Rubber Used in Bicycle Tires

Video Transcript

"Charles McMahon for Expert Village. We’re talking about the bicycle. You can get more information in my college level textbook. That is described on merionmedia.com. We talked about the tires. I wanted to tell you something about the rubber used in tires. Now the first rubber came from a tree, a rubber tree, they gave a latex and then it was melted together, but pure latex would just keep stretching, and stretching, stretching and obviously you know that rubber like a rubber band you stretch it, you let it go, it comes back, so you have to build a memory into the rubber. You have to make the rubber an elastomer, an elastomer is something you can stretch a whole lot and if you let the force off, it goes back to its original shape. Now the way you transform just latex rubber into an elastomer is to heat it up and throw in some sulphur, this is called vulcanizing and it was invented by Charles Goodyear back in 18 somethings. Basically the sulphur atoms, let me tell you about what happens when you take a polymer like a latex, it is a long chain molecule, very, very long, hold together with carbon bonds, so you take this chain and you stretch it, if you let the force off, it goes back into a little ball because of entropy, so rubber is an entropic material, the restoring force is not the same as in the metal, it is entropy. Now what you want to do is when you stretch it, you want to keep the chains from going past each other, because if they slide past each other then it won’t come back to its original shape. So you put in the sulphur atoms, the sulphur atoms get in between the chains, and form bonds from one chain to another and have all these little bonds, these cross lengths all along the chain, so that when you stretch the thing, the chains can’t slide past each other. They will slide a certain amount and then they are stuck by these bonds, by the sulphur bonds, and when you let the force off your chains will curl up so that’s how the rubber band works and that is how any rubber works as rubber in your tires. Now the sulphur is an important compound, another important compound is the filler. You mix in lampblack, a little tiny, tiny nanno-scale carbon globs, you mix that in, lampblack, carbon black and that is a strengthening mechanism, because the chains bond to these little things and around each particle of carbon black the chains can’t stretch so much. So this is part of the strengthening mechanism. If you leave the carbon black out then the vulcanized rubber is not nearly as stiff and strong as it would be if you filled it with the carbon black. "