Flat Cups

Public summary: 

Flatten a cup then stretch a cup and flatten it and stretch and flat. Look how plastic is recycled, what it's made of, and how we make things.

Flatten a cup, stretch a cup.
Useful information
Kit List: 

Polystyrene Cups (the thin ones without air in like for soft drinks work best)
Heat Gun
Pliers/test tube grippers
Wooden mould
Rubber bands
Further ideas are get some playdough like substance to show all the types of moulding.

Packing Away: 

Put cups and mould in misc and then heat gun, pliers in tool box maybe?


What is this made out of? *hold out cup*
Plastic, it's actually polystyrene. Compare to the other polystyrene cup with air in it. They use similar amounts of plastic one has extra air. They're used differently.
Looking at our cup. How do we think it was made?
There's lots of ways to make things from plastic:
- Injection moulding - fill a mould with the hot plastic, let it set then unscrew mould. You can see mould lines so this wasn't how this cup was made;
- Extrusion moulding - the hot plastic is forced through a mould to form constant shapes like pipes. This wasn't how this cup was made as it changes shape;
- Blow moulding - blow out the shape using air pressure, good for bottles;
- 3D printing - built up slice by slice.

These cups were made by stretching hot plastic disks over a mould and cooling them.

We can demonstrate this. Hot plastic is a bit like a rubber band, so we can stretch it. When it cools it's still stretched out but keeps its shape. Take the cup in the pliers and point the heat gun at it. Heat it evenly and stop before the edges start to curl up. When it gets hot it's going to return to its original shape, a small disk. When it cools it'll stay in this disk shape. You can then heat it up again and stretch it over a mould, keep it held while it cools and it will hold its shape.

This is all because of the polymer form of plastics, the long chains can be stretched out.

Explain what a polymer is - maybe draw a monomer unit or find some string/something else that looks like a polymer chain. For older kids, can go into details of how monomers polymerise (stick to addition, as probably easier to explain than condensation) where the double bond opens out to form two single bonds to monomer units either side. Younger kids may not know what a molecule is - try not to confuse them with too much detail, and maybe just explain using your hand drawn/string model that the plastic is made up of long chains.

The chains want to curl up and tangle because this is entropically better (thermodynamically favourable) than the ordered stretched chains. You might want to say that the chains are "happier" when they're curled up to younger kids, but older ones might be more interested in the entropy-enthalpy stuff (minimising free energy, G = H - TS). For polymers, enthalpy term (H) is generally negligible and G depends on entropy mainly. Possibly discuss the idea that tangled chains are in a lower energy configuration. The contraction as a result of entropic driving force means the polymer acts as an entropy spring. Notice that the disk should actually be an oval, not perfectly circular because the sheet plastic that was stretched over the mould initially was made by rolling out the polymer, so you'll get contraction of the chains in the direction of rolling (I think). This should also work with other stuff, like yoghurt pots or something more exciting...
You can take this further by stretching out the rubber and then cooling it using liquid nitrogen, this is the same as plastic but at a shifted down temperature and you get the same effects! The rubber can be stretched into a hard cup shape, although it will be very cold! This happens because the behaviour of the polymer changes when cooled below the glass transition temperature - this is usually pretty low for polymers (well below room temperature), so rubber gets less rubbery as you cool it down because its polymer chains can't move as freely.

Risk Assessment
Date risk assesment last checked: 
Tue, 27/11/2018
Risk assesment checked by: 
Date risk assesment double checked: 
Thu, 10/01/2019
Risk assesment double-checked by: 
Grace Exley
Risk Assessment: 
Hazard Risk Likelihood Severity Overall Mitigation Likelihood Severity Overall
Heat gun Fire risk and also the possibility of burns 3 3 9 Demonstrator to control heat gun. Do not leave on. Keep flammable material away from the heat gun. Use stand instead of lying heat gun on a surface. Do not touch the heat gun.
In case of burns, run affected area under cold tap for 10 minutes. Call a first aider. Follow venue RA protocols in case of fire.
2 3 6
Liquid nitrogen cooled stuff Tissue damage as a result of touching cooled objects. 3 4 12 Don’t allow people to touch very cold objects. Demonstrator to wear gloves where appropriate.
Call a first aider in the event of an accident.
In the event that they touch anything, call a first aider.
2 3 6
This experiment contains mains electrical parts, see separate risk assessment.