Thermoplastic Starch – Potato Plastic
Objective: to learn how to make a plastic film from potato starch
Introduction:
Biodegradable plastics can be derived from many starch-based materials. In this lab, we
will make a plastic film from potato starch and test its properties. Potato starch is a
polymer made of long chains of glucose units joined together. Specifically, it contains
two polymers:
1. Amylopectin – a highly branched polymer of glucose units
2. Amylose – a straight chain of glucose units
The amylopectin prevents the starch from becoming plastic-like. In this lab, acetic acid is
used to break down the amylopectin and change the structure and properties of the
polymer.
We will make two different batches of the potato plastic. One batch will contain
glycerin, which acts as a plasticizer, and in the other batch the glycerin will be left out.
Amylopectin
Amylose
Materials:
Corn starch
Glycerin
Acetic acid
Sodium bicarbonate
Water
Hot plate
Balance
50 mL beaker
250 mL beaker
Stir rod
Baking pan
Procedure #1:
1. Add 15 g of potato starch to 105 mL of water in a 250 mL beaker.
2. Add 10 mL of glycerin to the water solution and stir the contents. Solution should
look opaque.
3. In a separate 50 mL beaker add 2.5 g of sodium bicarbonate to 30 mL of water.
4. Place the 250 mL beaker of the starch solution on a hot plate set at 130 ◦C and
continue to stir.
5. Continue to stir until the mixture becomes thick and almost transparent (about 5-7
min).
6. Add the solution of sodium bicarbonate and water to the starch solution and
continue to stir for an additional 1-2 min.
7. Pour the mixture onto a baking pan in a thin sheet.
8. Place in oven to dry.
Pictures of the batch that contains glycerin:
Heating of starch solution
Potato plastic spread out on pan
before drying
Potato plastic after drying
Procedure #2:
1. Repeat the steps described above but leave out the glycerin. Make sure you label
your mixtures so that you know which one contains the glycerin and which one
does not.
Potential discussion questions:
1. Describe carefully the properties of each of the substances you have made.
2. Why is acetic acid used to make potato starch?
3.
What difference did the addition of glycerin make?
4. Why does adding the glycerin change the properties of the plastic?
Discussion
Students should be able to see a difference in the two films that are made. The one
without the glycerin is much more brittle. Because acetic acid is added to break down the
amylopectin, the straight chains of the amylose can line up together. This makes the film
crystalline, causing it to be brittle.
When glycerin is added to the mixture, the film is more flexible. Plasticizers work by
embedding themselves between the chains of polymers, increasing the spacing and free
volume between chains, which prevents close packing of the polymer chains. This
decreases the glass transition temperature of the polymer and results in a lower degree of
crystallinity in the film. Due to this, the film is softer and more flexible.