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 （2011.8.7）

Natural rubber, also called India rubber or caoutchouc, is an elastomer (an elastic hydrocarbon polymer) that was originally derived from latex, a milky colloid produced by some plants. The plants would be ‘tapped’, that is, an incision made into the bark of the tree and the sticky, milk colored latex sap collected and refined into a usable rubber. The purified form of natural rubber is the chemical polyisoprene, which can also be produced synthetically. Natural rubber is used extensively in many applications and products, as is synthetic rubber. It is normally very stretchy and flexible and extremely waterproof.

Varieties

The commercial source of natural rubber latex is the para rubber tree (Hevea brasiliensis), a member of the spurge family, Euphorbiaceae. This is largely because it responds to wounding by producing more latex, also this means that the tree is able to photosynthesise more.

Other plants containing latex include gutta-percha (Palaquium gutta),^[1] rubber fig (Ficus elastica), Panama rubber tree (Castilla elastica), spurges (Euphorbia spp.), lettuce, common dandelion (Taraxacum officinale), Russian dandelion (Taraxacum kok-saghyz), Scorzonera (tau-saghyz), and guayule (Parthenium argentatum). Although these have not been major sources of rubber, Germany attempted to use some of these during World War II when it was cut off from rubber supplies^{[citation needed]}. These attempts were later supplanted by the development of synthetic rubbers. 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 initially grew in South America. Charles Marie de La Condamine is credited with introducing samples of rubber to the Académie Royale des Sciences of France in 1736.^[2] 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.^[2]

When samples of rubber first arrived 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 Ceylon (Sri Lanka), Indonesia, Singapore and British Malaya. Malaya (now Malaysia) was later to become the biggest producer of rubber. About 100 years ago, the Congo Free State in Africa was also a significant source of natural rubber latex, mostly gathered by forced labour. 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 the 19th and early 20th century, it was often called "India rubber." In 2010, India's natural rubber consumption stood at 0.978 million tons per year, with production at 0.893 million tons; the rest was imported with an import duty of 20%.

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 sensitive to ozone cracking.

Solvents

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.

Chemical makeup

Latex is a natural polymer of isoprene (most often 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 is also created synthetically, producing what is sometimes referred to as "synthetic natural rubber".

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

Natural rubber is an elastomer and a thermoplastic. However, it should be noted that 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.

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 metals, strain energy is stored thermally.

In its relaxed state, rubber consists of long, coiled-up polymer chains that are interlinked at a few points. Between a pair of links, each monomer can rotate freely about its neighbour, thus giving each section of chain leeway to assume a large number of geometries, like a very loose rope attached to a pair of fixed points. At room temperature, rubber stores enough kinetic energy so that each section of chain oscillates chaotically, like the above piece of rope being shaken violently. The entropy model of rubber was developed in 1934 by Werner Kuhn.

When rubber is stretched, the "loose pieces of rope" are taut and thus no longer able to oscillate. Their kinetic energy is given off as excess heat. Therefore, the entropy decreases when going from the relaxed to the stretched state, and it increases during relaxation. This change in entropy can also be explained by the fact that a tight section of chain can fold in fewer ways (W) than a loose section of chain, at a given temperature (nb. entropy is defined as S=k*ln(W)). Relaxation of a stretched rubber band is thus driven by an increase in entropy, and the force experienced is not electrostatic, rather it 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. (Metals, for example, become softer as temperature increases). 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. Note that 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 counterintuitive, 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 more disulfide bonds between chains, so it shortens each free section of chain. The result is that the chains tighten more quickly for a given length of 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 is a property it shares with most elastomers. At very low temperatures, rubber is rather brittle; it will break into shards when struck or stretched. 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.

Current sources

Close to 21 million tons of rubber were produced in 2005 of which around 42% was natural. Since the bulk of the rubber produced is the synthetic variety which is derived from petroleum, the price of even natural rubber is determined to a very large extent by the prevailing global price of crude oil.^[3][4] Today Asia is the main source of natural rubber, accounting for around 94% of output in 2005. The three largest producing countries, Thailand, Indonesia (2.4m tons)^[5] 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 (a) rainfall of around 250 cm evenly distributed without any marked dry season and with at least 100 rainy days per year (b) temperature range of about 20°C to 34°C with a monthly mean of 25°C to 28°C (c) high atmospheric humidity of around 80% (d) bright sunshine amounting to about 2000 hours per year at the rate of 6 hours per day throughout the year and (e) 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.

