The Discovery of Vulcanization

Besides balls, the Native Americans used to make lots of other handy things from rubber, such as water-proof shawls, a kind of Wellington boots, flasks, etc. They had not quite reached perfection though. All the stu↵ was rather sticky and short-lasting. Even worse, on a hot day it would melt altogether!

The Europeans took over, and kept on trying to find a better use for such an unusual substance. But it was not that easy.

The problem is that natural rubber is not really a solid. Any external force (such as gravity) makes it flow, albeit slowly. Strictly speaking, it is a liquid polymer melt in the viscous state. Therefore, any natural rubber product keeps changing shape. Exactly how much it “wobbles” or “oozes”, depends strongly on the temperature. High above room temperature, nat-ural rubber is more of a liquid. At low temperatures, the oozing nearly stops. But the high elasticity disappears as well, and the rubber hardens.

An amusing true-life story about the rise and fall of a man named Charles Mackintosh, from Glasgow, Scotland, illustrates the problem. The clever fellow decided to use rubber in the production of raincoats. A thin layer of rubber was placed between two layers of fabric. It worked out very nicely, and the raincoats (called macintoshes) became very popular in the notoriously wet Britain. Mackintosh rapidly became rich, shortly after he started his business in the winter of 1820. However, when summer came along, the temperature rose, and all the rubber flowed out of the macin-toshes. The poor inventor went bankrupt, and the whole idea of padding coats with rubber was abandoned for many years.

Not for too long though. A breakthrough occurred in 1839 when the American C. Goodyear suggested the process for vulcanizing rubber. At the molecular level, rubber consists of polymer chains with frequent double bonds (Figure 7.2). The vulcanization involves adding sulphur atoms to the rubber. They form covalent bonds between the chains (Figure 7.2 b), so the chains become linked together by sulphur bridges. You get a

a c b

Fig. 7.2 A cartoon illustrating vulcanization and elasticity of vulcanized network. Panel (a) depicts a system of polymer chains prior to vulcanization. Panel (b) shows the same polymers after vulcanization: chains are cross-linked and form now a network (cross-linkers are shown as grey balls). Panel (c) gives an idea of network behavior upon deformation. In this figure, the sample is stretched in horizontal direction and shrinks somewhat in vertical direction to maintain practically unchanged volume (unchanged area in the figure). The deformation is achieved by chains uncoiling to a necessary extent, by releasing some of their loops and wiggles, along the direction of stretching; chains going in perpendicular direction become somewhat more crumpled. The main point is that lengths of chains practically do not change, only their shapes rearrange.

polymer network. It is not fluid, even at relatively high temperatures, when a normal polymer melt of unattached chains would start flowing (due to intense thermal motion, making the chains move with respect to each other). At the same time, there is nothing to stop such a network from expanding. Under strain all the chains would stretch as in Figure 7.2 c, so it is still highly elastic. Of course, Goodyear had no idea that rubber was a polymer. (This was discovered almost a hundred years later.) He did not even dream of explaining the vulcanization in the way we have just done.

But his invention started the era of commercial use of new, vulcanized rubber.

The story of the discovery is very interesting in its own right. Charles Goodyear (1800–1860) was not a scientist in the modern sense of the word.

His education was not very deep, and his aspirations were mainly directed toward business. Once he happened to buy a lifebuoy of india (natural) rubber. The unusual material captured the inventor’s imagination. He became literally obsessed with the idea of making rubber strong and pliable.

There was hardly anything Goodyear did not try! He mixed rubber with turpentine, soot, and oil. He burnt it in the oven, as the Native Americans were said to have made some progress by keeping rubber in the bright sun. There were times when Goodyear thought he had succeeded. Then he would persuade investors to support the enterprize, and immediately set up production on a really American scale. Alas, every time the rubber would

114 Giant Molecules: Here, There, and Everywhere

start to run. The products would ooze away, sometimes giving out such a horrible smell that they had to be buried in the ground! The debts were left unpaid, and Goodyear’s numerous children had to live in poverty, and for a while he was even imprisoned for debt. But nothing could stop him.

To be fair, we need to say that Goodyear was not the only person to work on improving india rubber. There was even a kind of “rubber fever”

in the 1820s and early 1830s in North America. However, it was none other than Goodyear who eventually made the breakthrough.

It was entirely accidental. One day he was mixing rubber with sulphur and various other ingredients when he dropped some on top of a hot stove.

The next morning the stove had cooled, and one side of the rubber lump, which was next to the sulphur, had become unrecognizable. It looked like the normal rubber we now use every day. At this stage, Goodyear did something most important, which makes him really deserve the fame, not just credit for being a lucky guy who got the answer by chance. He noticed what had happened, realized its significance, and drew the right conclusion.

The recipe for success had to do both, mixing rubber with sulphur and then heating it.

This is what Goodyear wrote about his discovery: “I was encouraged in my e↵orts by the reflection that what is hidden and unknown and cannot be discovered by scientific research, will most likely be discovered by accident, if at all, by the man who applies himself most perseveringly to the subject, and is most observing of everything related thereto¹.”

Goodyear died almost as poor as he had been in his youth. Nevertheless, his invention became widely popular even during his lifetime. The method of vulcanization that he designed has survived till now with hardly any changes. Furthermore, many of Goodyear’s ideas on how to obtain di↵erent sorts of rubber with particular features are now successfully exploited. For instance, incorporating an inert filler (such as carbon black), results in a very hard and robust rubber that is especially good for tires. (The way it works is that little particles of soot fill in the mesh of the network. This makes it harder to squash.) To obtain the opposite e↵ect, a plasticizer (e.g.

some oil that would help the particles of filler move along the network) is added. This gives rubber that is easily worn away, like that used to make erasers and the like.

Thus, since the second half of the 19th century, the rubber industry has developed very rapidly. The latex of Hevea brasiliensis, growing in the

1Quoted from the book [53], p. 124.

wild, had long remained the only raw material for the industry. However, in 1870 the English smuggled about 100,000 Hevea seeds from Brazil.

Then young trees were cultivated from the seeds in British botanical gardens. They gave birth to vast plantations of rubber trees in the colonies, mainly in Malaysia, Indonesia and Ceylon (Sri Lanka). By the First World War, only a negligible part of the world production of rubber was from Brazil.