Natural rubber

Polymer harvested from certain trees

"Rubber" and "India rubber" redirect here. For other uses, see Rubber (disambiguation)

"Caoutchouc" redirects here. For the painting by Francis Picabia, see Caoutchouc (Picabia)

This article is about the polymeric material "natural rubber". For man-made rubber materials, see Synthetic rubber

Pieces of natural vulcanized rubber at Hutchinson's Research and Innovation Center in France. Latex being collected from a tapped rubber tree, Cameroon

Rubber, also called India rubber, latex, Amazonian rubber, caucho, or caoutchouc,[1] as initially produced, consists of polymers of the organic compound isoprene, with minor impurities of other organic compounds. Thailand, Malaysia, Indonesia, and Cambodia are four of the leading rubber producers.[2][3][4]

Types of polyisoprene that are used as natural rubbers are classified as elastomers.

Currently, rubber is harvested mainly in the form of the latex from the Pará rubber tree (Hevea brasiliensis) or others. The latex is a sticky, milky and white colloid drawn off by making incisions in the bark and collecting the fluid in vessels in a process called "tapping". The latex then is refined into the rubber that is ready for commercial processing. In major areas, latex is allowed to coagulate in the collection cup. The coagulated lumps are collected and processed into dry forms for sale.

Natural rubber is used extensively in many applications and products, either alone or in combination with other materials. In most of its useful forms, it has a large stretch ratio and high resilience and also is water-proof.[citation needed]

Industrial demand for rubber-like materials began to outstrip natural rubber supplies by the end of the 19th century, leading to the synthesis of synthetic rubber in 1909 by chemical means.[citation needed]

Varieties

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Amazonian rubber tree (Hevea brasiliensis)

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The major commercial source of natural rubber latex is the Amazonian rubber tree (Hevea brasiliensis),[1] a member of the spurge family, Euphorbiaceae. Once native to Brazil, the species is now pan-tropical. This species is preferred because it grows well under cultivation. A properly managed tree responds to wounding by producing more latex for several years.[citation needed]

Congo rubber (Landolphia owariensis and L. spp.)

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Congo rubber, formerly a major source of rubber, which motivated the atrocities in the Congo Free State, came from vines in the genus Landolphia (L. kirkii, L. heudelotis, and L. owariensis).[5]

Dandelion

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Dandelion milk contains latex. The latex exhibits the same quality as the natural rubber from rubber trees. In the wild types of dandelion, latex content is low and varies greatly. In Nazi Germany, research projects tried to use dandelions as a base for rubber production, but failed.[6] In 2013, by inhibiting one key enzyme and using modern cultivation methods and optimization techniques, scientists in the Fraunhofer Institute for Molecular Biology and Applied Ecology (IME) in Germany developed a cultivar of the Kazakh dandelion (Taraxacum kok-saghyz) that is suitable for commercial production of natural rubber.[7] In collaboration with Continental Tires, IME began a pilot facility.

Other

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Many other plants produce forms of latex rich in isoprene polymers, though not all produce usable forms of polymer as easily as the Pará.[8] Some of them require more elaborate processing to produce anything like usable rubber, and most are more difficult to tap. Some produce other desirable materials, for example gutta-percha (Palaquium gutta)[9] and chicle from Manilkara species. Others that have been commercially exploited, or at least showed promise as rubber sources, include the rubber fig (Ficus elastica), Panama rubber tree (Castilla elastica), various spurges (Euphorbia spp.), lettuce (Lactuca species), the related Scorzonera tau-saghyz, various Taraxacum species, including common dandelion (Taraxacum officinale) and Kazakh dandelion, and, perhaps most importantly for its hypoallergenic properties, guayule (Parthenium argentatum). The term gum rubber is sometimes applied to the tree-obtained version of natural rubber in order to distinguish it from the synthetic version.[10]

History

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The first use of rubber was by the indigenous cultures of Mesoamerica. The earliest archeological evidence of the use of natural latex from the Hevea tree comes from the Olmec culture, in which rubber was first used for making balls for the Mesoamerican ballgame. Rubber was later used by the Maya and Aztec cultures – in addition to making balls, Aztecs used rubber for other purposes, such as making containers and to make textiles waterproof by impregnating them with the latex sap.[11][12]

Charles Marie de La Condamine is credited with introducing samples of rubber to the Académie Royale des Sciences of France in 1736.[13] In 1751, he presented a paper by François Fresneau to the Académie (published in 1755) that described many of rubber's properties. This has been referred to as the first scientific paper on rubber.[13] In England, Joseph Priestley, in 1770, observed that a piece of the material was extremely good for rubbing off pencil marks on paper, hence the name "rubber". It slowly made its way around England. In 1764, François Fresnau discovered that turpentine was a rubber solvent. Giovanni Fabbroni is credited with the discovery of naphtha as a rubber solvent in 1779.[citation needed] Charles Goodyear redeveloped vulcanization in 1839, although Mesoamericans had used stabilized rubber for balls and other objects as early as 1600 BC.[14][15]

