How long does helium last in a mylar balloon and does it deflate or stop floating because the helium escapes, or does helium degrade?

Helium can’t degrade, it merely filters out a few molecules at a time, leaving behind other gasses or deflating the balloon completely.

I’m curious too. I could see the knot at the bottom leaking. But what if the opening was fused shut with heat? And where does helium come from?

The helium does not degrade. It escapes through the knot, and even through the surface of the balloon.

Helium is mined from underground, although it is also present in the atmosphere.

Most mylar balloons maintain their shapes for 3–4 days. I’ve had some last a couple of weeks though. One cool thing to do is to watch how a mylar balloon will change in volume by transferring it to a colder space (it will seemingly shrink in size and appear half deflated) and then go right back to normal if you move it back to the warmer space.

Well, I’m not familiar with balloon types, but I know the principles are the same.

Helium is a very small element, and when we encounter it, the molecule is usually He2.
Because it is a ‘noble gas’ the outer shell of the atom’s electrons is filled, and thusly the helium atom is not very reactive at all.

So, to answer your question, the helium does not ‘degrade’ or react with something else.

What happens, is the helium molecules escape between the balloon’s ‘skin’.
Why?
Two reasons.
1) The helium molecules are smaller than the Mylar (or rubber, or whatever) ones, so there are spaces where the helium could slide through. Generally at the seams, or places like that.

2) The air pressure inside the balloon is greater than the pressure outside, so the gas inside wants to escape. Since there are small holes they can slip out of, the helium atoms do so until the pressure inside the balloon is the same as the pressure outside.

I could go into more detail, but I feel that this ‘dissertation’ should suffice.

Though if you want to know more, look up, or ask a chemistry teacher about the ‘Gas Laws’. That would prove a good starting point.

Is there anything that @pdworkin doesn’t know?!

How long it stays aloft varies a lot. I had one come down from my ceiling two days ago that had been up there since October 25th. But others of the same lot (from a party)—duplicates of that one, same size and shape and even color—drifted down several weeks ago. I think that means that one factor is how completely they were filled.

From the same party, the plain round ones stayed up the longest, and the leaf-shaped ones came down first. The leaf ones were the heaviest overall and, because of their irregular outline, probably had longer seams and more actual uninflated edge area than the round ones. So weight per inflatable capacity is probably another factor.

Of course if there is anything hanging from them, like ribbons or string, that will weigh them down too.

Finally, if yours are like mine, they are just tied shut with a ribbon. I expect that the helium is mostly escaping not through the mylar but through the ribbon-tied seal. How tightly they are tied is going to affect the leakage.

sorry, I don’t mean to be a know-it-all. I just spend too much time on here in order to avoid what I should be doing.

Naw, I appreciate your knowledge. (I should be doing my work now as well)

Thank you all for the erudite responses. Is there a balloon material that’s less permeable? Could last longer?

I don’t know for certain, but maybe weather balloons are made to hold their helium longer. It’s worth checking out.

Basically, helium gets out of a balloon the same way ice crystals get inside a bag of frozen peas, one non-reactive molecule at a time sneaking through the gaps between larger molecules.

There are tricks to make helium balloons last longer. You can spray the outside with a form of gel such as Hi-Float that slows the leaking of helium through the material of the balloon. You can also coat the inside of the balloon with a gel called Super Hi-Float, which again makes the balloon better resistant to helium molecules sneaking out between the molecules making up the balloon. You can also use special balloon material that is actually a sandwich of different materials that also stops the tiny helium molecules from making their way out between the molecules of the balloon.

This site gives all sorts of tips for keeping balloons aloft longer.

And weather balloons are made out of a special non-woven and seamless fabric made with urethane to make them inflatable and less permeable than simple latex or mylar balloons. This company is one that makes them in the U.S.

Like hydrogen, some of the helium will escape from the atmosphere into interplanetary space.

The helium slowly escapes in fine holes through the material it’s made out of.

@RAMesesII “the molecule is usually He2” Nope, it’s going to be ‘He’ single atoms by themselves. The noble gasses rarely combine with anything else including themselves.