Natural rubber

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

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

Latex being collected from a tapped rubber tree

Rubber tree plantation of Thailand （7 August 2011）

Natural rubber, also called India rubber or caoutchouc, as initially produced, consists of suitable polymers of the organic compound isoprene with minor impurities of other organic compounds plus water. Forms of polyisoprene that are useful as natural rubbers are classified as elastomers. Currently the rubber is harvested mainly in the form of the latex from certain trees. The latex is a sticky, milky colloid drawn off by making incisions into the bark and collecting the fluid in vessels. This process is called "tapping". The latex then is refined into rubber ready for commercial processing. 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, high resilience, and is extremely waterproof.

Varieties

The major commercial source of natural rubber latex is the Pará rubber tree (Hevea brasiliensis), a member of the spurge family, Euphorbiaceae. This species is widely used because it grows well under cultivation and a properly managed tree responds to wounding by producing more latex for several years.

Many other plants produce forms of latex rich in isoprene polymers, though not all produce usable forms of polymer as easily as the Pará rubber latex does; 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) and chicle from Manilkara species. Others that have been commercially exploited, or at least have shown promise as sources of rubber, 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 Russian dandelion (Taraxacum kok-saghyz), and guayule (Parthenium argentatum). To distinguish the tree-obtained version of natural rubber from the synthetic version, the term gum rubber is sometimes used.

Discovery of commercial potential

The para rubber tree is indigenous to South America. Charles Marie de La Condamine is credited with introducing samples of rubber to the Académie Royale des Sciences of France in 1736. In 1751, he presented a paper by François Fresneau to the Académie (eventually published in 1755) which described many of the properties of rubber. This has been referred to as the first scientific paper on rubber. In England, it was observed by Joseph Priestley, in 1770, that a piece of the material was extremely good for rubbing off pencil marks on paper, hence the name rubber. Later it slowly made its way around England.

South America remained the main source of the limited amounts of latex rubber that were used during much of the 19th century. In 1876, Henry Wickham gathered thousands of para rubber tree seeds from Brazil, and these were germinated in Kew Gardens, England. The seedlings were then sent to India, Ceylon (Sri Lanka), Indonesia, Singapore and British Malaya. Malaya (now Malaysia) was later to become the biggest producer of rubber. In the early 1900s, the Congo Free State in Africa was also a significant source of natural rubber latex, mostly gathered by forced labor. Liberia and Nigeria also started production of rubber.

In India, commercial cultivation of natural rubber was introduced by the British planters, although the experimental efforts to grow rubber on a commercial scale in India were initiated as early as 1873 at the Botanical Gardens, Calcutta. The first commercial Hevea plantations in India were established at Thattekadu in Kerala in 1902.

In Singapore and Malaya, commercial production of rubber 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. Because of his very fervent promotion of this crop, he is popularly remembered by the nickname "Mad Ridley".

Properties

Rubber latex

Rubber exhibits unique physical and chemical properties. Rubber's stress-strain behavior exhibits the Mullins effect, the Payne effect, and is often modeled as hyperelastic. Rubber strain crystallizes.

Owing to the presence of a double bond in each repeat unit, natural rubber is susceptible to vulcanisation and sensitive to ozone cracking.

There are two main solvents for rubber: turpentine and naphtha (petroleum). The former has been in use since 1764 when François Fresnau made the discovery. Giovanni Fabbroni is credited with the discovery of naphtha as a rubber solvent in 1779. 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 while it is being transported from its collection site.

Elasticity

In most elastic materials, such as metals used in springs, the elastic behavior is caused by bond distortions. When force is applied, bond lengths deviate from the (minimum energy) equilibrium and strain energy is stored electrostatically. Rubber is often assumed to behave in the same way, but it turns out this is a poor description. Rubber is a curious material because, unlike in metals, strain energy is stored thermally.

In its relaxed state, rubber consists of long, coiled-up chains. When rubber is stretched, the chains are taut. Their kinetic energy is released as heat. The entropy decreases during elongation but increases during relaxation. This change in entropy related to the changes in degrees of freedom. Relaxation of a stretched rubber band is thus driven by an increase in entropy, and the force experienced is a result of the thermal energy of the material being converted to kinetic energy. Rubber relaxation is endothermic, and for this reason the force exerted by a stretched piece of rubber increases with temperature. The material undergoes adiabatic cooling during contraction. This property of rubber can easily be verified by holding a stretched rubber band to your lips and relaxing it. Stretching of a rubber band is in some ways equivalent to the compression of an ideal gas, and relaxation is equivalent to its expansion. A compressed gas also exhibits "elastic" properties, for instance inside an inflated car tire. The fact that stretching is equivalent to compression may seem somewhat counter-intuitive, but it makes sense if rubber is viewed as a one-dimensional gas. Stretching reduces the "space" available to each section of chain.

