Wickham took great care in transporting his cargo, packing his oily seeds carefully to prevent them from becoming rancid or germinating. Early one morning in June 1876 he arrived at the home of the eminent botanist Joseph Hooker, curator of the Royal Botanical Gardens at Kew, outside London. A propagating house was established, and the rubber tree seeds were planted. A few days later some of these seeds began to germinate, the forerunners of more than nineteen hundred rubber tree seedlings that were to be sent to Asia—the start of another great rubber dynasty. The first seedlings, sealed in miniature greenhouses and carefully tended, were shipped to Colombo in Ceylon (now Sri Lanka).
Very little was known at the time about the growth habits of the rubber tree or how growing conditions in Asia would affect the production of latex. Kew Gardens established a program of intensive scientific study of all aspects of cultivation of Hevea brasiliensis and found that, contrary to current belief, well-cared-for trees could be tapped daily for latex. Cultivated trees would begin producing after four years, whereas the age for starting to tap wild rubber trees had always been assumed to be around twenty-five years.
The first two rubber plantations were established in Selangor, in what is now western Malaysia. In 1896 clear, amber-colored Malayan rubber first arrived in London. The Dutch soon established plantations in Java and Sumatra, and by 1907 the British had some ten million rubber trees, planted in orderly rows, spread over 300,000 acres in Malaya and Ceylon. Thousands of immigrant workers were imported, Chinese into Malaya and Tamils into Ceylon, to supply the labor force necessary for the cultivation of natural rubber.
Africa was also affected by the demand for rubber, in particular the central African region of the Congo. During the 1880s, Leopold II of Belgium, finding that the British, French, Germans, Portuguese, and Italians had already partitioned much of the western, southern, and eastern African continent, colonized areas of less coveted central Africa, where for centuries the slave trade had reduced the region’s population. The nineteenth-century ivory trade had an equally devastating effect, disrupting traditional ways of life. A favorite method of ivory traders was to capture local people, demand ivory for their release, and force whole villages into dangerous elephant-trapping expeditions to save their families. As ivory became scarce and the worldwide price of rubber increased, traders switched to requiring, as ransom, red rubber from the wild rubber vine that grew in the forests of the Congo basin.
Leopold used the rubber trade to finance the first formal colonial rule in central Africa. He leased enormous tracts of land to commercial companies such as the Anglo-Belgian India Rubber Company and the Antwerp Company. Profit from rubber depended on volume. Sap collecting became compulsory for the Congolese, and military force was used to convince them to abandon their agricultural livelihoods to harvest rubber. Whole villages would hide from the Belgians to avoid being enslaved. Barbaric punishments were common; those who didn’t gather enough rubber could have their hands cut off by machete. Despite some humanitarian protest against Leopold’s regime, other colonizing nations allowed companies leasing their rubber concessions to use forced labor on a large scale.
HISTORY AFFECTS RUBBER
Unlike other molecules, rubber was as much changed by history as history changed it. The word rubber now applies to a variety of polymer structures whose development was hastened by events of the twentieth century. The supply of plantation-grown natural rubber quickly surpassed the supply of natural rubber from the Amazon rain forests, and by 1932, 98 percent of rubber came from plantations in Southeast Asia. Its dependence on this source was of great concern to the government of the United States, as—despite a program of stockpiling rubber—much more rubber was needed for the nation’s growing industrialization and transportation sector. After the December 1941 Japanese attack on Pearl Harbor brought the United States into World War II, President Franklin Delano Roosevelt appointed a special commission to investigate proposed solutions to looming wartime rubber shortages. The commission concluded that “if we fail to secure quickly a large rubber supply, our war effort and our domestic economy both will fail.” They dismissed the idea of extracting natural rubbers from a variety of different plants grown in different states: rabbit brush in California, dandelions in Minnesota. Though Russia actually did use its native dandelions as an emergency source of rubber during the war, Roosevelt’s commission felt that latex yields from such sources would be low and the quality of the latex questionable. The only reasonable solution, they thought, was the manufacture of synthetic rubber.
