Science 7–10 · Year 10

Making a plastic from potato starch and changing it with glycerol

Materials

PracticalLow risk

The idea

A polymer's properties depend on how its chains are held together, so adding a plasticiser such as glycerol turns a brittle starch film into a flexible one, and a starch plastic can be broken down by soil organisms that cannot digest polyethylene.

Safety card

Low riskAn adult supervises

Hazards

hot starch mixture spits and jumps if it boils dry
hydrochloric acid and sodium hydroxide at 0.1 mol/L are mild irritants
hot glassware

Controls

eye protection throughout; stir continuously and keep the heat low
beaker tongs; hotplate on a heat-resistant mat
films disposed of in compostable waste, as the RSC advises

Note

NSW Department of Education, Chemical Safety in Schools (CSIS) 2021 Technical Update: Section 1.7 (risk assessment before use) and Volume 2 Appendix D (generic assessment advice, user codes and DoE categories). Complete a RiskAssess or CSIS site-specific risk assessment before the lesson.

What you need

potato starch 2.5 g per batch (two batches), water 25 mL per batch
hydrochloric acid 0.1 mol/L, 3 mL per batch; sodium hydroxide 0.1 mol/L, about 3 mL per batch; universal indicator paper
glycerol (propane-1,2,3-triol) 2 mL for one batch only
250 mL beaker, hotplate or tripod and gauze with Bunsen burner, glass rod, two labelled Petri dishes or tiles, measuring cylinders
for the follow-on test: damp garden soil in two pots and damp washed sand in two more, all kept equally moist, and two strips of polyethylene bag of similar size to the starch film

How to do it

Mix 2.5 g of starch, 25 mL of water and 3 mL of 0.1 mol/L hydrochloric acid in the beaker; for the second batch also add 2 mL of glycerol.
Heat to boiling and boil gently for 15 minutes, stirring; never let it boil dry.
Neutralise with 0.1 mol/L sodium hydroxide, a little at a time, checking with indicator paper after each addition.
Pour each batch into its labelled dish and leave to dry (about a day on a warm windowsill, two days at room temperature).
Compare the two films: bend each until it cracks, stretch a strip, and record flexibility, strength and appearance.
Follow-on test: bury a weighed piece of the film made without glycerol and a weighed strip of polyethylene in the damp garden soil, one to a pot, and identical pieces in the damp washed sand; keep all four pots equally moist, then dig each piece up carefully, rinse, dry and weigh it after 2 and 4 weeks. The sand pots are the control the soil test needs: a gelatinised starch film disperses in water on its own, so without them mass lost in damp soil cannot be told apart from mass washed away. The glycerol film is not used here: glycerol is about half of its dry mass and dissolves readily in water, so it would wash out into damp soil whether or not soil organisms broke the starch down.

What you should see

Both batches thicken into a translucent gel and dry to a clear film. The film made without glycerol is far more brittle and cracks when bent; the film with glycerol bends and stretches, because glycerol acts as a plasticiser: the RSC explains that it attracts water, which stops crystalline regions forming between the starch chains and so prevents brittleness. In the follow-on test the starch film softens and loses mass in the damp sand as well as in the soil, because a gelatinised starch film disperses in water without any help from organisms; only the loss in the soil over and above the loss in the sand is evidence that soil organisms broke it down, and the polyethylene keeps its mass in both. How fast depends on the warmth and moisture of the soil. The learner knows it worked when the two films differ clearly in the bend test and the only difference between the batches was the glycerol.

What changes

What you change: glycerol added (0 mL or 2 mL)
What you measure: flexibility of the dried film (bend and stretch tests)
What you keep the same: starch mass
water and acid volumes
boiling time
drying conditions and film thickness

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

All plastics are made from oil.
A plastic labelled biodegradable breaks down anywhere, quickly.
Adding a liquid to a plastic must make it weaker.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science 7–10 Syllabus (2023)SC5-MAT-01 SC5-WS-04
Australian Curriculum v9AC9S10I02AC9S10I05

Sources

The pages the author read to write this activity.

All Concept Studio activities