Vulcanization of rubber creates disulfide bonds between chains, so it limits the degrees of freedom. The result is that the chains tighten more quickly for a given strain, thereby increasing the elastic force constant and making rubber harder and less extensible.

When cooled below the glass transition temperature, the quasi-fluid chain segments "freeze" into fixed geometries and the rubber abruptly loses its elastic properties, although the process is reversible. This property it shared by most elastomers. At very low temperatures, rubber is rather brittle. This critical temperature is the reason winter tires use a softer version of rubber than normal tires. The failing rubber o-ring seals that contributed to the cause of the Challenger disaster were thought to have cooled below their critical temperature; the disaster happened on an unusually cold day.

Chemical makeup

Latex is the polymer cis-1,4-polyisoprene) – with a molecular weight of 100,000 to 1,000,000. 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 created synthetically, producing what is sometimes referred to as "synthetic natural rubber", but the synthetic and natural routes are completely different

Chemical structure of cis-polyisoprene, the main constituent of natural rubber. Synthetic cis-polyisoprene and natural cis-polyisoprene are derived from different precursors.

Some natural rubber sources called gutta-percha are composed of trans-1,4-polyisoprene, a structural isomer that has similar, but not identical, properties.

Natural rubber is an elastomer and a thermoplastic. Once the rubber is vulcanized, it will turn into 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 like carbon black, factice, whiting, and a host of others.

Biosynthesis

Rubber particles are formed in the cytoplasm of specialized latex producing cells called laticifers within rubber synthesizing plants. 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 due to the high negative charge along the double bonds of the rubber polymer backbone. 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. 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. The farnesyl pyrophosphate can bind to rubber transferase to elongate a new rubber polymer.

The required isopentenyl pyrophosphate is obtained from the mevalonate (MVA) pathway, which is derives from acetyl-CoA in the cytosol. In plants, isoprene pyrophosphate can also be obtained from 1-deox-D-xyulose-5-phosphate/2-C-methyl-D-erythritol-4-phosphate pathway within plasmids. 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 have shown numerous small molecular weight proteins with unknown function. It is possible that the proteins serve as cofactors as there is a reduction of synthetic rate with complete removal.

Current sources

Close to 21 million tons of rubber were produced in 2005, of which approximately 42% was natural. Since the bulk of the rubber produced is of the synthetic variety, which is derived from petroleum, the price of natural rubber is determined, to a large extent, by the prevailing global price of crude oil. Today, Asia is the main source of natural rubber, accounting for about 94% of output in 2005. The three largest producing countries, Thailand, Indonesia (2.4m tons) 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 due to the existence of South American leaf blight, and other natural predators of the rubber tree.

Cultivation

Rubber is generally cultivated in large plantations. See the coconut shell used in collecting latex, in plantations in Kerala, India

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

The soil requirement of the plant is generally 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 consist of:

• Rainfall of around 250 cm evenly distributed without any marked dry season and with at least 100 rainy days per year.
• Temperature range of about 20°C to 34°C with a monthly mean of 25°C to 28°C.
• High atmospheric humidity of around 80%.
• Bright sunshine amounting to about 2000 hours per year at the rate of 6 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 of dry rubber per hectare per year, when grown under ideal conditions and ideal field.

Collection

A woman in Sri Lanka in the process of harvesting rubber

In places like Kerala, where coconuts are in abundance, the half shell of coconut is used as the collection container for the latex but glazed pottery or aluminium or plastic cups are more common elsewhere. 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. 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 alternate or third daily, although there are many variations in timing, length and number of cuts. The latex, which contains 25–40% dry rubber, is in the bark, so the tapper must avoid cutting right through to the wood or the growing cambial layer will be damaged and the renewing bark will be badly deformed, making later tapping difficult. It is usual to tap a pannel 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 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 will 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, then start collecting the latex at about midday. Some trees will continue to drip after the collection and this leads to a small amount of cup lump which 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.

The latex will coagulate in cup if kept for long. The latex has to be collected before coagulation. The collected latex is transferred in to coagulation tanks for the preparation of dry rubber or transferred into air-tight containers with sieving for ammoniation. Ammoniation is necessary to preserve the latex in colloidal state for long.

Latex is generally processed into either latex concentrate for manufacture of dipped goods or it can be 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. The processing of the rubber for these grades is a size reduction and cleaning process to remove contamination and prepare the material for the final stage of drying.

The dried material is then baled and palletized for storage and shipment in various methods of transportation.

Transportation

Natural rubber latex is shipped from factories in South-West Asia, South America and North Africa to destinations around the world. As cost of natural rubber has risen significantly, the shipping methods which offer the lowest cost per unit (kg, tonne or pound) are preferred. Depending on the destination, warehouse availability, transportation conditions, some methods are more suitable to certain buyers than others. In international trade, latex rubber is mostly shipped in 20 foot ocean containers. Inside the ocean container, various types of smaller containers are used by factories to store latex rubber.