Attempts to make synthetic rubber by the polymerization of isoprene had hitherto been without success. The problem was rubber’s cis double bonds. When natural rubber is produced, enzymes control the polymerization process so that the double bonds are cis. No such control was available for the synthetic process, and the result was a product in which the double bonds were a randomly arranged mix of both the cis and trans form.
A similar variable isoprene polymer was already known to occur naturally in latex from the South American sapodilla tree, Achras sapota. It was known as “chicle” and had long been used for making chewing gum. The chewing of gum appears to have been an ancient practice; pieces of chewed tree resins have been unearthed with prehistoric artifacts. Ancient Greeks would chew the resin of the mastic tree, a shrub found in parts of the Middle East, Turkey, and Greece, where it is still chewed today. In New England local Indians chewed the hardened sap of the spruce tree, a habit adopted by European settlers. Spruce gum had a distinctive and overpowering flavor. But it often contained difficult-to-remove impurities, so gum made of paraffin wax became more popular among colonists.
Chicle, chewed by the Mayan people of Mexico, Guatemala, and Belize for at least a thousand years, was introduced to the United States by General Antonio López de Santa Anna, conqueror of the Alamo. As president of Mexico, around 1855, Santa Anna agreed to land deals in which Mexico relinquished all territories north of the Rio Grande; as a result he was deposed and exiled from his homeland. He hoped the sale of chicle—as a substitute for rubber latex—to American rubber interests would allow him to raise a militia and reclaim the presidency of Mexico. But he did not count on the random cis and trans double bonds of chicle. Despite numerous efforts by Santa Anna and his partner Thomas Adams, a photographer and an inventor, gum chicle could not be vulcanized to an acceptable rubber substitute, nor could it be effectively blended with rubber. Chicle appeared to have no commercial value until Adams saw a child buying a penny’s worth of paraffin chewing gum in a drugstore and remembered that the natives of Mexico had chewed chicle for years. He decided that this might be the solution for the supply of chicle stored in his warehouse. Chicle-based gum, sweetened with powdered sugar and variously flavored, soon became the basis of a growing chewing gum industry.
Though it was issued to troops during World War II to keep the men alert, chewing gum could hardly be considered a strategic material in wartime. Experimental procedures trying to make rubber from isoprene produced only chicle-like polymers, so an artificial rubber made from materials other than isoprene still had to be developed. Ironically, the technology for the process that made this possible came from Germany. During World War I, German supplies of natural rubber from Southeast Asia had been subjected to an Allied blockade. In response, the large German chemical companies had developed a number of rubberlike products, the best of which was styrene butadiene rubber (SBR), which had properties very similar to those of natural rubber.
Styrene was first isolated in the late eighteenth century, from the balsam of the oriental sweet gum tree, Liquidamber orientalis, native to south-western Turkey. After a few months, it was noted, the extracted styrene would become jellylike, indicating that it was beginning to polymerize.
This polymer is today known as polystyrene and is used for plastic films, packing materials, and “Styrofoam” coffee cups. Styrene—prepared synthetically as early as 1866—and butadiene were the starting materials used by the German
chemical company IG Farben in the manufacture of artificial rubber. The ratio of butadiene (CH2=CH-CH=CH2) to styrene is about three to one in SBR; though the exact ratio and structure are variable, it is thought that the double bonds are randomly cis or trans.
Partial structure of styrene butadiene rubber (SBR), also known as government rubber styrene (GR-S) or Buna-S. SBR can be vulcanized with sulfur.
In 1929 the Standard Oil Company of New Jersey formed a partnership with IG Farben based on shared processes relating to synthetic oil. Part of the agreement specified that Standard Oil would have access to certain of IG Farben’s patents, including the SBR process. IG Farben was not obligated to share its technical details, however, and in 1938 the Nazi government informed the company that the United States was to be denied any information on Germany’s advanced rubber-manufacturing technology.
