Guest post by Rowena Fletcher-Wood

Perhaps, if you spend enough time looking, you can find anything. So it was for Charles Goodyear, a would-be inventor who, at the expense of everything else, bounced back after every failure, devoting his life to transforming natural rubber into a commercially useful material. He saw the potential immediately – just not the chemistry.

The rubber in Goodyear’s hands during the early 1830s wasn’t a particularly useful material. It was temperamental: whilst it exhibited promising properties including elasticity, hydrophobicity, adhesiveness and electrical insulation, when it got hot it would melt and turn into a horrible sticky slime, and when it got cold in the chilly English weather it would become brittle and readily crack.

Looking at the structure of rubber, it all makes sense: a natural cis polymer of isoprene, this allowed it to stretch (whereas the trans polymer of isoprene, gutta-percha, is crystalline) and the chains could readily flow past each other, especially when warmed. Equally, when solidified, splits could propagate rapidly and directionally between the chains of polymers. Goodyear put a lot of time and effort into trying to mop up the runny rubber by mixing it with various different dry powders and attempting to reform it into a ball. But it would take chemical rather than physical methods to get this compound to bend to his will.

Rubber was introduced to Europe by Charles Marie de La Condamine in 1736 and named by Joseph Priestley, who first studied it in the UK. Made from latex sap, it is a gooey, milky white colloid containing around 35% polyisoprene molecules and 5% impurities – mostly natural organic impurities like proteins, sugars and fatty acids – and inorganic salts, all suspended in a solution. This colloid is tapped from the Hevea brasilienesis rubber tree by cutting long diagonal strips in the bark and letting it run. Astonishingly, after 3 hours of tapping, a tree produces only enough latex to fill a cup. Eventually the latex coagulates, much like blood clotting over a wound, and is pressed to dry it of excess liquids.

As pressing had worked before, Goodyear was convinced that his rubber just needed to be dried out a little more, and no matter how many times he failed, he bounced back and tried again, much like the rubber he was toying with. One of the drying powders Goodyear used was sulfur. It didn’t work, but he persisted. He persisted so much that he did months of jail time for debt, only to come back out and begin again. He was a man driven by obsession, passionate about a utopian vision of ubiquitous rubber (he may have dreamed of tyres, marigolds and Wellington boots.)

Then, one day, he accidentally dropped his precious sulfur-dried rubber on the stove, filling the air with the horrible eggy reek of burning sulfur. Somehow, Goodyear managed to get the blackened and pungent rubber back off the stove, and to examine what he’d done to it. What he found was exactly what he’d been looking for all these years, what he’d got into debt and gone to prison for – vulcanised rubber. The new vulcanised rubber did neither melt nor crack, was harder wearing and more chemically resistant than its precursor, whilst remaining springy and becoming even more waterproof. He was elated.

Although Goodyear never really understood the structural changes he had made to rubber, its interesting characteristics are today widely appreciated. By creating sulfur crosslinks between the polyisoprene, the rubber essentially becomes one big molecule. When deformed, the sulfur crosslinks make it spring back into shape and stop it from running fluidly in the heat, or interrupt the propagation of cracks, making a tough, energy absorbent material. Hard to break down, the sulfur crosslinks do make rubber hard to recycle and impossible to reshape: light crosslinking allows a compromise between unvulcanised thermoplastic or vulcanised thermosetting properties and other chemical modifiers may be added, as today they often are.

After his years of debt and obsession, Goodyear’s clumsy breakthrough came too late to free him of his financial constraints, and he died in debt in 1860. The ‘Goodyear Tire and Rubber Company’ was founded in 1898, and named in his honour.

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Bouncing back, 10.0 out of 10 based on 2 ratings
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