Balloon Science:
Rubber Chemistry 101


The existence of balloons is predicated on the existence of rubber. Quite obviously, if rubber didn't exist, all we'd have are those mylar bags, and what fun would that be? To get the most out of the balloon experience, it's helpful to be at least partially versed in the chemistry and physics that make rubber what it is.

Natural rubber is primarily made up of a chemical called cis-polyisoprene. This can be made in the laboratory using a Ziegler-Natta catalyst to facilitate the cis polymerization of isoprene (don't worry if this doesn't make sense-- laboratory latex isn't what makes up most balloons). The chemical structure of cis-polyisoprene is shown below. (C represents an atom of carbon; H an atom of hydrogen.)

Rubber is a polymer, that is, a long chain-like molecule made up of repeating units (in this case, the repeating unit is called isoprene). Most rubber chains extend to be very long, usually at least 3000 times as long as the chain shown at left.

The special thing about rubber is the unique shape the polymer cis-polyisoprene forms. While the molecule drawn at left looks flat and boring, the actual molecule in three dimensions forms a shape that strongly resembles a coiled spring. A three-dimensional view is shown below (gray sticks represent carbons, blue hydrogens). Click the thumbnail for a bigger view.

The spring shape of rubber molecules is the reason rubber is elastic. In a way few other materials can, a piece of rubber can stretch to many times its original size and return to its original size when released (well, almost-- see Hysteresis).

However, natural rubber in its raw state is the consistency of putty-- it stretches a little, but tears apart easily. This is because while the "springs" are stretchy, there is nothing holding one "spring" to another, and when pulled, the "springs" slide past one another and the material pulls apart. The process of vulcanization (developed by Charles Goodyear) uses sulfur and heat to create cross-links between chains. This creates a substance that is still stretchy due to the spring-shaped polymers, but also strong due to the links between the individual rubber molecules. Balloon latex is lightly vulcanized, creating these cross-links at 1 to 2 percent of available sites. This makes for a very elastic rubber that is comparatively not that strong, making it possible to be inflated by mouth. Car tires and inner-tubes are more heavily vulcanized, usually around 5 to 10 percent, and so are less elastic than balloon rubber but very strong (try supporting a two-ton truck with balloons instead of tires, and you'll appreciate what vulcanization can do!).

At left is a view of vulcanized rubber (gray balls are carbon atoms, blue hydrogen, yellow sulfur). Notice how the disulfide bonds hold one "spring" to the other; this extends out in all directions, holding thousands and millions of these springy molecules together and strengthening the rubber. Again, click on the picture for a larger view.

So now that we know what rubber is, so what? We've found out why rubber is stretchy, and that covers the most important thing balloonists hold dear about balloons. But the molecular structure of rubber can explain many more phenomena about balloons than simply their stretchiness.

Why do balloons shrink over time?

Anyone that's ever kept helium balloons around for any long period of time knows that several hours after they're inflated, they begin to shrink and shrivel as they lose their helium. Eventually, enough helium leaks out that the balloon is no longer buoyant, and the shriveling balloon drifts to the ground.

The reason why this happens lies in the structure of rubber. The sulfur cross-links between spring-shaped molecules make the rubber strong, but also make it porous. In particular, helium (which is a very small molecule) can worm its way through these pores and escape the balloon-- leaking out into the surrounding air and causing the balloon to shrink. This happens with balloons filled with air as well, but since air molecules (nitrogen, oxygen, and carbon dioxide) are much larger than helium molecules, the leakage is much slower. The diagram at right shows the leakage of helium (pink) through a vulcanized sheet of rubber. Click on the image for a larger view.

Why are inflated balloons shiny?

Unstretched rubber

Stretched rubber

One of the aspects of balloons that make them so beautiful to look at is their shine. A properly inflated balloon, no matter the brand or color, will always have a glistening shine to it. Furthermore, the tighter a balloon is inflated, the shinier it becomes.

The reason a material shines is that it reflects light coherently, that is, two parallel light rays remain parallel after being reflected. Mirrors reflect light in this way, as does glass, and any smooth metal. All these materials have a surface that is uniform and smooth, allowing reflected light to remain parallel. Rough surfaces scatter light in many different directions, making them poor reflecting material.

Rubber that is not stretched, such as that of an uninflated balloon, is highly coiled and presents an uneven, rough surface for light that scatters incoming light rays and makes the rubber appear soft (above left-- click for a better view).

Stretched rubber, such as an inflated balloon, elongates the rubber molecule and making it straighter. This straighter, smoother surface reflects light very well. In particular, the tighter the balloon rubber is stretched, the straighter the rubber molecules and the better the reflection of incoming light, making overinflated balloons much shinier than underinflated ones.

However, keep an inflated balloon around long enough and it will lose its shine-- partially because it shrinks (see above), but mostly due to the oxidation of the rubber by molecular oxygen in the air (see Oxidation). This puts a layer of oxidized rubber on the outside of the balloon, and since oxidized rubber is chalky and molecularly rough, the light can no longer be reflected as coherently as in a new balloon.