Uses

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

The use of rubber is widespread, ranging from household to industrial products, entering the production stream at the intermediate stage or as final products. Tires and tubes are the largest consumers of rubber. The remaining 44% are taken up by the general rubber goods (GRG) sector, which includes all products except tires and tubes.

Prehistoric uses

The first use of rubber was by the Olmecs, who centuries later passed on the knowledge of natural latex from the Hevea tree in 1600 BC to the ancient Mayans. They boiled the harvested latex to make a ball for a Mesoamerican ballgame.

Manufacturing

Other significant uses of rubber are door and window profiles, hoses, belts, matting, flooring and dampeners (antivibration mounts) for the automotive industry. Gloves (medical, household and industrial) and toy balloons are also large consumers of rubber, although the type of rubber used is that of the concentrated latex. Significant tonnage of rubber is used as adhesives in many manufacturing industries and products, although the two most noticeable are the paper and the carpet industries. Rubber is also commonly used to make rubber bands and pencil erasers. Many aircraft tires and inner tubes are still made of natural rubber due to the high cost of certification for aircraft use of synthetic replacements.

Textile applications

Rubber produced as a fiber sometimes called elastic, has significant value for use in the textile industry because of its excellent elongation and recovery properties. For these purposes, manufactured rubber fiber is made as either an extruded round fiber or rectangular fibers that are cut into strips from extruded film. Because of its low dye acceptance, feel and appearance, the rubber fiber is either covered by yarn of another fiber or directly woven with other yarns into the fabric. In the early 1900s, for example, 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. Seeking a way to address these shortcomings, the textile industry has 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.

Vulcanization

Main article: Vulcanization

Natural rubber is often vulcanized, 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 perishing. The development of vulcanization is most closely associated with Charles Goodyear in 1839. Carbon black is often used as an additive to rubber to improve its strength, especially in vehicle tires.

Allergic reactions

Main article: Latex allergy

Some people have a serious latex allergy, and exposure to certain natural latex rubber products such as latex gloves can cause anaphylactic shock. Depending on how the latex is processed, the antigenic proteins found in Hevea latex may be significantly reduced, as in Vytex Natural Rubber Latex or Talalay foam rubber.

Guayule latex is hypoallergenic and is being researched as a substitute to the allergy-inducing Hevea latexes. Unlike the tappable Hevea tree, these relatively small shrubs must be harvested whole and latex extracted from each cell.

Some allergic reactions are not from the latex but from residues of other ingredients used to process the latex into clothing, gloves, foam, etc. These allergies are usually referred to as multiple chemical sensitivity (MCS).

See also

• Akron, Ohio, center of the rubber industry in the USA
• Condoms, also called "rubbers"
• Emulsion dispersion
• Fordlândia, failed attempt to establish a rubber plantation in Brazil
• Reinforced rubber
• Resilin, a rubber substitute
• 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

Sources

• Rubbery Materials and their Compounds by J.A Brydson
• Rubber Technology by Maurice Morton
• Hobhouse, Henry (2003, 2005). Seeds of Wealth: Five Plants That Made Men Rich. Shoemaker & Hoard. pp. 125–185. ISBN 1-59376-089-2. {{cite book}}: Check date values in: |date= (help)

Additional resources

• Database for natural rubber, by National Laboratories of Medicine

Bibliography

• Ascherson, Neal: The King Incorporated, Allen & Unwin, 1963. ISBN 1-86207-290-6 (1999 Granta edition).
• Hochschild, Adam: King Leopold’s Ghost: A Story of Greed, Terror, and Heroism in Colonial Africa, Mariner Books, 1998. ISBN 0-330-49233-0.
• Petringa, Maria: Brazza, A Life for Africa. Bloomington, IN: AuthorHouse, 2006. ISBN 978-1-4259-1198-0

External links

History

• History of the International Rubber Industry from 1870–1930 (Economic History Association)
• A Brief History of Rubber
• An Introduction to Rubber Technology
• Rubber timeline
• Historical Rubber Prices

Associations

• International Rubber Research and Development Board (IRRDB)
• Association of Natural Rubber Producing Countries (ANRPC)
• International Rubber Study Group (IRSG)
• Associação Paulista de Produtores e Beneficiadores de Borracha (APABOR), Brazil (Portuguese)
• Malaysian Rubber Board (LGM)
• Rubber Board Of India
• Thailand Rubber Association (TRA)
• Rubber Research Institute of Sri Lanka

Other

• Rubber-Chemical Compatibility Guide
• Basic Rubber Testing
• Fertilizer–free rubber tree planting, - BioRubber