Natural rubber

• 1. Introduction

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  Natural rubber begins as a sticky, milky, white fluid that bleeds from a cut in tree bark. Tappers collect it drop by drop into vessels, a process they call tapping. The substance is a colloid of polymers built from the organic compound isoprene, carrying only minor impurities. Joseph Priestley noticed in 1770 that a piece of it rubbed pencil marks off paper, and the name rubber stuck. But this humble eraser material once carried such value that it drove atrocities across two continents. How did a fluid drawn from a spurge tree become buoyant, waterproof, and so coveted that empires fought over its seeds? Why does a stretched balloon, after days, wither at a stubborn volume and shrink the moment a warm hand touches it? And how did chopped-off hands and slave raids become entangled with a material now found in surgeons' gloves and car tires?

• 2. Trees, Vines, And Dandelions

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  The Amazonian rubber tree, Hevea brasiliensis, is the major commercial source of natural rubber latex and a member of the spurge family, Euphorbiaceae. Once native to Brazil, the species is now pan-tropical. Growers prefer it because it thrives under cultivation, and a properly managed tree responds to wounding by producing more latex for several years.

  Congo rubber came not from trees but from vines in the genus Landolphia, including L. kirkii, L. heudelotii, and L. owariensis. Once a major source, it motivated the atrocities in the Congo Free State.

  Dandelion milk also contains latex, and that latex shows the same quality as rubber from rubber trees. Wild dandelions carry little of it, and the amount varies greatly. In Nazi Germany, research tried and failed to base rubber production on dandelions. In 2013, scientists at the Fraunhofer Institute for Molecular Biology and Applied Ecology in Germany inhibited one key enzyme and used modern cultivation and optimization to develop a cultivar of the Kazakh dandelion, Taraxacum kok-saghyz, that seems suitable for commercial production. Working with Continental Tires, the institute began a pilot facility.

  Many other plants make latex rich in isoprene polymers, though few yield usable rubber as easily as the Pará. Gutta-percha from Palaquium gutta and chicle from Manilkara species offer other desirable materials. The rubber fig, the Panama rubber tree, and various spurges have all been exploited or shown promise. Perhaps most important is guayule, Parthenium argentatum, prized for its hypoallergenic properties.

• 3. Balls, Erasers, And Empire

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  The earliest known use of rubber belongs to the indigenous cultures of Mesoamerica. Archeological evidence traces natural latex from the Hevea tree to the Olmec, who first used rubber to make balls for the Mesoamerican ballgame. The Maya and Aztec cultures followed, and the Aztecs went further, making containers and waterproofing textiles by soaking them in latex sap. Mesoamericans had used stabilized rubber as early as 1600 BC.

  Charles Marie de La Condamine is credited with introducing rubber samples to the Académie Royale des Sciences of France in 1736. He wrote the word as caoutchouc, taken from cahuchu in the language of the Manina people in Quito. In 1751 he presented a paper by François Fresneau to the Académie, published in 1755, describing many of rubber's properties, sometimes called the first scientific paper on rubber.

  François Fresnau discovered in 1764 that turpentine dissolved rubber, and Giovanni Fabbroni is credited with finding naphtha as a solvent in 1779. Charles Macintosh experimented with naphtha and invented waterproof rubberized fabric; the heart of his patent was cementing two thicknesses of cloth together with natural rubber. Charles Goodyear redeveloped vulcanization in 1839.

  South America remained the main source of latex rubber through much of the 19th century. No laws expressly prohibited exporting seeds or plants. In 1876, Henry Wickham smuggled 70,000 Amazonian rubber tree seeds from Brazil to Kew Gardens in England. Only 2,400 germinated. Seedlings then went to India, British Ceylon, the Dutch East Indies, Singapore, and British Malaya. Malaya, now Peninsular Malaysia, would become the biggest producer of rubber.

• 4. Blood On The Vines

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  In the early 1900s, the Congo Free State in Africa became a significant source of natural rubber latex, mostly gathered by forced labor. King Leopold II's colonial state brutally enforced production quotas because of the high price of rubber. Enforcers removed the hands of victims to prove they had been killed. Soldiers returned from raids with baskets full of chopped-off hands. Villages that resisted were razed to encourage compliance.

  The Amazon rubber boom struck indigenous populations too. Correrias, or slave raids, were frequent in Colombia, Peru, and Bolivia, where many were captured or killed. The best-known South American case came from the Putumayo genocide. Between the 1880s and 1913, Julio César Arana and his company, which became the Peruvian Amazon Company, controlled the Putumayo river.

  W. E. Hardenburg, Benjamin Saldaña Rocca, and Roger Casement were influential in exposing these atrocities. Casement was also prominent in revealing the Congo atrocities to the world. Days before entering Iquitos by boat, Casement wrote a bitter line. "Caoutchouc was first called 'india rubber,' because it came from the Indies, and the earliest European use of it was to rub out or erase. It is now called India rubber because it rubs out or erases the Indians."

• 5. Mad Ridley And The Planters

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  Commercial cultivation in India began with British planters, though experimental efforts started as early as 1873 at the Calcutta Botanical Garden. The first commercial Hevea plantations were established at Thattekadu in Kerala in 1902. Plantations later expanded to Karnataka, Tamil Nadu, and the Andaman and Nicobar Islands. Today India is the world's third-largest producer and fourth-largest consumer of rubber.