South America remained the main source of latex rubber used during much of the 19th century. The rubber trade was heavily controlled by business interests but no laws expressly prohibited the export of seeds or plants. In 1876, Henry Wickham smuggled 70,000 Amazonian rubber tree seeds from Brazil and delivered them to Kew Gardens, England. Only 2,400 of these germinated. Seedlings were then sent to India, British Ceylon (Sri Lanka), Dutch East Indies (Indonesia), Singapore, and British Malaya. Malaya (now Peninsular Malaysia) was later to become the biggest producer of rubber.[16]

In the early 1900s, the Congo Free State in Africa was also a significant source of natural rubber latex, mostly gathered by forced labor.[citation needed] King Leopold II's colonial state brutally enforced production quotas. Tactics to enforce the rubber quotas included removing the hands of victims to prove they had been killed. Soldiers often came back from raids with baskets full of chopped-off hands. Villages that resisted were razed to encourage better compliance locally.[citation needed] (See Atrocities in the Congo Free State for more information on the rubber trade in the Congo Free State in the late 1800s and early 1900s.)

The rubber boom in the Amazon also similarly affected indigenous populations to varying degrees. Correrias, or slave raids were frequent in Colombia, Peru and Bolivia where many were either captured or killed. The most well known case of atrocities generated from rubber extraction in South America came from the Putumayo genocide. Between the 1880s–1913 Julio César Arana and his company that would become the Peruvian Amazon Company controlled the Putumayo river. W.E. Hardenburg, Benjamin Saldaña Rocca and Roger Casement were influential figures in exposing these atrocities. Roger Casement was also prominent in revealing the Congo atrocities to the world. Days before entering Iquitos by boat Casement wrote "'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."[17][18]

"Enslaved natives with a load of rubber weighing 75 kilos, they have journeyed 100 kilometers with no food given"

In India, commercial cultivation was introduced by British planters, although the experimental efforts to grow rubber on a commercial scale were initiated as early as 1873 at the Calcutta Botanical Garden. The first commercial Hevea plantations were established at Thattekadu in Kerala in 1902. In later years the plantation expanded to Karnataka, Tamil Nadu and the Andaman and Nicobar Islands of India. Today, India is the world's 3rd largest producer and 4th largest consumer of rubber.[19]

In Singapore and Malaya, commercial production was heavily promoted by Sir Henry Nicholas Ridley, who 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 causing serious harm to the tree.[20] Because of his fervent promotion of this crop, he is popularly remembered by the nickname "Mad Ridley".[21]

Pre–World War II

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Before World War II significant uses included door and window profiles, hoses, belts, gaskets, matting, flooring, and dampeners (antivibration mounts) for the automotive industry. The use of rubber in car tires (initially solid rather than pneumatic) in particular consumed a significant amount of rubber. Gloves (medical, household, and industrial) and toy balloons were large consumers of rubber, although the type of rubber used is concentrated latex. Significant tonnage of rubber was used as adhesives in many manufacturing industries and products, although the two most noticeable were the paper and the carpet industries. Rubber was commonly used to make rubber bands and pencil erasers.

Rubber produced as a fiber, sometimes called 'elastic', had significant value to the textile industry because of its excellent elongation and recovery properties. For these purposes, manufactured rubber fiber was made as either an extruded round fiber or rectangular fibers cut into strips from extruded film. Because of its low dye acceptance, feel and appearance, the rubber fiber was either covered by yarn of another fiber or directly woven with other yarns into the fabric. Rubber yarns were used in foundation garments. While rubber is still used in textile manufacturing, its low tenacity limits its use in lightweight garments because latex lacks resistance to oxidizing agents and is damaged by aging, sunlight, oil and perspiration. The textile industry turned to neoprene (polymer of chloroprene), a type of synthetic rubber, as well as another more commonly used elastomer fiber, spandex (also known as elastane), because of their superiority to rubber in both strength and durability.

Properties

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Rubber latex

Rubber exhibits unique physical and chemical properties. Rubber's stress–strain behavior exhibits the Mullins effect and the Payne effect and is often modeled as hyperelastic. Rubber strain crystallizes. Because there are weakened allylic C-H bonds in each repeat unit, natural rubber is susceptible to vulcanisation as well as being sensitive to ozone cracking. The two main solvents for rubber are turpentine and naphtha (petroleum). Because rubber does not dissolve easily, the material is finely divided by shredding prior to its immersion. An ammonia solution can be used to prevent the coagulation of raw latex. Rubber begins to melt at approximately 180 °C (356 °F).

Elasticity

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Rubber latex elasticity

On a microscopic scale, relaxed rubber is a disorganized cluster of erratically changing wrinkled chains. In stretched rubber, the chains are almost linear. The restoring force is due to the preponderance of wrinkled conformations over more linear ones. For the quantitative treatment see ideal chain, for more examples see entropic force.

Cooling below the glass transition temperature permits local conformational changes but a reordering is practically impossible because of the larger energy barrier for the concerted movement of longer chains. "Frozen" rubber's elasticity is low and strain results from small changes of bond lengths and angles: this caused the Challenger disaster, when the American Space Shuttle's flattened o-rings failed to relax to fill a widening gap.[22] The glass transition is fast and reversible: the force resumes on heating.

