Biodegradable plastics can be derived from many starch-based materials. Starch is a natural polymer, and can be readily attacked and broken down by microbes. In industry, starch is broken down to make lactic acid, which is then polymerised to form the biodegradable plastic polylactide (PLA). However, it is also possible to make your own biodegradable plastic sample directly from potato starch, using basic laboratory equipment.
Grate the potato and add it to the mortar.
Add about 100 cubic centimetres of distilled water and grind the potato mixture thoroughly. Pour the liquid off through a tea strainer into a beaker. Repeat this procedure twice more.
Leave the liquid to settle in the beaker. The white starch should sink to the bottom. Decant the water carefully. Add another 100 cubic centimetres of distilled water, stir, leave to settle and decant.
Measure 22 cubic centimetres of distilled water and add it to a beaker together with 4 grams of the potato slurry from Section 1. Add 3 cubic centimetres of dilute hydrochloric acid. The acid removes part of the starch called amylopectin, which inhibits the formation of a plastic film. Add 2 cubic centimetres of glycerol to act as a plasticiser, making the plastic less brittle.
Set up the tripod and gauze over the Bunsen burner. Place the beaker on top of the gauze and balance a watch glass over the beaker to act as a lid.
Heat the mixture to boiling and allow it to boil for around 15 minutes. Take care the mixture does not boil dry.
Neutralise the mixture by adding dilute sodium hydroxide, drop by drop, using a pipette. Test the pH of the mixture after each addition by dipping a glass rod into the liquid and touching it to a sheet of universal indicator paper. Continue adding drops of sodium hydroxide until the mixture is neutral (pH 7).
Pour the mixture into a petri dish and use a glass rod to ensure it is evenly spread over the surface. Allow to dry overnight on a sunny windowsill or radiator, or you can dry the plastic in a drying cabinet.
Wear eye protection throughout this experiment. Dilute sodium hydroxide and dilute hydrochloric acid are irritants.
Tips and warnings
- Wear eye protection throughout this experiment. Dilute sodium hydroxide and dilute hydrochloric acid are irritants.
Things you need
- Large potato
- Pestle and mortar
- Distilled water
- Tea strainer
- Stirring rod
- Measuring cylinders
- Mass balance
- Watch glass
- Bunsen burner and heatproof mat
- Tripod and gauze
- Petri dish
- Universal indicator paper
- Dropping pipettes
- Dilute hydrochloric acid
- Dilute sodium hydroxide