When Charles Goodyear figured out how to take the smelly sap from some trees and turn it into the rubber of industry, the finished, stretchy product bounced a crazy diverse number of materials out of their soon-to-be former jobs. Like sheep intestines. Ick.
Before rubber, wheels were made of wood and steel. That's how we rolled. Slick or bumpy roads did not go over well for our vehicles or for our backs. Rubber changed all that. Now more than half of all rubber produced is turned into to tyres, and alternatives are unfathomable for anything but kids' toys.
You know what else rubber replaced? Fish membranes and sheep intestines. Before rubbers were made of rubber, people had to get creative with their contraception. (So don't complain about wearing them now, people. It could be worse. Sheep. Guts.) Rubber condoms put the offals out of business before latex, another rubber product, stepped in to make safer sex more comfortable.
But even before Goodyear, scientists knew that what came out of this particular kind of Brazilian tree was special, they just didn't know the extent of it. Early on, the raw material was used for waterproofing. And you know how the British call erasers "rubbers"? Well there's a good reason for it. An English chemist named Joseph Priestley realised in 1770 that the malleable substance was good for rubbing out pencil marks, which gave rubber its name. But problems remained — namely it was sticky, smelly and perishable. It also became stiff in cold weather and turned soft in the heat.
Charles Goodyear — who did not found the company, but was the inspiration for the name, BTW — figured out in 1839 that heating the rubber and mixing it with sulfur fixed all these problems.
Needless to say, Goodyear's rubber-processing innovation, called vulcanisation, was a BFD.
When rubber's long chain molecules are mixed with sulfur, the latter isn't just dissolved or dispersed. Instead, the long chains become cross-linked, which strengthens rubber's support system. Even crazier: "These sulfur linkages can actually break and repair themselves," explains Michael Kerns, a manager of global materials science group at Goodyear. The cross-linked structure also allows the rubber to resist flowing, which is how rubber is able to handle all that friction-created heat. Only when it gets really, really hot, explains Kerns, does it begin to break down.
Rubber also accepts additives to reinforce its features. Carbon black, for instance, is a standard reinforcement agent. It's also the thing that gives tyre rubber its recognisable colour. "The earliest tyres were actually white," explains Kerns.
Rubber became so key in the automobile industry that we could no longer rely on natural sources to provide it. The kind that's tapped from trees only grows within certain latitudes and under certain conditions, so the supply isn't guaranteed for countries that live outside that magical rubber-producing zone (ahem, us). This became very clear when Japan entered World War II and Asian sources of rubber became unavailable to the Allies. Think about it: every wheel for every passenger vehicle, tank and aeroplane relied on the material, and our supplier was suddenly out of touch. Switching back to something like wood obviously wasn't an option. Desperate need drove innovation, and a big government push birthed the synthetic rubber industry practically overnight. And by the 1960s, production of synthetic rubber overtook production from natural sources.
Still, tree-tapped rubber is pretty special — in something like its resistance to cracking and tearing, its abilities still elude synthetics. This is an unfair comparison, but think for a moment about how something like glass deals with a flaw; a small stress can cause the whole thing to shatter. Natural rubber, on the other hand, actively works against imperfections on the molecular level. "When a crack starts spreading, rubber actually crystallises to resist further tearing," explains Kerns.
Scientists aren't entirely clear on how exactly its composition makes it so special, but it has something to do with the elastic polymer called polyisoprene. Inside natural rubber, the polymer's chains are laid out in a highly regular pattern that works against external strain. So for applications where the strain is great, like when big construction tyres take on rocks or when aircraft tires take on a runway, natural rubber is still used almost exclusively. (For passenger vehicles, though, synthetics work well. In fact, scientists are constantly pushing them to work harder for us, which can improve gas mileage and tire-life. )
Our electronics also love synthetics. It's a popular bumper for our phones because it can handle being dropped, thrown, or smacked against the ground, unlike other materials that crack under the pressure. It sits beneath our laptops so our computers don't slide of the table when we lean into them. It's a good protector for other reasons too. Rubber's flexibility and ability to manage heat make it perfect for swaddling electrical wires.
Basically, rubber takes all these materials that have a hard time dealing with us or the world, and it mediates their interactions to make them less brutal for everybody. Not such a bad friend to have around.
The Materialist is a regular column about the materials that make up the things we love, want or just plain can't live without.