The parallel chains of stretched rubber are susceptible to crystallization. This takes some time because turns of twisted chains have to move out of the way of the growing crystallites. Crystallization has occurred, for example, when, after days, an inflated toy balloon is found withered at a relatively large remaining volume. Where it is touched, it shrinks because the temperature of the hand is enough to melt the crystals.

Vulcanization of rubber creates di- and polysulfide bonds between chains, which limits the degrees of freedom and results in chains that tighten more quickly for a given strain, thereby increasing the elastic force constant and making the rubber harder and less extensible.

Malodour

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Raw rubber storage depots and rubber processing can produce malodour that is serious enough to become a source of complaints and protest to those living in the vicinity.[23] Microbial impurities originate during the processing of block rubber. These impurities break down during storage or thermal degradation and produce volatile organic compounds. Examination of these compounds using gas chromatography/mass spectrometry (GC/MS) and gas chromatography (GC) indicates that they contain sulfur, ammonia, alkenes, ketones, esters, hydrogen sulfide, nitrogen, and low-molecular-weight fatty acids (C2–C5).[24][25] When latex concentrate is produced from rubber, sulfuric acid is used for coagulation. This produces malodourous hydrogen sulfide.[25] The industry can mitigate these bad odours with scrubber systems.[25]

Chemical makeup

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Chemical structure of cis-polyisoprene, the main constituent of natural rubber. Synthetic cis-polyisoprene and natural cis-polyisoprene are derived from distinct precursors, isopentenyl pyrophosphate and isoprene.

Rubber is the polymer cis-1,4-polyisoprene – with a molecular weight of 100,000 to 1,000,000 daltons. Typically, a small percentage (up to 5% of dry mass) of other materials, such as proteins, fatty acids, resins, and inorganic materials (salts) are found in natural rubber. Polyisoprene can also be created synthetically, producing what is sometimes referred to as "synthetic natural rubber", but the synthetic and natural routes are distinct.[10] Some natural rubber sources, such as gutta-percha, are composed of trans-1,4-polyisoprene, a structural isomer that has similar properties. Natural rubber is an elastomer and a thermoplastic. Once the rubber is vulcanized, it is a thermoset. Most rubber in everyday use is vulcanized to a point where it shares properties of both; i.e., if it is heated and cooled, it is degraded but not destroyed. The final properties of a rubber item depend not just on the polymer, but also on modifiers and fillers, such as carbon black, factice, whiting and others.

Biosynthesis

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Rubber particles are formed in the cytoplasm of specialized latex-producing cells called laticifers within rubber plants.[26] Rubber particles are surrounded by a single phospholipid membrane with hydrophobic tails pointed inward. The membrane allows biosynthetic proteins to be sequestered at the surface of the growing rubber particle, which allows new monomeric units to be added from outside the biomembrane, but within the lacticifer. The rubber particle is an enzymatically active entity that contains three layers of material, the rubber particle, a biomembrane and free monomeric units. The biomembrane is held tightly to the rubber core by the high negative charge along the double bonds of the rubber polymer backbone.[27] Free monomeric units and conjugated proteins make up the outer layer. The rubber precursor is isopentenyl pyrophosphate (an allylic compound), which elongates by Mg2+-dependent condensation by the action of rubber transferase. The monomer adds to the pyrophosphate end of the growing polymer.[citation needed] The process displaces the terminal high-energy pyrophosphate. The reaction produces a cis polymer. The initiation step is catalyzed by prenyltransferase, which converts three monomers of isopentenyl pyrophosphate into farnesyl pyrophosphate.[28] The farnesyl pyrophosphate can bind to rubber transferase to elongate a new rubber polymer.

The required isopentenyl pyrophosphate is obtained from the mevalonate pathway, which derives from acetyl-CoA in the cytosol. In plants, isoprene pyrophosphate can also be obtained from the 1-deox-D-xyulose-5-phosphate/2-C-methyl-D-erythritol-4-phosphate pathway within plasmids.[29] The relative ratio of the farnesyl pyrophosphate initiator unit and isoprenyl pyrophosphate elongation monomer determines the rate of new particle synthesis versus elongation of existing particles. Though rubber is known to be produced by only one enzyme, extracts of latex host numerous small molecular weight proteins with unknown function. The proteins possibly serve as cofactors, as the synthetic rate decreases with complete removal.[30]

Production

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Rubber is generally cultivated in large plantations. The image shows a coconut shell used in collecting latex, in plantations in Kerala, India. Sheets of natural rubber

More than 28 million tons of rubber were produced in 2017, of which approximately 47% was natural. Since the bulk is synthetic, which is derived from petroleum, the price of natural rubber is determined, to a large extent, by the prevailing global price of crude oil.[31][32] Asia was the main source of natural rubber, accounting for about 90% of output in 2021.[33] The three largest producers, Thailand, Indonesia,[34] and Malaysia, together account for around 72% of all natural rubber production. Natural rubber is not cultivated widely in its native continent of South America because of the South American leaf blight, and other natural predators there.

Cultivation

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Rubber latex is extracted from rubber trees. The economic life of rubber trees in plantations is around 32 years, with up to 7 years being an immature phase and about 25 years of productive phase.