  In Singapore and Malaya, Sir Henry Nicholas Ridley promoted commercial production heavily. He served as the first Scientific Director of the Singapore Botanic Gardens from 1888 to 1911. He distributed rubber seeds to many planters and developed the first technique for tapping trees for latex without serious harm to the tree. His fervent promotion of the crop earned him the nickname Mad Ridley.

  Before World War II, significant uses included door and window profiles, hoses, belts, gaskets, matting, flooring, and antivibration mounts for the automotive industry. Car tires, at first solid rather than pneumatic, consumed a large amount of rubber. Gloves and toy balloons were big consumers, though those use concentrated latex. Rubber served as adhesive across many industries, most noticeably paper and carpet, and made rubber bands and pencil erasers. As a fiber sometimes called elastic, it gave the textile industry excellent elongation and recovery, and rubber yarns went into foundation garments. The industry later turned to neoprene and spandex for their superior strength and durability.

• 6. Why Rubber Springs Back

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  On a microscopic scale, relaxed rubber is a disorganized cluster of erratically changing wrinkled chains. When stretched, those chains become almost linear. The restoring force comes from the dominance of wrinkled conformations over linear ones.

  Cooling below the glass transition temperature permits small local changes but makes reordering practically impossible, because longer chains face a larger energy barrier to concerted movement. Frozen rubber has low elasticity, and strain then comes only from small shifts in bond lengths and angles. This caused the Challenger disaster, when the Space Shuttle's flattened o-rings failed to relax and fill a widening gap. The glass transition is fast and reversible, and the force resumes on heating.

  The parallel chains of stretched rubber can crystallize, though this takes time as twisted chains move out of the way of growing crystallites. This is why an inflated toy balloon, after days, is found withered at a relatively large remaining volume. Where it is touched, it shrinks, because the warmth of a hand is enough to melt the crystals.

  Vulcanization creates di- and polysulfide bonds between chains. These limit the degrees of freedom and make chains tighten more quickly for a given strain. The result is harder, less extensible rubber with a higher elastic force constant. Rubber begins to melt at about 180 degrees Celsius, and its two main solvents are turpentine and naphtha.

• 7. From Bark To Bale

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  Rubber tapping normally happens early in the morning, when the internal pressure of the tree is highest. A tapper makes a slanting slash in the bark with a small hatchet, releasing latex from ducts in the inner bark, the cambium. Because the cambium controls growth, cuts must be precise, neither too many nor too deep, or they could stunt or kill the tree. A good tapper can tap a tree every 20 seconds on a standard half-spiral system, and a daily task runs between 450 and 650 trees.

  The latex-bearing tubes spiral upward to the right, so cuts are usually made upward to the left to cross more tubes. Trees release latex for about four hours before the flow stops, as the latex coagulates on the cut and blocks the tubes. Tappers rest and eat, then begin collecting the field latex around midday. In Kerala and Sri Lanka, half a coconut shell once served as the collection cup, later replaced by glazed pottery, aluminium, or plastic cups held by a spring-loaded wire that stretches as the tree grows.

  The four types of field coagula are cuplump, treelace, smallholders' lump, and earth scrap. Cup lump, found in the collection cup at the next visit, is of higher purity and greater value than the others. Tree lace, peeled from the previous cut, usually carries more copper and manganese, both pro-oxidants that damage the dry rubber. Earth scrap gathers at the tree's base, mixed with soil and contaminants, and is of low quality.

  Field latex goes into coagulation tanks for dry rubber, or into air-tight containers for ammoniation. Ammoniation, invented by patent lawyer and United States Rubber Company vice-president Ernest Hopkinson around 1920, preserves latex in a colloidal state for longer. Coagulated latex becomes block rubbers such as SVR 3L or SVR CV, or Ribbed Smoke Sheet grades, while cup lump feeds TSR10 and TSR20 grades. The dried material is then baled and palletized for storage and shipment.

• 8. A Material Under Pressure

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  More than 29 million metric tonnes of rubber were produced in 2022, of which over half, 15.1 million tonnes, was natural. Because the rest is synthetic and derived from petroleum, the price of natural rubber tracks the global price of crude oil. Asia supplied about 90 percent of output in 2021, and the three largest producers, Thailand, Indonesia, and Vietnam, together made around 61 percent of all natural rubber in 2022. The native continent of South America barely cultivates it, held back by the South American leaf blight and other predators.

  The economic life of a plantation rubber tree runs about 32 years, with up to 7 years of immaturity and roughly 25 productive years. High-yielding clones can give more than 2,000 kilograms of dry rubber per hectare per year under ideal conditions. Rubber's link to deforestation made it one of seven commodities in the 2023 EU Regulation on Deforestation-free products.

  Supply faces growing concern from plant disease, climate change, and volatile prices. Producers are mostly small family plantations serving large industrial aggregators. When a rival crop like palm oil grows more profitable, farmers may tear out their rubber trees. During the COVID-19 pandemic in 2020 and 2021, demand for rubber gloves surged and prices spiked about 30 percent, worsened by plantations that had been replaced over the previous 5 to 10 years.

  Some people have a serious latex allergy, and exposure to products like latex gloves can cause anaphylactic shock. Vulcanization reduces the antigenic proteins in Hevea latex by about 99.9 percent, though it does not eliminate them. Latex from non-Hevea sources such as guayule can be used by those allergic to Hevea, a quiet reminder of why the desert shrub still matters.