IG Farben did eventually release the SBR patent to Standard Oil, certain that it contained insufficient technical information for the Americans to use in making their own rubber. But this judgment proved wrong. The chemical industry in the United States mobilized, and development of an SBR manufacturing process proceeded rapidly. In 1941 American synthetic rubber production was only eight thousand tons, but by 1945 it had expanded to over 800,000 tons, a significant proportion of the country’s total rubber consumption. The production of such huge quantities of rubber in such a short period of time has been described as the second greatest feat of engineering (and chemistry) of the twentieth century, after the building of the atomic bomb. Over the following decades other synthetic rubbers (neoprene, butyl rubber, and Buna-N) were created. The meaning of the word rubber came to include polymers made from starting materials other than isoprene but with properties closely related to those of natural rubber.
In 1953, Karl Ziegler in Germany and Giulio Natta in Italy further refined the production of synthetic rubber. Ziegler and Natta independently developed systems that produced either cis or trans double bonds depending on the particular catalyst used. It was now possible to make natural rubber synthetically. The so-called Ziegler-Natta catalysts, for which their discoverers received the 1963 Nobel Prize in chemistry, revolutionized the chemical industry by allowing the synthesis of polymers whose properties could be precisely controlled. In this way rubber polymers could be made that were more flexible, stronger, more durable, stiffer, less likely to be affected by solvents or ultraviolet light, and with greater resistance to cracking, heat, and cold.
Our world has been shaped by rubber. The collecting of raw material for rubber products had an enormous effect on society and the environment. The felling of the rubber trees in the Amazon basin, for example, was just one episode in the exploitation of the resources of tropical rain forests and destruction of a unique environment. The shameful treatment of the area’s indigenous population has not changed; today prospectors and subsistence farmers continue to invade the traditional lands of the descendants of the native peoples who harvested latex. The brutal colonization of the Belgian Congo left a legacy of instability, violence, and strife that is still very much present in the region today. The mass migrations of workers to the rubber plantations of Asia over a century ago continue to affect the ethnic, cultural, and political face of the countries of Malaysia and Sri Lanka.
Our world is still shaped by rubber. Without rubber the enormous changes brought on by mechanization would not have been possible. Mechanization requires essential natural or man-made rubber components for machines—belts, gaskets, joints, valves, o-rings, washers, tires, seals, and countless others. Mechanized transportation—cars, trucks, ships, trains, planes—has changed the way we move people and goods. Mechanization of industry has changed the jobs we do and the way we do them. Mechanization of agriculture has allowed the growth of cities and changed our society from rural to urban. Rubber has played an essential part in all these events.
Our exploration of future worlds may be shaped by rubber, as this material—an essential part of space stations, space suits, rockets and shuttles—is now enabling us to explore worlds beyond our own. But our failure to consider long-known properties of rubber has already limited our push to the stars. Despite NASA’s sophisticated knowledge of polymer technology, rubber’s lack of resistance to cold—a trait known to La Condamine, to Macintosh, to Goodyear—doomed the space shuttle Challenger on a chilly morning in January 1986. The launch temperature was 36°F, 15° lower than the next-coldest previous launch. On the shuttle system’s solid rocket motor aft field joint, the rubber o-ring in the shade on the side away from the sun was possibly as cold as 28°F. At that frigid temperature it would have lost its normal pliability and, by not returning to its proper shape, led to failure of a pressure seal. The resulting combustion gas leak caused an explosion that took the lives of the seven Challenger astronauts. This is a very recent example of what we might now call the Napoleon’s buttons factor, the neglect of a known molecular property being responsible for a major tragic event: “And all for the want of an O-ring.”
9. DYES
DYES COLOR OUR clothes, our furnishings, our accessories, and even our hair. Yet even as we ask for a different shade, a brighter hue, a softer tint, or a deeper tone, we rarely give a passing thought to the variety of compounds that allow us to indulge our passion for color. Dyes and dyestuffs are composed of natural or man-made molecules whose origins stretch back thousands of years. The discovery and exploitation of dyes has led to the creation and growth of the biggest chemical companies in the world today.