The soil requirement is well-drained, weathered soil consisting of laterite, lateritic types, sedimentary types, nonlateritic red or alluvial soils.

The climatic conditions for optimum growth of rubber trees are:

• Rainfall of around 250 centimetres (98 in) evenly distributed without any marked dry season and with at least 100 rainy days per year
• Temperature range of about 20 to 34 °C (68 to 93 °F), with a monthly mean of 25 to 28 °C (77 to 82 °F)
• Atmospheric humidity of around 80%
• About 2,000 hours sunshine per year at the rate of six hours per day throughout the year
• Absence of strong winds

Many high-yielding clones have been developed for commercial planting. These clones yield more than 2,000 kilograms per hectare (1,800 lb/acre) of dry rubber per year, under ideal conditions.

Collection

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Vintage tobacco card, Tapping a Rubber Tree, India, Products of the World series, Player's Cigarettes, 1909

In places such as Kerala and Sri Lanka, where coconuts are in abundance, the half shell of coconut was used as the latex collection container. Glazed pottery or aluminium or plastic cups became more common in Kerala-India and other countries. The cups are supported by a wire that encircles the tree. This wire incorporates a spring so it can stretch as the tree grows. The latex is led into the cup by a galvanised "spout" knocked into the bark. Rubber tapping normally takes place early in the morning, when the internal pressure of the tree is highest. A good tapper can tap a tree every 20 seconds on a standard half-spiral system, and a common daily "task" size is between 450 and 650 trees. Trees are usually tapped on alternate or third days, although many variations in timing, length and number of cuts are used. "Tappers would make a slash in the bark with a small hatchet. These slanting cuts allowed latex to flow from ducts located on the exterior or the inner layer of bark (cambium) of the tree. Since the cambium controls the growth of the tree, growth stops if it is cut. Thus, rubber tapping demanded accuracy, so that the incisions would not be too many given the size of the tree, or too deep, which could stunt its growth or kill it."[35]

A woman in Sri Lanka harvesting rubber, c. 1920

It is usual to tap a panel at least twice, sometimes three times, during the tree's life. The economic life of the tree depends on how well the tapping is carried out, as the critical factor is bark consumption. A standard in Malaysia for alternate daily tapping is 25 cm (vertical) bark consumption per year. The latex-containing tubes in the bark ascend in a spiral to the right. For this reason, tapping cuts usually ascend to the left to cut more tubes. The trees drip latex for about four hours, stopping as latex coagulates naturally on the tapping cut, thus blocking the latex tubes in the bark. Tappers usually rest and have a meal after finishing their tapping work and then start collecting the liquid "field latex" at about midday.

Field coagula

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Mixed field coagula.

The four types of field coagula are "cuplump", "treelace", "smallholders' lump", and "earth scrap". Each has significantly different properties.[36] Some trees continue to drip after the collection leading to a small amount of "cup lump" that is collected at the next tapping. The latex that coagulates on the cut is also collected as "tree lace". Tree lace and cup lump together account for 10%–20% of the dry rubber produced. Latex that drips onto the ground, "earth scrap", is also collected periodically for processing of low-grade product.

Cup lump

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Cup lump rubber coagula in a Myanmar road stall.

Cup lump is the coagulated material found in the collection cup when the tapper next visits the tree to tap it again. It arises from latex clinging to the walls of the cup after the latex was last poured into the bucket, and from late-dripping latex exuded before the latex-carrying vessels of the tree become blocked. It is of higher purity and of greater value than the other three types.

'Cup lumps' can also be used to describe a completely different type of coagulate that has collected in smallholder plantations over a period of 1–2 weeks. After tapping all of the trees, the tapper will return to each tree and stir in some type of acid, which allows the newly harvested latex to mix with the previously coagulated material. The rubber/acid mixture is what gives rubber plantations, markets, and factories a strong odor.

Tree lace

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Tree lace is the coagulum strip that the tapper peels off the previous cut before making a new cut. It usually has higher copper and manganese contents than cup lump. Both copper and manganese are pro-oxidants and can damage the physical properties of the dry rubber.

Smallholders' lump

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Additional reading:
Why Can't Natural Rubber Be Used to Make Products?
The effect of all kinds of packing element sealing ring
What is the Carbon Steel Butt Welding Pipe Reducer?
What Does Silicone Rubber React With?
Natural and synthetic rubber

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Smallholders' lump is produced by smallholders, who collect rubber from trees far from the nearest factory. Many Indonesian smallholders, who farm paddies in remote areas, tap dispersed trees on their way to work in the paddy fields and collect the latex (or the coagulated latex) on their way home. As it is often impossible to preserve the latex sufficiently to get it to a factory that processes latex in time for it to be used to make high quality products, and as the latex would anyway have coagulated by the time it reached the factory, the smallholder will coagulate it by any means available, in any container available. Some smallholders use small containers, buckets etc., but often the latex is coagulated in holes in the ground, which are usually lined with plastic sheeting. Acidic materials and fermented fruit juices are used to coagulate the latex — a form of assisted biological coagulation. Little care is taken to exclude twigs, leaves, and even bark from the lumps that are formed, which may also include tree lace.

