Eli5 why can’t tires be melted down and reused?

Question

Eli5 why can’t tires be melted down and reused?

In: 808

Answer 1

I can offer an engineering ELI5 view here, I used to work in a rubber research lab as a test engineer.

When you look at new rubber (sometimes called green rubber) on a very small scale it is separate, long chains of molecules before you heat it. These chains can be thought of as cooked spaghetti. When you make rubber, it goes through a process called vulcanization. Basically you heat it up to change the structure of your spaghetti shaped molecules.

During vulcanization your floppy cooked spaghetti begin to make bonds to its neighbors. So visualize your cooked spaghetti making more connecting spaghetti strings to each other. Eventually you have a big mass of spaghetti that is all connected and you let that connected mass dry out. You’re basically left with one big ugly spaghetti noodle. That’s your finished product of rubber.

Now undoing this process is tricky, the spaghetti-like molecules don’t want to unbind and go back to being separate, the new spaghetti bonds are rather strong. If you try and heat it up again your rubber will burn before it melts. The combustion reaction destroys your big ugly spaghetti noodle and you’re left with a pile of not so useful ash/mush and nasty smoke. This is because the melting point of a rubber that goes through vulcanization is higher than its combustion point.

One of the cornerstones of recycling plastics and rubbers is trying to melt them down to reshape them. There are two big classes of polymers (rubbers, plastics, and a few other things like them) and those are thermoset or thermoplastics.

The thermosets are like our ugly spaghetti noodles, they have new spaghetti string bonds that make them nearly impossible to reshape or melt down.

The thermoplastics are kinda like the unvulcanized rubber in structure, lots of cooked noodles that DO NOT make links between the noodles when heated. They have a lower melting point than combustion point so you can melt them down and reshape them. Though sometimes those points are closer than you would like, it can be exciting when you overcook your thermoplastic spaghetti molecules.