Latex is a colloidal emulsion, a suspension of natural rubber particles in water. Many tropical trees and shrubs produce latex, including Ficus elastica, the houseplant usually referred to as the “rubber plant.” In parts of Mexico latex is still harvested in the traditional manner from wild rubber trees, Castilla elastica. All members of the widely distributed Euphorbia (milkweed or spurge) family are latex producers, including the familiar Christmas poinsettia, the cactuslike succulent Euphorbias from desert regions, deciduous and evergreen shrubby Euphorbias, and “Snow-on-the-Mountain,” an annual, fast-growing North American Euphorbia. Parthenium argentatum, or guayule, a shrub that grows in the southern United States and northern Mexico, also produces much natural rubber. Though it is neither tropical nor a Euphorbia, the humble dandelion is yet another latex producer. The single greatest producer of natural rubber is a tree that originated in the Amazon region of Brazil, Hevea brasiliensis.
CIS AND TRANS
Natural rubber is a polymer of the molecule isoprene. Isoprene, with only five carbon atoms, is the smallest repeating unit of any natural polymer, making rubber the simplest natural polymer. The first chemical experiments on the structure of rubber were conducted by the great English scientist Michael Faraday. Nowadays more often considered a physicist than a chemist, Faraday thought of himself a “natural philosopher,” the boundaries between chemistry and physics being less distinct during his time. Though he is mainly remembered for his physics discoveries in electricity, magnetism, and optics, his contributions to the field of chemistry were substantial and included establishing the chemical formula of rubber as a multiple of C5H8 in 1826.
By 1835 it had been shown that isoprene could be distilled from rubber, suggesting that it was a polymer of repeating C5H8 or isoprene units. Some years later this was confirmed when isoprene was polymerized to a rubberlike mass. The structure of the isoprene molecule is usually written as with two double bonds on adjacent carbon atoms. But rotation occurs freely around any single bond between two carbon atoms, as shown.
So these two structures—and all the other possible twistings around this single bond—are still the same compound. Natural rubber is formed when isoprene molecules add to one another in an end-to-end fashion. This polymerization in rubber produces what are called cis double bonds. A double bond supplies rigidity to a molecule by preventing rotation. The result of this is that the structure on the left, below, known as the cis form, is not the same as the structure on the right, known as the trans form.
For the cis structure, the two H atoms (and also the two CH3 groups) are both on the same side of the double bond, whereas in the trans structure the two H atoms (and also the two CH3 groups) are on different sides of the double bond. This seemingly small difference in the way various groups and atoms are arranged around the double bond has enormous consequences for the properties of the different polymers from the isoprene molecule. Isoprene is only one of many organic compounds with cis and trans forms; they often have quite different properties.
Below four isoprene molecules are shown ready to link up end to end, as indicated with the double-headed arrows, to form the natural rubber molecule.
In the next drawing, the dashed lines indicate where the chain continues with the polymerization of further isoprene molecules.
Natural rubber
New double bonds form when isoprene molecules combine; they are all cis with respect to the polymer chain, that is, the continuous chain of carbon atoms that makes the rubber molecule is on the same side of each double bond.
The carbons of the continuous chain are on the same side of this double bond, so this is a cis structure.
This cis arrangement is essential for the elasticity of rubber. But natural polymerization of the isoprene molecule is not always cis. When the arrangement around the double bond in the polymer is trans, another natural polymer with very different properties from those of rubber is produced. If we use the same isoprene molecule but twisted to the position shown,
and then have four molecules like this add in an end-to-end fashion, joining as indicated again by double-headed arrows;
the result is the trans product.
The continuous carbon chain crosses from one side of this double bond to the other, so this is a trans structure.
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.
RUBBER’S PROMOTERS
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 la
unched 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.
WHAT MAKES IT STRETCH?
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.
The extended cis isomer chain of the rubber molecule cannot fit closely to another rubber molecule, and so few cross-links occur. On stretching, the molecules slip past one another.
The zigzag trans isomer chains can fit closely together, allowing for many cross-links between adjacent molecules. This prevents them from slipping; gutta-percha and balata don’t stretch.
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.
Molecules of rubber with disulfur (-S-S-) cross-links that hinder slippage
After Goodyear’s discovery vulcanized rubber became one of the important commodities of the world and a vital material in wartime. As little as 0.3 percent of sulfur changes the limited temperature range of the elasticity of natural rubber so that it is no longer sticky when warm and brittle when cold. Soft rubber, used to make rubber bands, contains about 1 to 3 percent sulfur; rubber made with 3 to 10 percent sulfur has more cross-links, is more inflexible, and is used for vehicle tires. With even more cross-links, rubber becomes too rigid to be used in applications where flexibility is required, although ebonite—developed by Goodyear’s brother, Nelson—a very hard, black material used as an insulator, is vulcanized rubber containing 23 to 35 percent sulfur.
RUBBER AFFECTS HISTORY
Once the possibilities of vulcanized rubber were recognized, the demand for it began in earnest. Although many tropical trees yield rubberlike latex products, the Amazon rainforests had a monopoly on the Hevea species. Within a very few years so-called rubber barons became extremely wealthy from the work of indentured laborers, mainly natives of the Amazon basin region. While it has not generally been recognized as such, this system of indebted bondage must be considered close to slavery. Once workers were signed on, they were advanced credit to buy equipment and supplies from their employer. Their debt increased as wages never quite covered costs. Rubber harvesters worked from sunrise to sunset tapping rubber trees, collecting latex, curing the coagulating mass over dense smoky fires, and hauling solid balls of blackened latex to waterways for shipping. During the December-to-June rainy season, when latex would not congeal, the workers remained in their dismal camps, guarded by brutal overseers who would not hesitate to shoot would-be escapers.
Less than 1 percent of the trees in the Amazon basin were rubber trees. The best trees gave only about three pounds of rubber a year. A good tapper could manage to produce about twenty-five pounds of smoke-cured rubber a day. Balls of cured latex were taken downstream by canoe to trading posts and eventually reached the city of Manaus, nine hundred miles inland from the At
lantic Ocean, on the Negro River, eleven miles above its confluence with the Amazon River. Manaus grew from a small tropical river town to a boom city on the basis of rubber. The huge profits made by the hundred or so rubber barons—mainly Europeans—and the disparity between their luxurious lifestyle and the wretched conditions of the indentured workers laboring upstream were most obvious in Manaus. Enormous mansions, fancy carriages, luxury stores carrying every manner of exotic goods, manicured gardens, and every other indication of wealth and prosperity could be found in Manaus at the height of the Amazon rubber monopoly between 1890 and 1920. A great opera house featured top stars from the theaters of Europe and America. At one time Manaus even had the distinction of having the greatest number of diamond purchases in the world.
But the rubber bubble was about to burst. As early as the 1870s Britain began to worry about the continual felling of wild rubber trees in tropical forests. Increased yields of latex could be drained from each fallen tree, up to one hundred pounds compared with the three pounds per year from tapping. The Castilla tree, a species that produced an inferior grade of natural rubber known as Peruvian slab, which was used for household goods and children’s toys, faced extinction from this practice. In 1876 an Englishman, Henry Alexander Wickham, left the Amazon on a chartered ship carrying seventy thousand seeds from Hevea brasiliensis, which later proved to be the most prolific source of rubber latex. Amazonian forests had seventeen different species of Hevea trees, and it is not clear whether Wickham knew that the oily seeds he collected were from the most promising species or whether luck played a part in his collecting choice. Nor is it clear why his chartered ship was not searched by Brazilian officials except, possibly, that the authorities thought the rubber tree could not grow anywhere other than the Amazon basin.
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