Earth scrap

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Earth scrap is material that gathers around the base of the tree. It arises from latex overflowing from the cut and running down the bark, from rain flooding a collection cup containing latex, and from spillage from tappers' buckets during collection. It contains soil and other contaminants, and has variable rubber content, depending on the amount of contaminants. Earth scrap is collected by field workers two or three times a year and may be cleaned in a scrap-washer to recover the rubber, or sold to a contractor who cleans it and recovers the rubber. It is of low quality.

Processing

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Removing coagulum from coagulating troughs.

Latex coagulates in the cups if kept for long and must be collected before this happens. The collected latex, "field latex", is transferred into coagulation tanks for the preparation of dry rubber or transferred into air-tight containers with sieving for ammoniation. Ammoniation, invented by patent lawyer and vice-president of the United States Rubber Company Ernest Hopkinson around 1920, preserves the latex in a colloidal state for longer periods of time. Latex is generally processed into either latex concentrate for manufacture of dipped goods or coagulated under controlled, clean conditions using formic acid. The coagulated latex can then be processed into the higher-grade, technically specified block rubbers such as SVR 3L or SVR CV or used to produce Ribbed Smoke Sheet grades. Naturally coagulated rubber (cup lump) is used in the manufacture of TSR10 and TSR20 grade rubbers. Processing for these grades is a size reduction and cleaning process to remove contamination and prepare the material for the final stage of drying.[37]

The dried material is then baled and palletized for storage and shipment.

Molecular Structure

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Rubber is a natural polymer of isoprene (polyisoprene), and an elastomer (a stretchy polymer). Polymers are simply chains of molecules that can be linked together. Rubber is one of the few naturally occurring polymers and prized for its high stretch ratio, resilience, and water-proof properties. Other examples of natural polymers include tortoise shell, amber, and animal horn.[38] When harvested, latex rubber takes the form of latex, an opaque, white, milky suspension of rubber particles in water. It is then transformed through industrial processes to the common solid form so commonly seen today.

Vulcanized rubber

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Torn latex rubber dry suit wrist seal

Natural rubber is reactive and vulnerable to oxidization, but it can be stabilized through a heating process called vulcanization. Vulcanization is a process by which the rubber is heated and sulfur, peroxide, or bisphenol are added to improve resistance and elasticity and to prevent it from oxidizing. Carbon black, which can be derived from a petroleum refinery or other natural incineration processes, is sometimes used as an additive to rubber to improve its strength, especially in vehicle tires.[39][40]

During vulcanization, rubber's polyisoprene molecules (long chains of isoprene) are heated and cross-linked with molecular bonds to sulfur, forming a 3-D matrix. The optimal percentage of sulfur is approximately 10%. In this form, the polyisoprene molecules orientation is still random but they become aligned when the rubber is stretched. This sulfur vulcanization makes the rubber stronger and more rigid, but still very elastic.[41] And through the vulcanization process, the sulfur and latex are meant to be totally used up in individual form.

Transportation

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Natural rubber latex is shipped from factories in Southeast Asia, South America, and West and Central Africa to destinations around the world. As the cost of natural rubber has risen significantly and rubber products are dense, the shipping methods offering the lowest cost per unit weight are preferred. Depending on destination, warehouse availability, and transportation conditions, some methods are preferred by certain buyers. In international trade, latex rubber is mostly shipped in 20-foot ocean containers. Inside the container, smaller containers are used to store the latex.[42]

Rubber shortage and global economics

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There is growing concern for the future supply of rubber due to various factors, including plant disease, climate change, and the volatile market price of rubber.[43][44][45][46] Producers of natural rubber are mostly small family-held plantations, often serving large industrial aggregators. High volatility in the price of rubber affects rubber plantation investment, and farmers may remove their rubber trees if the international market spot price of a seemingly more profitable crop, (for example palm oil) surges in relation to rubber.

For instance, during the 2020 and 2021 international COVID-19 pandemic, demand for rubber gloves surged, leading to a spike in rubber prices of about 30%. In addition to the pandemic, demand exceeded supply in part because long term plantations had been torn out and replaced with other crops over the previous 5-10 years, and other areas were affected by climate-fueled natural disasters. In this environment, producers did increase their prices in keeping with supply and demand dynamics, putting upward price pressure on the whole downstream supply chain.[46]

Uses

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Compression molded (cured) rubber boots before the flashes are removed

Uncured rubber is used for cements;[47] for adhesive, insulating, and friction tapes; and for crepe rubber used in insulating blankets and footwear. Vulcanized rubber has many more applications. Resistance to abrasion makes softer kinds of rubber valuable for the treads of vehicle tires and conveyor belts, and makes hard rubber valuable for pump housings and piping used in the handling of abrasive sludge.