The extraction and preparation of dyestuffs, mentioned in Chinese literature as long ago as 3000 B.C., may have been man’s earliest attempts at the practice of chemistry. Early dyes were obtained mainly from plants: their roots, leaves, bark, or berries. Extraction procedures were well established and often quite complicated. Most substances did not adhere permanently to untreated fibers; fabrics first had to be treated with mordants, compounds that helped fix the color to the fiber. Although early dyes were highly sought after and very valuable, there were numerous problems with their use. They were frequently difficult to obtain, their range was limited, and the colors were not strong or faded quickly to dull, muddy shades in sunlight. Early dyes were rarely colorfast, bleeding out at every wash.
PRIMARY COLORS
Blue, in particular, was a much-sought-after color. Compared with red and yellow, blue shades are not common in plants, but one plant, Indigofera tinctoria, a member of the legume family, was known to be a major source of the blue dye indigo. Named by the famed Swedish botanist Linnaeus, Indigofera tinctoria grows up to six feet tall in both tropical and subtropical climates. Indigo is also produced in more temperate regions from Isatis tinctoria, one of the oldest dye plants of Europe and Asia, known as “woad” in Britain and “pastel” in France. On his travels seven hundred years ago, Marco Polo was reputed to have seen indigo being used in the Indus valley; hence the name indigo. But indigo was also prevalent in many other parts of the world, including Southeast Asia and Africa, well before the time of Marco Polo.
The fresh leaves of indigo-producing plants do not appear to be blue. But after fermentation under alkaline conditions followed by oxidation, the blue color appears. This process was discovered by numerous cultures around the world, possibly when the plant leaves were accidentally soaked in urine or covered with ashes, then left to ferment. In these circumstances the conditions necessary for production of the intense blue color of indigo would be present.
The indigo precursor compound, found in all indigo-producing plants, is indican, a molecule that contains an attached glucose unit. Indican itself is colorless, but fermentation under alkaline conditions splits off the glucose unit to produce the indoxol molecule. Indoxol reacts with oxygen from the air to produce blue-colored indigo (or indigotin, as chemists call this molecule).
Indigo was a very valuable substance, but the most expensive of the ancient dyes was a very similar molecule known as Tyrian purple. In some cultures the wearing of purple was restricted by
law to the king or emperor; hence the other name for this dye—royal purple—and the phrase “born to the purple,” implying an aristocratic pedigree. Even today purple is still regarded as an imperial color, an emblem of royalty. Mentioned in writings dating to around 1600 B.C., Tyrian purple is the dibromo derivative of indigo; that is, an indigo molecule that contains two bromine atoms. Tyrian purple was obtained from an opaque mucus secreted by various species of a marine mollusk or snail, most commonly of the genus Murex. The compound secreted by the mollusk is, as in the indigo plant, attached to a glucose unit. It is only through oxidation in the air that the brilliant color of Tyrian purple develops.
Bromine is rarely found in terrestrial plants or animals, but as there is a lot of bromine, as well as chlorine and iodine, in seawater, it is not that surprising to find bromine incorporated into compounds from marine sources. What is perhaps surprising is the similarity of these two molecules, given their very different sources—that is, indigo from a plant and Tyrian purple from an animal.
Mythology credits the discovery of Tyrian purple to the Greek hero Hercules, who observed his dog’s mouth becoming stained a deep purple color as the animal crunched on some shellfish. The manufacture of the dye is believed to have started in the Mediterranean port city of Tyre in the Phoenician Empire (now part of Lebanon). An estimated nine thousand shellfish were needed to produce one gram of Tyrian purple. Mounds of shells from Murex brandaris and Purpura haemastoma can still be found on the beaches of Tyre and Sidon, another Phoenician city involved in the ancient dye trade.
To obtain the dye, workers cracked open the shell of these mollusks and, using a sharp stick, extracted a small veinlike gland. Cloth was saturated with a treated solution from this gland, then exposed to the air for the color to develop. Initially the dye would turn cloth a pale yellow-green shade, then gradually blue, and then a deep purple. Tyrian purple colored the robes of Roman senators, Egyptian pharaohs, and European nobility and royalty. It was so sought after that by A.D. 400 the species of shellfish that produced it were in danger of becoming extinct.
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