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.
The 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 responds to wounding by producing more latex.
Other plants containing latex include gutta-percha (Palaquium gutta), rubber fig (Ficus elastica), Panama rubber tree (Castilla elastica), spurges (Euphorbia spp.), lettuce, common dandelion (araxacum officinale), Russian dandelion (Taraxacum kok-saghyz), Scorzonera (tau-saghyz), and guayule (Parthenium argentatum). Although historically not major sources of rubber, Germany attempted to use some of these during World War II when it was cut off from rubber supplies. 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. In 1751, he presented a paper by François Fresneau to the Académie (eventually published in 1755) which described many of the properties of rubber. Fresneau’s article is commonly referred to as the first scientific paper on rubber.
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.
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 remaining balance was imported with an import duty of 20%.
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.
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.
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.
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. 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, similar to a 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 unable to oscillate. Their kinetic energy is emitted 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.
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. Currently Asia is the main source of natural rubber, accounting for approximately 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.
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 and ideal field.
In locations like Kerala, where coconuts are in abundance, the half shell of a 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 panel 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 ontainers with sieving for ammoniation. Ammoniation is necessary to preserve the latex in colloidal state for longer periods.
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.
The dried material is then baled and palletized for storage and shipment via various methods of 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.
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.
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 sport.
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 with the paper and the carpet industries being the two most noticeable. 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.
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.
Rubbery Materials and their Compounds by J.A Brydson
Rubber Technology by Maurice Morton
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