# Why does a balloon suddenly pops?

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I was just at a graduation party when a balloon suddenly pops. Nothing in it’s way or such. It just suddenly pops. How and why?

In: Other

You may not have seen it, but sometimes hair or dust particles can cause a balloon to pop. Those things float around in the air. Also hot temperature can make a balloon pop since the rubber becomes weak due to the heat

This usually happens when the balloon is over-inflated and has a tendency toward collapse. The air pressure inside the balloon is higher than air and thus becomes too much for the elastic balloon material to handle it.

They run into something called “critical crack length”.

For any material in tension, like a balloon skin, it takes some energy to expand an existing crack (basically, how hard it is to tear the rubber). But when the crack grows a little bit, the rubber immediately around it relaxes a little (because it’s not in tension anymore), which releases energy. Through a bunch of messy math, you can show that the amount of energy released depends on how big the crack already is (bigger cracks release more energy when they grow than small ones) and how much tension there is (more tension = more energy release).

So small cracks can’t release enough energy to tear the rubber and they just sit there. Big cracks can release *more* energy than they take to tear the rubber and they grow *very* fast. The breakeven point is called the “critical crack length”, and it depends on how much tension the rubber is under. More tension = smaller crack will go critical.

Now the weird part…virtually all real materials have tiny cracks/holes in them. Most of the time you never notice this in everyday life…it only shows up in materials that are stressed up close to their limits. But balloons are one of those materials that operate up near their limit…if you overinflate the balloon, one of the tiny pre-existing cracks will eventually go critical and “Pop!”.

But just sitting there, something else is going on…creep. Some materials, including rubber, can slowly deform under constant load. Not all materials do this (most metals don’t at the stress we use them), but rubber and many plastics do. So you inflate a balloon and it’s all cool and none of the cracks/holes are critical. It just sits there. Except it’s creeping…the cracks & holes are sloooooowly getting bigger. Eventually one might get big enough to go critical, then “Pop!”. Or it’ll get close, and then something really subtle like a bump or gust or dust or hair or temperature swing will alter the material properties just enough that a subcritical crack goes critical…and Pop!.