Efect Fade

History

Natural rubber is an elastic hydrocarbon polymer that naturally occurs as a milky colloidal suspension, or latex, in the sap of some plants.

The major commercial source of natural rubber latex is the Para rubber tree, Hevea brasiliensis . This is largely because it responds to wounding by producing more latex. Henry Wickham gathered thousands of seeds from Brazil in 1876 and they were germinated in Kew Gardens, England. The seedlings were sent to Ceylon (Sri Lanka), Indonesia, Singapore and British Malaya. Malaya(now Malaysia) was later to become the biggest producer of rubber. Liberia and Nigeria are examples of African rubber-producing countries.

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, its price is determined to a very large extent by the prevailing global price of crude oil. Of course, this also reflects on the price of natural rubber, which in some fields stands in direct competition to synthetic rubber, however, the ultimate situation on the NR market is largely influenced by supply and demand, which presently shows a considerable deficit thereby exercising substantial pressure on the price of natural rubber which right now stands at an all-time high and is likely to remain there for some time.

Today Asia is the main source of natural rubber, accounting for around 94% of output in 2005. The three largest producing countries (Indonesia, Malaysia and Thailand) together account for around 72% of all natural rubber production.

Rubber latex is extracted from Rubber trees. The economic life period of rubber trees in plantations is around 32 years – 7 years of growth 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 tree consist of  Rainfall of around 250 cm evenly distributed without any marked dry season and with at least 100 rainy days per annum  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 annum at the rate of 6 hours per day throughout the year and  Absence of strong winds.

Chemical makeup

Aside from a few natural product impurities, natural rubber is essentially a polymer of isoprene units, a hydrocarbon diene monomer. The rubber polymer represents all cis poly isoprene chains. This fact is responsible for its elastomer behaviour. The all trans configuration of poly isoprene shows mere plastic characteristics. A well known natural product is gutta-percha. The first one shows a glass transition temperature of –730C against –530C of the latter. However it should be born in mind that only by vulcanisation the elastomeric properties become permanent, more or less independent of temperature.

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 neighbor. This gives 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.

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. 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. 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 in 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 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. Vulcanisation creates the sulphur bonds between the chains and thus shortens the free sections of the chains. As the result the chains tighten more quickly on a given strain. This increases the elongation retraction or compres­sion deflection behaviour of the rubber. The amount of sulphur employed for vulcanisation results in a more or less flexible article

When cooled below the glass transition temperature, the quasi-fluid chain segments "freeze" into fixed geometries and the rubber abruptly loses its elastic properties, though the process is reversible. This is a property it shares with most elastomers. At very cold temperatures rubber is actually rather brittle; it will break into shards when struck or stretched.

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