Penny le Couteur & Jay Burreson (25 page)

This trans isoprene polymer occurs naturally in two substances, gutta-percha and balata. Gutta-percha is obtained from latex of various members of the
Sapotaceae
family, particularly the
Palaquium
tree native to the Malay peninsula. About 80 percent of gutta-percha is the trans polymer of isoprene. Balata, made of similar latex from the
Mimusops globosa,
native to Panama and the northern parts of South America, contains the identical trans polymer. Both gutta-percha and balata can be melted and molded, but after exposure to the air for some time, they become hard and hornlike. As this change does not occur when these substances are kept under water, gutta-percha was used extensively as underwater cable coating during the late nineteenth and early twentieth centuries. Gutta-percha was also used by the medical and dental professions in splints, catheters, and forceps, as a poultice for skin eruptions, and as a filling for cavities in teeth and gums.

The peculiar properties of gutta-percha and balata are probably most appreciated by golfers. The original golf ball was wooden, usually made out of elm or beech. But by sometime around the early part of the eighteenth century, the Scots had invented the “feathery,” a leather outer casing stuffed with goose feathers. A feathery could be hit about twice as far as a wooden ball, but it would become soggy and performed poorly in wet weather. Featheries also had a tendency to split and were more than ten times as expensive as wooden balls.

In 1848 the
gutty
was introduced. Made from gutta-percha that had been boiled in water, molded into a sphere by hand (or later in metal molds), and then allowed to harden, the gutty quickly became popular. But it too had disadvantages. The trans isomer of isoprene tends to become hard and brittle with time, so an older gutta-percha golf ball was likely to break up in midair. The rules of golf were changed to allow play to continue if this happened by substituting a new ball at the position where the largest piece of the old ball had fallen. Balls that became scuffed or scored were observed to travel farther, so factories started to prescore new balls, eventually leading to the dimpled ball of today. At the end of the nineteenth century, isoprene's cis isomer also invaded golf when a ball with rubber wound around a gutta-percha core was introduced; the cover was still made of gutta-percha. A variety of materials are used in modern golf balls; even now many include rubber in their construction. The trans isoprene polymer, often from balata rather than gutta-percha, may still be found in the covers.

Michael Faraday was not alone in experimenting with rubber. In 1823, Charles Macintosh, a Glasgow chemist, used naphtha (a waste product from the local gas works) as a solvent to convert rubber into a pliable coating for fabric. Waterproofed coats made from this treated fabric were known as “macintoshes,” and raincoats are still called this (or “macs”) in Britain today. Macintosh's discovery led to an increased use of rubber in engines, hoses, boots, and overshoes as well as hats and coats.

A period of rubber fever hit the United States in the early 1830s. But despite waterproof qualities, the popularity of these early rubberized garments declined as people realized they became iron-hard and brittle in the winter and melted to a smelly gluelike mess in the summer. Rubber fever was over almost as soon as it began, and it seemed that rubber would remain a curiosity, its only practical use as an eraser. The word
rubber
had been coined, in 1770, by the English chemist Joseph Priestley, who found that a small piece of caoutchouc rubbed out pencil marks more effectively than the moistened bread method then in use. Erasers were marketed as India Rubbers in Britain, furthering the mistaken perception that rubber came from India.

Just as the first round of rubber fever waned, around 1834, American inventor and entrepreneur Charles Goodyear began a series of experiments that launched a much more prolonged period of worldwide rubber fever. Goodyear was a better inventor than entrepreneur. He was in and out of debt all his life, went bankrupt a number of times, and was known to refer to debtors' prisons as his “hotels.” He had the idea that mixing a dry powder with rubber could absorb the excess moisture that made the substance so sticky during hot weather. Following this line of reasoning, Goodyear tried mixing various substances with natural rubber. Nothing worked. Every time the right formulation seemed to be achieved, summertime proved him wrong; rubber-impregnated boots and clothes sagged into an odorous mess whenever the temperature soared. Neighbors complained about the smell from his workshop and his financial backers retreated, but Goodyear still persisted.

