Balloon Science:
Balloon Physics 101

Besides the useful chemical properties of rubber, the physical properties of rubber make balloons such an interesting and fun hobby. A balloonist with a knowledge of rubber physics is able to exploit that knowledge to get the most out of balloons-- for example, the physical properties of rubber allow balloons to inflate successively larger on repeated inflations, allow the inflator to know when to stop blowing, and for a balloon whose inflated surface area can be 100 square feet to be stored uninflated in your pocket.

The most important physical measurement a balloonist should be familiar with is the stress-strain curve of rubber. I'll frame this discussion in terms of balloons; stress and strain are important in all other materials as well.

So consider the following illustrative examples before we move on:

The stress-strain relationship in a balloon is sigmoidal (it has an S-shape), similar to the figure below.

The way to interpret this diagram is as follows (it should already be second nature to anyone who's blown up a balloon, especially blown one up to popping):

Balloon rubber is an elastic material (this should be obvious). In other words, after a balloon is stretched or inflated, it returns back to its original shape and size when the stress is removed... at least ideally. In reality, rubber doesn't always go back to its exact original shape and size when it is unstretched. In fact, if you've ever blown up a balloon, let it sit for a while and then deflated it, you've noticed that the deflated balloon is a bit bigger than it was before. This effect is called creep or hysteresis. Technically, it means that the current stress and strain of a balloon depend on its history: whether or not it was previously inflated, how big, how old the balloon is, and so on.

A Hysteresis Example. The first time a balloon is inflated (up the top half of the above hysteresis loop), allowed to remain at the upper-right point and the deflated (along the bottom half of the loop), the uninflated stage of the balloon (stress equals zero) happens at a higher strain-- the uninflated balloon is now larger than it was before.

Balloonists can exploit hysteretic effects in one fun way: using repeated inflations and deflations, a balloon can inflate larger and larger each time. Using this technique, an 11-inch balloon (for example) can be blown up to 14 inches or more. Consider the following example from my own experience.

An example of uninflated hysteresis-- these two balloons are both 12" Perfect Products brand from the same bag. The red one is fresh out of the bag, while the pink one has been tightly inflated more than a dozen times.

The two balloons from at left, both from the same bag and originally the same size, blown up to approximately the same stress-- the pink one, having been inflated many times before, is an amazing amount bigger than the red one, blown up for the first time. Notice how the sizes and shapes of the bulb and the neck differ: the fresh balloon has a rounder bulb and narrower neck than the "experienced" one, with a more elongated bulb and wide neck.