Balloons are made of latex rubber. Latex rubber is a polymer – that is a material made from long molecules that tangle up like spaghetti. And just like spaghetti, there are spaces in between the molecules that can let smaller molecules through.
The real question about balloons is why it takes so long for the air to leak out. In 1831, a man named Thomas Graham figured it out.
It turns out that all gas molecules have the same energy if they are at the same temperature. The energy of a gas molecule is a combination of how heavy it is, and how fast it is moving. That means that if there are two molecules at the same temperature, but one is lighter than the other, the lighter one must be moving faster.
Fast moving molecules leak faster than slow moving ones.
The lightest gas molecule is hydrogen. It has a weight of 2. The second lightest is helium, which has a weight of 4. To find out how much faster hydrogen leaks from a balloon than helium, we divide the square root of the heavier weight by the square root of the lighter one. So hydrogen leaks 1.414 times as fast as helium.
An oxygen molecule has a weight of 32. It stays in the balloon four times as long as hydrogen. Air is mostly nitrogen molecules, with a weight of 28. Nitrogen stays in the balloon 3.74 times as long as hydrogen.
We would expect butane, a gas whose molecules have four carbons and ten hydrogens, weighing 58, to stay in the balloon over five times longer than hydrogen, and 1.414 times longer than air (which has an average weight of about 29). But butane leaks out of the balloon surprisingly fast. I suspect that the butane is reacting with the latex rubber, and either making it stretch out more, so it is thinner, or by actually dissolving the rubber a little bit, so the long molecules are separated by butane molecules, which can then leak out quickly.