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 trees' 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 basically a size reduction and cleaning process to remove contamination and prepare the material for the final stage of drying.^[6]

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

Transportation

Natural Rubber Latex is shipped from their factories in 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.^[7]

1. Steel Drums:

55 gallon steel drum205 kilogram or 55 gallon drum steel drums are the traditional method of packaging of natural latex. The drum is typically made of steel with a rubbed outer to improve rigidity and durability. A 20 foot container can store 80 of such 55 gallon drums for a net weight of 16.40 metric ton (16,400 kilogram or 36,080 "wet" pound) of latex. The use of drums allows bulk transportation to save cost yet permits flexibility in retail distribution. A broker can sell in small quantity such as 1 or 2 drums.

2. ISO Tanks:

These tanks are in fact the ocean container itself. They all have standard size of 20 foot x 8 foot x 8’6 foot. Their capacity is between 17,500 to 25,000 liters depending on the cylinder size and shape. These ISO tanks can also be used for transportation of liquid chemicals and food stuffs. This is the costliest shipping method, but also provides most convenient handling in some countries. Usually the buyer has to buy the entire tank to enjoy the benefit of bulk saving.

3. Flexitank or Flexibag inside 20 foot container:

Flexitanks offer a much cheaper method of shipping than ISO tank, and they are widely used nowadays. The flexitank itself is a big PE bag with a valve to for loading and later, unloading of the liquid content. The PE bag is placed inside a 20 foot container with proper reinforcement near the container doors. The latex is then pumped into the PE bag (picture 3), which is "blown up" as the amount of liquid increases. Maximum of 21 tons of liquid latex can be stored inside a flexitank as this is a limit weight allowable on public streets and highways in many countries. The container doors are closed, sealed, and are ready for load onto sailing ship. At destination, the rubber latex is pumped out using hydrolic pumps. The flexitank is discarded after use.

4. Other Methods:

Totes made of PE or corrugated paper of different sizes are sometimes used to transport latex rubber. Their footprints (bases) are often similar to the size of a standard pallet (40 in x 48 in). However, these totes are less common as the other methods make the majority of shipment volume in international trades.

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 sport.^[8]

Manufacturing

Other significant uses of rubber are door and window profiles, hoses, belts, matting, flooring and dampeners (antivibration mounts) for the automotive industry in what is known as the "under the bonnet" products. 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

Additionally, 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 form 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.^[9] 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 rubber latex products such as latex gloves can cause anaphylactic shock. Guayule latex is hypoallergenic and is being researched as a substitute to the allergy-inducing Hevea latexes. Unlike the sappable Hevea tree, these relatively small shrubs must be harvested whole and latex extracted from each cell. Chemical processes may also be employed to reduce the amount of antigenic protein in Hevea latex, resulting in alternative Hevea-based materials such Vytex Natural Rubber Latex that, while not completely hypoallergenic, do provide lessened exposure to latex allergens.

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

Notes

1. ^ Burns, Bill. "The Gutta Percha Company". History of the Atlantic Cable & Undersea Communications. http://www.atlantic-cable.com/Article/GuttaPercha/. Retrieved 2009-02-14. 
2. ^ ^{a b} Untitled Document
3. ^ Overview of the Causes of Natural Rubber Price Volatility
4. ^ Short run and long run effects of the world crude oil prices on the Malaysian natural rubber and palm oil export prices
5. ^ bloomberg.com
6. ^ Technical Grades and Basis for Grading by ASTM D2227 - Basic Rubber Testing
7. ^ Transportation of Natural Rubber - Industry Source
8. ^ The Mayan-Olmec Connection
9. ^ Slack, Charles. "Noble Obsession: Charles Goodyear, Thomas Hancock, and the Race to Unlock the Greatest Industrial Secret of the Nineteenth Century". Hyperion 2002. [ISBN 9780786867899]

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. 

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
• 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
• Revista Lateks Brazilian rubber magazine (Portuguese)