The flexibility of rubber is appealing in hoses, tires and rollers for devices ranging from domestic clothes wringers to printing presses; its elasticity makes it suitable for various kinds of shock absorbers and for specialized machinery mountings designed to reduce vibration. Its relative gas impermeability makes it useful in the manufacture of articles such as air hoses, balloons, balls and cushions. The resistance of rubber to water and to the action of most fluid chemicals has led to its use in rainwear, diving gear, and chemical and medicinal tubing and as a lining for storage tanks, processing equipment and railroad tank cars. Because of their electrical resistance, soft rubber goods are used as insulation and for protective gloves, shoes, and blankets; hard rubber is used for articles such as telephone housings and parts for radio sets, meters, and other electrical instruments. The coefficient of friction of rubber, which is high on dry surfaces and low on wet surfaces, leads to its use for power-transmission belting, highly flexible couplings,[48] and for water-lubricated bearings in deep-well pumps. Indian rubber balls or lacrosse balls are made of rubber.

Compression molding machine for rubber parts

Around 25 million tonnes of rubber are produced each year, of which 30 percent is natural.[49] The remainder is synthetic rubber derived from petrochemical sources. The top end of latex production results in latex products such as surgeons' gloves, balloons, and other relatively high-value products. The mid-range which comes from the technically specified natural rubber materials ends up largely in tires but also in conveyor belts, marine products, windshield wipers, and miscellaneous goods. Natural rubber offers good elasticity, while synthetic materials tend to offer better resistance to environmental factors such as oils, temperature, chemicals, and ultraviolet light. "Cured rubber" is rubber that has been compounded and subjected to the vulcanisation process to create cross-links within the rubber matrix. Rubber can be added to cement to improve its properties.[50]

Allergic reactions

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Some people have a serious latex allergy, and exposure to natural latex rubber products such as latex gloves can cause anaphylactic shock. The antigenic proteins found in Hevea latex are greatly reduced by about 99.9 percent (though not eliminated)[51] through vulcanization processing.

Latex from non-Hevea sources, such as guayule, can be used without allergic reaction by persons with an allergy to Hevea latex.[52]

Some allergic reactions are not to the latex itself, but from residues of chemicals used to accelerate the cross-linking process. Although this may be confused with an allergy to latex, it is distinct from it, typically taking the form of Type IV hypersensitivity in the presence of traces of specific processing chemicals.[51][53]

Microbial degradation

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Natural rubber is susceptible to degradation by a wide range of bacteria.[54][55][56][57][58][59][60][61] The bacteria Streptomyces coelicolor, Pseudomonas citronellolis, and Nocardia spp. are capable of degrading vulcanized natural rubber.[62]

See also

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References

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Citations

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Sources

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Further reading

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• Dean, Warren. (1997) Brazil and the Struggle for Rubber: A Study in Environmental History. Cambridge University Press.
• Grandin, Greg. Fordlandia: The Rise and Fall of Henry Ford's Forgotten Jungle City. Picador Press 2010. ISBN 978-0-312-42962-1
• Weinstein, Barbara (1983) The Amazon Rubber Boom 1850–1920. Stanford University Press.
• Tully, John A. The Devil's Milk; A Social History of Rubber. New York: Monthly Review Press, 2011.

Synthetic rubbers

Synthetic rubbers are made in chemical plants using petrochemicals as their starting point. One of the first (and still one of the best known) is neoprene (the brand name for polychloroprene), made by reacting together acetylene and hydrochloric acid. Emulsion styrene-butadiene rubber (E-SBR), another synthetic rubber, is widely used for making vehicle tires.

For the rest of this article, we'll concentrate mostly on natural rubber.

How is rubber made?

Photo: Traditional rubber tapping using a machete, photographed in the 1920s. The latex drips down the cuts into the can on the ground. Photo by Bain News Service courtesy of US Library of Congress Prints & Photographs Division .

It takes several quite distinct steps to make a product out of natural rubber. First, you have to gather your latex from the rubber trees using a traditional process called rubber tapping. That involves making a wide, V-shaped cut in the tree's bark. As the latex drips out, it's collected in a cup. The latex from many trees is then filtered, washed, and reacted with acid to make the particles of rubber coagulate (stick together). The rubber made this way is pressed into slabs or sheets and then dried, ready for the next stages of production.

By itself, unprocessed rubber is not all that useful. It tends to be brittle when cold and smelly and sticky when it warms up. Further processes are used to turn it into a much more versatile material. The first one is known as mastication (a word we typically use to describe how animals chew food). Masticating machines "chew up" raw rubber using mechanical rollers and presses to make it softer, easier to work, and more sticky. After the rubber has been masticated, extra chemical ingredients are mixed in to improve its properties (for example, to make it more hardwearing). Next, the rubber is squashed into shape by rollers (a process called calendering) or squeezed through specially shaped holes to make hollow tubes (a process known as extrusion). Finally, the rubber is vulcanized (cooked): sulfur is added and the rubber is heated to about 140°C (280°F) in an autoclave (a kind of industrial pressure cooker).

Photo: Vulcanized rubber is heated in a giant sealed "cooker" like this one, used for making earthmover tires, pictured at Firestone Tire Company in 1942. At that time, it was the biggest rubber vulcanizer in the world, standing some 2.5 stories high when opened wide. I've colored the people in the pictures red to give you an idea of the scale. Photos by Alfred T. Palmer courtesy of US Library of Congress.

Where does rubber come from?

