Po-ta-toes, boil them, mash them, make them into plastic
Students can begin either with potatoes or with commercially bought potato starch.
The practical is straightforward; the main hazard is the possibility of the mixture boiling dry.
Extracting the starch takes about 15–20 minutes and making the plastic about 20 minutes.
Using the activity
This can be used simply as a practical to enhance the teaching of a polymers or plastics topic, it can be used as an introduction to further work on biopolymers and bioplastics and/or it can be used as an example of the effects of plasticisers. A number of student sheets are provided so you can choose to do the whole activity or just selected parts of it.
Extracting starch from potatoes
For each group of students you will need:
- 100 g potatoes
- Tea strainer
- Distilled water
- Beaker, 400 cm3 x 2
- Pestle and mortar
Making the plastic film
For each group of students you will need:
- Beaker, 250 cm3
- Large watch glass
- Bunsen burner and heat proof mat
- Tripod and gauze
- Stirring rod
- Potato starch
- Propan-1,2,3-triol (glycerol)
- Hydrochloric acid 0.1 mol dm–3 (Minimal hazard)
- Sodium hydroxide 0.1 mol dm–3 (Irritant)
- Food colouring
- Petri dish or white tile
- Universal Indicator paper
- Eye protection
- Access to a balance
- Measuring cylinder, 25 cm3
- Measuring cylinder, 10 cm3
Health, safety and technical notes
- Read our standard health and safety guidance
- Wear eye protection.
- Propan-1,2,3-triol (glycerol) has no hazard classification but may be harmful if ingested in quantity
- If students have extracted their own potato starch they will need to use about 4 g of it for the next part of the experiment as it is a wet slurry rather than a dry powder. They should add about 22 cm3 water. If they do not have enough extract then a bit of bought potato starch added to the mix will be fine.
- If access to a balance is difficult then a heaped spatula of starch can be used rather than 2.5 g.
- If access to 10 cm3 measuring cylinders is difficult, 4 pipette squirts of hydrochloric acid and 3 squirts of propan-1,2,3-triol are suitable amounts.
- If you have a drying cabinet, the plastic film should dry in about 90 minutes at 100 °C.
- Warn students not to let the mixture boil dry because this can cause it to ‘pop’ and it shows a tendency to jump out of the beaker. For this reason, students should wear eye protection at all stages of the practical.
- Food colouring – while use of this is optional, it does enhance the product and the colour makes the plastic film look more like plastic. Only one drop is needed or the film is too dark.
- If too much water is used, the polymer does not solidify and remains a liquid.
Starch is made of long chains of glucose molecules joined together. Strictly, it contains two polymers: amylose, which is straight chained, and amylopectin, which is branched. When starch is dried from an aqueous solution it forms a film as a result of hydrogen bonding between the chains. However, the amylopectin inhibits the formation of the film.
The addition of hydrochloric acid breaks the amylopectin down, allowing a more satisfactory film formation. This is the product formed in the student activity without the addition of propan1,2,3-triol. The straight chains of the starch (amylose) can line up together and make a good film. However, it is brittle because the chains are so good at lining up – areas of the film can become crystalline, which causes the brittleness.
The addition of propan-1,2,3-triol has an effect because of its hydroscopic (water attracting) properties. Water bound to the propan-1,2,3-triol gets in amongst the starch chains and inhibits the formation of crystalline areas, preventing brittleness and resulting in more ‘plastic’ properties. In the notes for students, reference to water has been omitted to allow them to concentrate on the effect of the propan-1,2,3-triol itself.
What to do
- Grate about 100 g potato. The potato does not need to be peeled, but it should be clean.
- Put the potato into the mortar and add about 100 cm3 distilled water. Grind the potato carefully.
- Pour the liquid off through the tea strainer into the beaker, leaving the potato behind in the mortar. Add 100 cm3 water, grind and strain twice more.
- Leave the mixture to settle in the beaker for 5 minutes.
- Decant the water from the beaker, leaving behind the white starch which should have settled in the bottom. Add about 100 g distilled water to the starch and stir gently.
- Leave to settle again and then decant the water, leaving the starch behind.
- You can now use the starch to make a plastic film.
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