold) with the crushed polystyrene. If you have extra material, I recommend using another mold to
obtain a second foamed sample. Insert the bolts to secure the clamps. Suspend the mold with wire into one of the
community boiling water baths (use 1-liter beakers of boiling water on a hot plate for this purpose. Suspend the
molds from a clamp/ring stand assembly). Heat the samples for about 45 min (record the actual time that you used).
The material will expand in the mold. Following this heating period, remove the mold from the water bath, cool it
in cold water in the sink, or in an ice bath, and remove the expanded polystyrene from the mold. Allow the foam to
dry thoroughly in your in the backroom in the hood until the next lab period.
Third period
Weigh your dry expanded polystyrene sample. Now that you have the weight you will need to obtain the volume
before calculating the density of the foam. Obtain the volume (mL) of the foamed polystyrene (sphere or otherwise)
by water displacement in a large graduated cylinder. This can be done easily by adding water to a cylinder, noting
the volume, placing the foam in the cylinder and reading the new volume of water while pushing the foam gently
down into the cylinder. The difference in water volumes gives the volume of the foam in mL. If you have prepared
a sphere, calculate the volume (cm3) by using the formula for the volume of a sphere:
Volume = 4/3r3 where r = radius in cm
Now, using the weight and volume, calculate the density using the formula:
weight in grams
Density (g/mL or g/cm3) =
________________________
volume in mL or cm3
Lab Report (joint report with your partner)
1. Record the volume of polystyrene you put in the mold. Record the foaming time.
2. Record the weight of your foamed sample.
3. Record the volume of your foamed sample in mL obtained from water displacement. Calculate the
density of your foam using the formula provided above.
4. Draw the structure of polystyrene. Would the polystyrene sample be syndiotactic, isotactic or atactic?
5. Give a detailed stepwise mechanism for the free radical polymerization of styrene.
6. What is the purpose of the pentane?
7. Turn in your foamed polystyrene sample with your report.
Note 1. Adapted from the procedure of Pavia, Lampman, Kriz, and Engel "Introduction to Organic Laboratory
Techniques".
Note 2. The plastic bottles must have screw caps that fit well. These bottles can not be used again.
Note 3. Not recommended! The cast aluminum spherical molds from the Plastics Technology laboratory work
well. Lubricate them with silicone grease. Suspend the mold in the hot water bath using a piece of copper wire. The
airplane mold does not work well because the resulting plane is too fragile! The baseball mold is much too large to
use. My cylinder mold is tricky to use. I would not recommend using the cylinder unless it is lubricated with a
generous amount of silicone!
Note 4. Not recommended! We have had very limited success in using the microwave in the Technology Dept to
foam the polystyrene. The problem is that we don't get as good a quality of form using this method. We used a 118
mL Rubbermaid container with a polypropylene bottom and LDPE top (obtain at Target). Put several holes in the
top. Add about 18.3 g (essentially one batch; finely powdered material works best) of crushed polystyrene and 12
mL of water into the container. Tape the container securely with duct tape. Tape the container all the way around,
top to bottom, but avoid covering the holes. Don't overtape the container so that you can't see into the container to
watch it form. Microwave in the Tech Dept unit. Do this for 11 minutes on the high setting. Be sure to tape as
indicated above or the top of the container will come off during the microwaving process! We found that 1 minute at
medium gave no foaming, 1 minute high foams it, but not completely. 4 minutes works, but 11 minutes was better.
If the material has not formed completely, put it back into the microwave and continue heating. Dry the foam
completely until the next lab period.
Revised 2 April 2007