eli5 Why does paper explode when under pressure?


I was watching one of those hydraulic press videos and I want to know why this happens.

In: 0

Paper contains water, it has a moisture content of around 4% – 6%. Pressure creates friction, which creates heat. The more pressure, the more friction, the more heat. When the heat gets hot enough, the moisture in the paper exceeds the boiling point, and rapidly expands into steam. It’s the same phenomenon that happens when you pop a kernel of corn.

To the best of my knowledge: we don’t know (yet)! There are some hypotheses, and I am confident that people will test things to verify or disprove them. But right now it is too early, all there is are guesses. I would test things myself (yes, seriously), but I don’t own any kind of suitable press…

The popular explanation is that the mechanical structure of the paper fails. While paper seems featureless, it is still made of tiny wood fibers and cellulose. Under pressure, these are compressed, and at some point they break. Like a spring that was compressed, once it breaks all that energy is released in whatever direction it will go. So by this analogy, it’s the same as anything else exploding under pressure – it’s compressed, and at some point it breaks, and all that compression energy is released at once.

However, there isn’t much in the way of science about this. It comes out of reasonable assumptions, but the cellulose itself also stores some energy, and there could be other physical mechanisms for storing and releasing energy than those proposed. So take it with a reasonable grain of salt.