One line of experimentation did seem to offer hope. When treated with nitric acid, rubber would turn into a seemingly dry, smooth material that Goodyear hoped would stay that way even when the temperature fluctuated. He once again found financial backers, who managed to get a government contract for nitric acid-treated rubberized mailbags. This time Goodyear was certain that success was finally his. Storing the finished mailbags in a locked room, he took his family on a summer holiday. But on his return he found that his mailbags had melted into the familiar shapeless mess.

Goodyear's great discovery occurred in the winter of 1839, when he had been experimenting with powdered sulfur as his drying agent. He accidentally dropped some rubber mixed with sulfur on the top of a hot stove. Somehow he recognized potential in the charred glutinous mass that formed. He was now certain that sulfur and heat changed rubber in a way that he had been hoping to find, but he did not know how much sulfur or how much heat was necessary. With the family kitchen serving as his laboratory, Goodyear continued his experiments. Sulfur-impregnated rubber samples were pressed between hot irons, roasted in the oven, toasted over the fire, steamed over the kettle, and buried in heated sand.

Goodyear's perseverance finally paid off. After five years he had hit on a process that produced uniform results: a rubber that was consistently tough, elastic, and stable in hot and cold weather. But having shown his ability as an inventor with the successful formulation of rubber, Goodyear proceeded to demonstrate his inability as a business-man. The royalties he gained from his many rubber patents were minimal. Those to whom he sold the rights, however, made fortunes from them. Despite his taking at least thirty-two cases all the way to the U.S. Supreme Court and winning, Goodyear continued to experience patent infringement throughout his life. His heart was not in the business end of rubber. He was still infatuated by what he saw as the substance's endless possibilities: rubber banknotes, jewelry, sails, paint, car springs, ships, musical instruments, floors, wetsuits, life rafts—many of which later appeared.

He was equally inept with foreign patents. He sent a sample of his newly formulated rubber to Britain and prudently did not reveal any details of the vulcanization process. But Thomas Hancock, an English rubber expert, noticed traces of powdered sulfur on one of the samples. When Goodyear finally applied for a British patent, he found that Hancock had filed for a patent on the almost identical vulcanization process just weeks before. Goodyear, declining an offer of a half-share in Hancock's patent if he would drop his claim, sued and lost. In the 1850s, at a World's Fair in London and another in Paris, pavilions built completely of rubber showcased the new material. But Goodyear, unable to pay his bills when his French patent and royalties were canceled on a technicality, once again spent time in debtors' prison. Bizarrely, while he was incarcerated in a French jail, Goodyear was awarded the French Cross of the Legion of Honor. Presumably Emperor Napoleon III was recognizing the inventor and not the entrepreneur when he bestowed this medal.

Goodyear, who wasn't a chemist, had no idea why sulfur and heat worked so well on natural rubber. He was unaware of the structure of isoprene, unaware that natural rubber was its polymer and that, with sulfur, he had achieved the all-important cross-linking between rubber molecules. When heat was supplied, sulfur atoms formed cross-links that held the long chains of rubber molecules in position. It was more than seventy years after Goodyear's fortuitous discovery—named vulcanization after the Roman god of fire, Vulcan—before the English chemist Samuel Shrowder Pickles suggested that rubber was a linear polymer of isoprene and the vulcanization process was finally explained.

The elastic properties of rubber are a direct result of its chemical structure. Randomly coiled chains of the isoprene polymer, on being stretched, straighten out and align themselves in the direction of the stretch. Once the stretching force is removed, the molecules reform coils. The long flexible chains of the all-cis configuration of the natural rubber molecule do not lie close enough together to produce very many effective cross-links between the chains, and the aligned molecules can slip past one another when the substance is under tension. Contrast this with the highly regular zigzags of the all-trans isomer. These molecules can fit closely together, forming effective cross-links that prevent the long chains from slipping past one another—stretching is not possible. Thus gutta-percha and balata, trans isoprenes, are hard, inflexible masses, while rubber, the cis isoprene, is a flexible elastomer.

By adding sulfur to natural rubber and heating, Goodyear created cross-links formed through sulfur to sulfur bonds; heating was necessary to help the formation of these new bonds. Creating enough of these disulfur bonds allows rubber molecules to remain flexible but hinders them slipping past one another.