Flat Cups (or how I learned to stop plastic waste and love the environment)

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
Play doh and moulds

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.
Plastic is a polymer, that means long chains built up from individual monomers. Let the children each be a monomer, if they hold hands (with him ugh!) the they're a long polymer. Polystyrene monomers are phenylethene, this is two carbons with a double bond and a phenyl group (Benzene ring - a hydrogen).

Can they name any other plastics? See bottom for list and plastic facts.

Compare to the other insulated cup with air in it. What do they think this is made out of? Not only is it still plastic, it's also polystyrene! They both use similar amounts of plastic, of the same type, one has extra air. They're used differently. Lots of things are made out of polystyrene, we make several million tons of this one t
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.

So it would be very cool to show these ideas. Extrusion moulding is easy with playdoh, you can but syringe like things that extrude various shapes. It feels like injection moulding should also be fairly easy? We make a large mould, put some bolts and wing nuts through, clingfilm and oil so we can get it out then force play doh into a (largeish) hole. then unscrew and hope it works...
You can also do injection moulding with hot glue if the play doh doesn't work.

I don't think we can do the others easily with play doh.


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.

(Here's some by recycling code)

1 - PET and PETE - polyethylene terephthalate - Soft-drink bottles, containers for food and other consumer products, water bottles, peanut butter jars
2 - HDPE- high density polyethylene - Milk and juice bottles, dishwashing and laundry detergent, grocery bags
3 - PVC - polyvinyl chloride - Bottles, food trays, rigid sheets used for packaging, electrical insulation, irrigation pipes
4 - LDPE - low-density polyethylene - Squeeze bottles, wire insulation, grocery bags, trash bags, food storage bags
5 - PP - polypropylene - Fruit and vegetable packages, bottle caps, drinking straws (telltale sign: cloudy appearance)
6 - PS - polystyrene - Packaging, CD covers, Styrofoam, egg cartons, packing “peanuts,” plastic tableware, carryout containers
7 - A combination of plastics, or none of the above - Items already made from recycled goods, semi-rigid food storage, drinking cups

Other plastics are PVA (yes the glue!), plyurathane (foamy cushions), Epoxy, Silicone (a non carbon plastic), polyester, ABS, Teflon.

- manifacture 320 mil tonnes per year (2015 stat)
- most common are polyolefins - polypropylene and polyethylene
- pololefins are popular as they float, resistant to water, air, grease and cleaning solvents, come from cheap natural gas, and are robust enough to survive in the sun while still being easy to mould.
- polyolfins are bad as they degrade very slowly, meaning they last centuries in the natural enviroment. They also get abraded by waves into micro-particles which end up in animals and eventually us!
- polyolfins are also hard to recycle as they're often contaminated and can be damaged by Oxygen and heat during reprocessing.
- about 4% of our petroleum and natural gas production goes towards making plastics.
-about 40% of plastic use is on packaging!
- from the 1940s (development of plastics) to 2015, 8300 Mil tonnes new plastics made, 2000 Mil still in use and the rest is waste, 9% of this was recycled, 12% incinerated and the rest is either in landfill or in the sea :(

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.