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.

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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.

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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

 

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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