Natural rubber
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
Amazonian rubber tree (Hevea brasiliensis)
The major commercial source of natural rubber latex is the Amazonian rubber tree (
Congo rubber (Landolphia owariensis and L. spp.)
Dandelion
Other
Many other plants produce forms of latex rich in isoprene polymers, though not all produce usable forms of polymer as easily as the Pará.
History
The first use of rubber was by the indigenous cultures of
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
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]
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
Before World War II significant uses included door and window profiles, hoses, belts, gaskets,
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
Rubber exhibits unique physical and chemical properties. Rubber's stress–strain behavior exhibits the
Elasticity
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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
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
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
Rubber is the polymer cis-1,4-polyisoprene – with a
Biosynthesis
Rubber particles are formed in the
The required isopentenyl pyrophosphate is obtained from the
Production
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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
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
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
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
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
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
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
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
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
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
Rubber is a natural polymer of
Vulcanized rubber
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
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
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
This section needs additional citations for verification. (November 2016) |
This section possibly contains original research. (November 2016) |
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
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
Some people have a serious
Latex from non-
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
Natural rubber is susceptible to degradation by a wide range of bacteria.[54][55][56][57][58][59][60][61]
The bacteria
See also
- Akron, Ohio, center of the United States rubber industry
- Crepe rubber
- Ebonite
- Emulsion dispersion
- Fordlândia, failed attempt to establish a rubber plantation in Brazil
- Reinforced rubber
- Resilin, a highly elastic protein found in insects
- Rubber seed oil
- Rubber technology
- Stevenson Plan, historical British plan to stabilize rubber prices
- Charles Greville Williams, researched natural rubber being a polymer of the monomer isoprene
References
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- ^ Fadhlan Zuhdi, "The Indonesian natural rubber export competitiveness in global market." International Journal of Agriculture System 8.2 (2021): 130-139 online.
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The importance of developing alternative sources of natural rubber becomes clear when considering that there has been a shortage in the supply of this critical resource every year since 2004. By 2020, the global shortfall of natural rubber is projected to be more than the entire amount (1.2 million metric tonnes) the U.S. imports every year.
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- Hobhouse, Henry (2005) [2003]. Seeds of Wealth: Five Plants That Made Men Rich. Shoemaker & Hoard. pp. 125–185. ISBN 978-1-59376-089-2.
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- Morton, M. (2013). Rubber Technology. Springer Science & Business Media. ISBN 978-94-017-2925-3.
Further reading
- 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.
External links
- The dictionary definition of natural rubber at Wiktionary