Launching a successful high-altitude balloon is about more than just the payload: to make it to near-space, you’ve got to have a top-quality balloon that’s been treated right. That means avoiding getting it dirty or touching it without gloves. First, though, let’s get into what gives latex balloons their incredible ability to stretch.
The Chemistry
Latex — Natural Rubber — Cis-1,4-polyisoprene — all names for the same little molecule (by Smokefoot, licenced under the Creative Commons Attribution-ShareAlike 3.0 License.)
Latex is a polymer, a molecule formed from many repeating subunits in a long chain. It’s also an elastomer, so it can easily stretch and return to its original shape, much like a rubber band. When a balloon is uninflated all the polymers are tangled with each other, but when you stretch it out by inflating it the chains get pulled, straightened, and aligned. They naturally want to return to their tangled, or “relaxed” state, so it takes force in the form of internal helium pressure to stretch the balloon.
Latex being collected from a tapped rubber tree, Cameroon (by PRA, licenced under the Creative Commons Attribution-ShareAlike 3.0 License.)
Natural rubber comes from the sap of the rubber tree (Hevea brasiliensis), which grows in tropical climates. But when you first harvest the rubber sap, it’s very impure and crystallizes after only a few hours at -25C. You’d have a hard time flying a balloon up to 100,000 feet if it were made from pure natural rubber! Luckily balloon manufacturers add proprietary fillers (that white powder you may notice on the surface of your balloon) and slightly vulcanize the balloons. Vulcanization is a process where you heat rubber with sulfur, causing the polymer chains to link together. This creates a stronger, but stiffer material that lasts longer but won’t stretch as well. Latex high-altitude balloons can function for extended periods at below -60C.
Latex Damage
Even with filler, protectants, and vulcanization, latex balloons are fragile. Many chemical and physical factors can contribute to weakening your balloon and shortening your flight.
1. Temperature: The balloon won’t stretch as easily while cold, so store it at room temperature and try to fill the balloon while it’s above freezing
2. Hydrocarbon oils: Any oils swell and weaken the latex polymers — yes, that includes oils from your fingers! For best performance keep your latex balloon in its bag until right before launch and only touch it with latex or nitrile gloves
3. UV-radiation: Oxygen and UV radiation work together to form free radicals and break the polymer chains (all the details you could want and more in this paper). Keep your balloon in top shape by storing it in the dark, like in a cardboard box
4. Debris: When filling your balloon, make sure you’ve recently cleaned your tarp and nobody walks on it with their dirty shoes. A stray pebble or stick can easily poke a hole in a balloon if you don’t watch out
5. Be gentle while filling your balloon, and avoid pinching, twisting, poking, and other forms of stress which weaken the balloon
6. Ozone: Cis-1,4-polyisoprene is susceptible to an ozonalysis reaction, where the excess ozone in the atmosphere in the ozone layer reacts with the carbon-carbon double bond, breaking it and replacing it with two separate molecules with carbon-oxygen double bonds. This weakens the latex and reduces its life span
What finally ends up popping the balloon, however, is physics. An inflated balloon can almost triple in diameter between launch and when it pops. For a 200g balloon like the one provided in a complete Loonar high altitude balloon kit, that’s almost 10 feet, and for a larger 1500g balloon, that’s over 30 feet! As the balloon rises, the gas inside (usually helium) expands since the atmosphere is less dense at higher altitudes. As the gas pushes against the balloon from the inside and the countering force of the outside atmosphere becomes less and less, the balloon expands, the latex stretches, and when it can’t stretch any further it pops.
A high altitude balloon burst (image courtesy of N9XTN, Licenced under the Creative Commons Attribution-ShareAlike 4.0 License.)
What About After Landing?
Nearly everything seems to degrade the quality of latex balloons, so surely they’re biodegradable? It’s complicated; while pure natural rubber latex is biodegradable, the additives that balloon manufacturers mix in to make the balloons stronger also make them harder to biodegrade. Balloons can take over a year to fully break down, and plenty of animals might be curious enough to eat the torn bits of latex that fall to the ground along with your payload. The long, elastic strips could easily get tangled in their digestive systems. Do your part and pick up your balloon along with your payload when you recover it.
