Using a simple to make jig it is easy to turn a plastic bottle into a strong length of cordage. There are various ways of doing this. If you know of alternative and better ways please let me know.
I made a prototype jig from a couple of screws,some washers, and a craft knife blade. You can adjust the thickness of the cordage by clamping the blade between different layers.
Start by cutting a strip around the bottle. Feed it under the blade and then pull it through. You need to keep the bottle level as it rotates otherwise the cordage will break.
You can get around 15-20m of rope from a 2l plastic bottle. It is strong enough to be unbreakeable by pulling it apart (well for me it is 🙂 ) Here a length is suspending a 7kg bottle
Scientific investigations could be to see how the strength varies with thickness. Whether braiding the cord makes it stronger. Comparing different types of bottles
A video showing how to make a more sophisticated version is below
The cordage also shrinks when heated and this gives it many more applications