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Fantastic Shrinking Plastic

A melted yellow plastic ice-cream wrapper, next to an unmelted version. The melted one is about 25% the size of the original.

Are your ice block wrappers taking up too much space? Shrink them with this easy activity!

Australian Curriculum links

  • Science > Chemical Sciences > Year 3 > ACSSU046
  • Science > Chemical Sciences > Year 6 > ACSSU095

You'll need

  • Oven
  • Ice block wrapper
  • Water
  • Detergent
  • Paper towel
  • Baking tray
  • Aluminium foil
  • Oven mitt
  • Spoon
  • Scissors

Try this

Safety: This activity should be performed with adult supervision. This activity involves the use of an oven. Objects will become very hot in steps 7 and 8.

  1. Preheat the oven to 250°C.
  2. Being careful not to rip the ice block wrapper, open the wrapper and remove the ice block. Open the seams of the wrapper until the wrapper is a flat rectangle.
  3. Wash the wrapper with water and detergent.
  4. Thoroughly dry the wrapper with some paper towel.
  5. Cover the bottom of the baking tray with aluminium foil.
  6. Place the wrapper with the print-side of the wrapper facing upwards on the centre of the baking tray.
  7. Place the baking tray in the oven and leave for two to three minutes. Observe what happens to the wrapper during this time.
  8. Use the oven mitt to remove the baking tray from the oven.
  9. Flatten the edges of wrapper with the back of a spoon.
  10. Allow the wrapper to cool for five minutes.
  11. Cut off any rough edges of the wrapper using scissors.

Further investigation

Try this activity using different types of wrappers such as a wrapper from a chip packet.

What's happening?

Plastics are made from very long molecules called ‘polymers’. Polymers look a bit like a chain and are naturally found in a curled up and tangled state. During the creation of an ice block wrapper, the polymer chains are stretched out to form thin sheets of plastic. When heat is applied to the wrapper in the oven, the polymer chains start to gain energy and vibrate violently. As a result the polymer chains return to their original, curled-up state and the wrapper appears to shrink.

The manufacturers of the ice block wrappers selected polyethylene terephthalate, or PET, to make the wrappers because PET is inexpensive, easy to work with and lightweight. In the process of manufacturing ice block wrappers, PET polymer resins are melted and squeezed under high pressure to create long, thin sheets. The sheets are combined with other materials to improve the properties of the wrapper, such as strength and impermeableness. Graphics and product information are printed on the material whilst it is flat. Rolls of the flat material are thread into high-speed packaging machines that fold it together. The material is heat-sealed and then cut into individual bags. These bags are filled with ice blocks and heat-sealed again.

The heat-sealing quality of PET polymers allows us to shrink the ice block wrapper in the oven. During the process of making the wrappers, high pressure squeezing occurs, which stretches out the polymer chains and locks the polymer chains in a stretched state. If enough heat is applied to the wrapper, the heat energy allows the polymer chains to break free from the stretched state to return to their original smaller, curled up state. When the wrapper shrinks in size, the overall shape of the package remains the same because the polymer chains are still bound to each other and stacked in the multiple thick sheets that make up the packaging sheet. Since the labelling is just a very thin layer of ink, it also shrinks with the rest of the packaging sheet.

Real world links

PET is the easiest plastic to recycle. Recycled plastic can be used to make rubbish bins, park benches, playground equipment, decks and kayaks. However, when plastic is thrown away as waste, the plastic can take up to 50 to 80 years to decompose.