As its name suggests, the rubber tree Hevea brasiliensis originally came from Brazil, from where it was introduced to such countries of the Far East as Malaysia, Indonesia, Burma, Cambodia, China, and Vietnam. During World War II, supplies of natural rubber from these nations were cut off just when there was a huge demand from the military—and that accelerated the development of synthetic rubbers, notably in Germany and the United States. Today, most natural rubber still comes from the Far East, while Russia and its former republics, France, Germany, and the United States are among the world's leading producers of synthetic rubber. The world's largest single source of latex rubber is the Harbel Rubber Plantation near Monrovia in Liberia, Africa established in the 1920s and 1930s by the Firestone tire company.

Charts: Left: Where does rubber come from? Almost three quarters of the world's rubber is produced in Asia, with the rest split mostly between Europe (including Russia) and the Americas. Almost all of the rubber produced in Africa (which, here, includes the Middle East) is natural, whereas most American and all European-produced rubber is synthetic. Asia produces roughly 60% natural and 40% synthetic rubber. Right: Overall, the world now produces more synthetic than natural rubber. Both charts drawn using the latest available data from the International Rubber Study Group, 2020.

How does vulcanization make rubber stronger?

Rubber—the kind you get from a tree—starts off as white and runny latex. Even when it's set into a product, this latex-based, natural rubber is very squashy, pretty smelly, and not very useful. The kind of rubber you see in the world around you, in things like car and bicycle tires, is vulcanized: cooked with sulfur (and often other additives) to make it harder, stronger, and longer lasting.

So what's the difference between raw, latex rubber and cooked, vulcanized rubber? In its natural state, the molecules in rubber are long chains that are tangled up and only weakly linked together. It's relatively easy to pull them apart—and that's why latex rubber is so stretchy and elastic. When latex is vulcanized, the added sulfur atoms help to form extra bonds between the rubber molecules, which are known as cross-links. These work a bit like the trusses you see on a bridge, tying the molecules together and making them much harder to pull apart.

Artwork: Top: Natural, latex rubber is easy to pull apart because the long polymer molecules it contains (made from carbon and hydrogen atoms) are only weakly linked together. Bottom: When natural rubber is cooked with sulfur, the sulfur atoms form extra cross-links (shown here as yellow bars) "bolting" the molecules together and making them much harder to pull apart. This process is called vulcanization and it makes the strong, durable, black rubber we see on things like car tires.

What do we use rubber for?

Photo: Three everyday uses of rubber. Top: A latex pencil eraser; Middle: the tough vulcanized rubber drive belt from a vacuum cleaner; Bottom: the waterproof rubber gasket that seals a washing machine door tight.

The physical and chemical properties of a material dictate what we use it for. Even if you know absolutely nothing about the real-world uses of rubber, you can probably make some very good guesses. For example, everyone knows rubber is strong, stretchy, flexible (elastic), durable, and waterproof, so it's no surprise to find it used in things like waterproof clothes and wellington boots, sticking plasters, and adhesives.

The most important use of rubber is in vehicle tires; about half of all the world's rubber ends up wrapped around the wheels of cars, bicycles, and trucks! You'll find rubber in the hard, black vulcanized outsides of tires and (where they have them) in their inner tubes and liners. The inner parts of tires are usually made from a slightly different, very flexible butyl rubber, which is highly impermeable to gases (traps them very effectively), so tires (generally) stay inflated for long periods of time.

Photo: Swimming caps like this are made from soft and stretchy latex rubber.

The fact that rubber can be made either soft or hard greatly increases the range of things we can use it for. Soft and stretchy latex is used in all kinds of everyday things, from pencil erasers, birthday balloons, and condoms to protective gloves, adhesives (such as sticky white PVA), and paints. Harder rubbers are needed for tougher applications like roofing membranes, waterproof butyl liners in garden ponds, and those rigid inflatable boats (RIBs) used by scuba divers. Because rubber is strong, flexible, and a very poor conductor of heat and electricity, it's often used as a strong, thin, jacketing material for electrical cables, fiber-optic cables, and heat pipes. But the range of applications is truly vast: you'll find it in everything from artificial hearts (in the rubber diaphragms that pump blood) to the waterproof gaskets that seal the doors on washing machines!

Neoprene (polychloroprene) is best known as the heat-insulating, outer covering of wetsuits—but it has far more applications than most people are aware of. Medical supports of various kind use it because, tightly fitted, it compresses and warms injured bits of your body, promoting faster healing. Since it's flexible and waterproof, it's also widely used as a building material, for example, as a roof and floor sealant, and as a spongy absorber of sound and vibration in door and window linings.

Although the world has a vast appetite for new rubber, we also produce a huge quantity of rubber waste, especially from discarded vehicle tires—and that's becoming an important raw material in its own right. According to the Rubber Manufacturers Association, the United States alone produced almost 270 million waste rubber vehicle tires in 2011, which is about a third of all the tires used worldwide. While some of these are retreaded and others are ground up to make a low-grade aggregate that can be used for the floors in things like children's playgrounds, over half of them are wasted (either burned as a fuel or buried in landfills). Rubber manufacturers have recently turned their attention to recycling tires in all kinds of new ways, making everything from mouse mats and sports bags to shoe soles and car components.

