The bouncy science of toy superballs

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We all annoyed our parents with these and broke vases with these and chased these down the street into traffic and, on rainy days when we were bored, melted these in the microwave. But what actually makes superballs work?

Some portion of your childhood was spent seeing exactly how much damage could be done with a small, light rubberish ball that some probably well-meaning adult gave you in a birthday party gift bag. And it did have to do damage to make it entertaining. There's only so much fun to be had with bouncing a ball, even if that ball's coefficient of restitution is 0.9. The coefficient of restitution is physics-speak for what percentage of its initial height a ball reaches when it bounces. A dropped super ball will bounce to about ninety percent of the height it was originally dropped from, but it's when it was thrown down a hallway full of vases and glass picture frames that it's bouncy properties got really interesting, at least for kids.

For such a simple set-up, the ball was invented surprisingly late in our history. It wasn't until 1965 that materials science could come up with a cheap way to get maximum bounce. As many parents suspect, the missing ingredient was something infernal: sulfur. Scientist Norman Stingley was playing around with polybutadiene, a substance made up of long strings of carbon atoms. The strings tangled together, letting polybutadiene retain its shape without shattering, but the whole concoction needed something more. Stingley added a little heat and sulfur, and something diabolical happened.

Vulcanization, heating substances with sulphur, had been used before to make tires and raincoats. In the polybutadiene the process did what it always does, used the sulphur atoms to connect one string to the next at random points. Instead of a long string of tangled chains, which could be untangled, or at least pulled apart, the the substance became one big network of long strings tied together. It could be deformed, with force, but it would always snap back to where it started. What emerged was a hunk of material that was incredibly elastic. Happy with his discovery, Stingley named it Zectron, formed it into little lumps, marketed it with the Wham-O Manufacturing Company, and encouraged children around the world to hurl it around with reckless abandon.


Thanks to materials science, the world became less safe that day. But more fun.

Top image by Lenore Edman.

Via Indiana Public Media and Cool Science Club.