What Makes Rubber Rubbery?

What Makes Rubber Rubbery?


This video is sponsored by the Rubber Division
of ACS. Watching your favorite team throw the pigskin
around is an autumn weekend ritual — but there’s something they don’t want you
to know. As much fun as some people have watching guys
in tights fight over who gets to hold a weirdly shaped hand-egg, they’re LYING to you. That’s right — footballs aren’t made
of pigskin(1). Instead, sports balls of all varieties owe
their resilience and reliability to an unusual polymer, one whose derivatives and spinoffs
are everywhere you look, from cars to shoes to rocket fuel. Yes, rocket fuel. Meet the all star’s best friend: Rubber. For hundreds of years, indigenous peoples
of Central and South America like the Olmecs have known that you can extract a stretchy,
bouncy substance from the sap of certain trees, most notably Hevea brasiliensis. This is natural rubber latex. The sap contains polyisoprene, which is processed
into natural rubber, or dried and used as latex for things like surgical gloves. The polyisoprene strings are springy and flexible,
giving rubber its stretchy properties. Natural rubber has some flaws. It flows readily at high temperatures and
becomes rigid when it’s cold. That’s because the long strings of polyisoprene
are free to flow past each other inside the rubber, so they can ooze or solidify like
any other substance. Charles Goodyear found in 1839 that adding
sulfur to rubber, then heating the mixture, causes a chemical change to take place. Atoms of sulfur form bridges between the polyisoprene
chains, linking them together. They can no longer drift past each other. The flexible polyisoprene and tough sulfur
cross-links strike a balance between flexibility and rigidity that are the key to rubber’s
bouncy, stretchy properties. It can deform, but return to its original
shape time and time again. Like stretch armstrong. Remember him? When rubber became temperature-resistant,
aka ‘vulcanized’, that was its chance to take over the world. Not only was it suitable to be made into tires;
it could also be used to make engine turn belts and seal up gaps, even at high heat. Yup, rubber totally made cars a thing. But its cultural awesomeness doesn’t end
there. Most rubber these days isn’t derived from
trees. There are dozens of kinds of synthetic rubber,
and even more kinds of not-rubber-but-still-kinda-stretchy-and-useful materials, made from petroleum. One kind of synthetic rubber, polysulfide
rubber, or Thiokol is even used to bind rocket fuel together. But back to sports, because we know that’s
what you really watch science videos for. Most of your good ol’ sports balls are going
to have some rubber in them, or one of those similar stretchy substances. Footballs contain an air bladder made of polyurethane,
surrounded by leather made from cows — not pigs. Originally, footballs were probably made of
animal bladders, which might actually have come from pigs — but that’s still not their
skin. Rubber and then urethane replaced the bladders
— to the pigs’ relief. And while rubber or urethane holds its shape,
animal bladders don’t. It’s possible that the pointed shape of
modern footballs is just because the animal bladders were so wonky they gave up on trying
to have a round ball. How much rubber is in the ball affects what
it can do. This tennis ball is filled with air, which
is easily compressed, so it smushes flat when it bounces. The rubber snaps the ball back into shape,
but that requires energy. So some of the energy the ball had on the
way down is lost on the way back up. A super ball, on the other hand, is one giant
chunk of polybutadiene– another synthetic rubber.  And the whole thing is vulcanized. The tough sulfur bridges prevent the ball
from losing its shape too much, and most of the energy carries over to the next bounce. Basketballs bounce much like the tennis ball,
but the NBA apparently favors natural rubber and leather. And baseballs need to fly when you hit them,
but not so far that every hit is a home run. So while they have a rubber core, it’s wrapped
in string to deaden the bounciness a little, then in leather for grip. So there you
have it: All your fave sports rely on rubber, or rubber-ish things, to make the play. And you need rubber to drive to the ball park
and rubber to fly to space to watch space volleyball. I made that one up, but how dope would that
be? Thanks to the Rubber Division of the ACS for
making this video possible. Engage with them at rubber.org to learn more
about elastomer science, and to find out how to become part of this community. Sound off in the comments, and be sure to
hit like and subscribe as you’re scrolling down. See you out there, sports fans. And hey, thanks for watching.

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  1. Quick fun fact–if you’re allergic to latex, there are some other allergies that tend to go with it almost all of the time. If you’re allergic to bananas, it turns out you’re more likely to be allergic to latex gloves too.

    Outside of latex, what's your favorite application of rubber? I'm pretty transfixed on watching that tennis ball flatten out.

  2. what about the environmental impact of producing vulcanised rubber? and what is the best way to dispose of rubber?

  3. My favourite part of rubber's history is that Goodyear discovered it by accident. The story says he knocked sulfur and rubber onto a hot stove, walked away and came back later to find vulcanized rubber. Because sometimes that's science too.

  4. I live near Vulcan, there's no star trek themed hockey that I know of but they have a lot of star trek themed stuff. Even the funeral home has a mural of Spock painted on it.

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