Photo: Half of all rubber is used in vehicle tires, and hundreds of millions are wasted each year. I've made a very slight difference to the problem by buying this recycled rubber mouse mat, made from an old car tire. It's colored black because it's made from hard vulcanized rubber.

A brief history of rubber

• 1000CE: Indians living in Central and South America have learned how to made waterproof clothes and shoes using latex from rubber trees. They call rubber trees "cahuchu" (crying wood), which is why the French still call rubber caoutchouc (pronounced "cow-chew") today.
• 1731: During an expedition to South America, French explorer Charles Marie de La Condamine (1701–74) sends back samples of rubber to Europe, prompting intense scientific interest.
• 1770: The discoverer of oxygen, English scientist Joseph Priestley (1733–1804), finds he can use pieces of rubber to erase the marks made by pencil on paper. In England, erasers are still widely called "rubbers" today.
• 1791: Englishman Samuel Peal develops a method of waterproofing cloth with a rubber solution.
• 1818: Scottish medical student James Syme (1799–1870) uses rubber-coated cloth to make raincoats.
• 1823: Scotsman Charles Macintosh learns of Syme's discovery, refines it, and patents it, earning fame and fortune as the inventor of the rubberized, waterproof coat. Waterproof coats have been known as "Mackintoshes" (with a slight variation of spelling) ever since.
• 1829: English chemist and physicist Michael Faraday (1791–1867) analyzes samples of Hevea and works out that the chemical formula for isoprene-type rubber is C

  5

  H

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• 1839: American inventor Charles Goodyear (1800–1860) accidentally discovers how to vulcanize rubber after dropping a piece of the material (which has been treated with sulfur) onto a hot stove.

  Photo: In 1839, American inventor Charles Goodyear (1800–1860) developed the vulcanization (heat-treatment) process that makes rubber harder and more durable. He'd spent many years as a struggling inventor, trying desperately to turn rubber into a useful product, when he accidentally dropped some rubber on a hot stove and watched it "cook" itself into a much more useful form: the black, vulcanized material most of us know as rubber to this day. Despite developing one of the most useful materials of all time, Goodyear never made much money from his invention and died deeply in debt. Fortunately, his name lives on in the Goodyear tire company—and his superb contribution to materials technology has never been forgotten. Photo courtesy of US Library of Congress.

• 1830s~1840s: Botanist Thomas Lobb discovers a rubbery substance called Gutta-percha (Palaquium gutta) in Malaysia; Dr William Montgomerie, a surgeon working in the same region, sends samples back to Britain in 1843. According to a contemporary account by William Dalton, it has "remarkable properties, vast utility, and application to scientific and ornamental purposes" in everything from "boots and shoes" to "prevention of toothache."
• 1876: Intrepid English explorer Sir Henry Wickham (1846–1928) smuggles thousands of seeds from the rubber tree Hevea brasiliensis out of Brazil and back to England. The English grow the seeds at Kew Gardens just outside London and export them to various Asian countries, establishing the giant plantations that now supply much of the world's rubber.
• 1877: US rubber manufacturer Chapman Mitchell develops the first commercial process for recycling rubber from scratch.
• 1882: John Boyd Dunlop (1840–1921) invents the pneumatic (air-filled) rubber tire. The development of gasoline-powered cars with rubber tires leads to a huge increase in the need for rubber.
• 1883: US chemist George Oenslager (1873–1956) develops a much faster way of vulcanizing rubber using chemicals called organic (carbon-based) accelerators.
• 1906–12: Bayer, a German chemical company, develops methyl rubber (a polymer of methylisoprene). It becomes critically important to Germany during World War I when supplies of natural rubber are cut off, but falls out of fashion when better alternatives are eventually developed.
• 1910: English Chemist S.S. Pickles becomes the first person to propose (correctly) that rubber consists of long chains of isoprene. Technically, Hevea has the chemical name cis-1,4-polyisoprene, while Gutta-percha is a variation known as trans-1,4-polyisoprene.
• 1930: German chemical company IG Farben develops a type of general-purpose, synthetic rubber named Buna-S ("bu" from butadiene, "na" from the chemical symbol for sodium, and "S" for styrene). Technically, it's a copolymer of butadiene (75 percent) and styrene (25 percent), which is why it's now more generally known as styrene-butadiene or styrene-butadiene-rubber (SBR); it's also sold under tradenames such as Goodyear's Neolite®. Today, styrene-butadiene remains by far the world's most important synthetic rubber.
• 1930: A team of US chemists at the DuPont company, led by Wallace Carothers (1896–1937), develop a revolutionary synthetic rubber called polychloroprene and sold as neoprene. (Shortly afterward, the same group developed an even more revolutionary material: nylon.)
• 1940s: Synthetic rubbers are produced in the United States for the first time by companies such as Firestone, Goodyear, and Goodrich.

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Text copyright © Chris Woodford 2008, 2020. All rights reserved. Full copyright notice and terms of use.

Neolite is a registered trademark of The Goodyear Tire & Rubber Company Corporation.

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Woodford, Chris. (2008/2020) Rubber. Retrieved from https://www.explainthatstuff.com/rubber.html. [Accessed (Insert date here)]