Rubber Topics

• natural rubber latex
• henry wickham
• british malaya
• loose rope
• synthetic rubbers
• joseph priestley
• congo free state
• kew gardens
• gum rubber
• pencil marks
• poor description
• polyisoprene
• strain energy
• fatty acids
• bond lengths
• synthetic version
• minimum energy
• inorganic materials
• thermoset
• monomer

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The purified form of natural rubber is the chemical polyisoprene which can also be produced synthetically.

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.

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 out pencil marks on paper, hence the name rubber.

However in 1876, Henry Wickham gathered thousands of seeds from Brazil, and these were germinated in Kew Gardens, UK. 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 labor. Liberia and Nigeria also started production of rubber.

Because rubber does not dissolve easily, the material is finely divided by shredding prior to its immersion.

Typically, a few percent of other materials, such as proteins, fatty acids, resins and inorganic materials are found in natural rubber.

However, it should be noted that as 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.

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.

Between a pair of links each monomer can rotate freely about its neighbour. 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.

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.

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.

This increases the elastic force constant and makes rubber harder and less extendable.

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. This critical temperature is the reason that 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.

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. Today Asia is the main source of natural rubber, accounting for around 94% of output in 2005.

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 cups are supported by a wire that encircles the tree.This wire incorporates a spring so that it can stretch as the tree grows. The latex is led into the cup by a galvanised "spout" that has been 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 annum. 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 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 coagulated latex can then be processed into the higher grade technically specified block rubbers such as TSR3L or TSRCV or used to produce Ribbed Smoke Sheet grades.

The processing of the rubber for these grades is basically a size reduction and cleaning process in order to remove contamination and prepare the material for the final stage drying.

Tires and tubes are the largest consumers of rubber, accounting for around 56% total consumption in 2005. The remaining 44% are taken up by the general rubber goods (GRG) sector, which includes all products except tires and tubes.

Gloves (medical, household and industrial) are also large consumers of toy balloons and 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 industry. Rubber is also commonly used to make rubber bands and pencil erasers.

Vulcanization greatly improved the durability and utility of rubber from the 1830s on. The development of vulcanization is most closely associated with Charles Goodyear Slack, Charles.

Source: Wikipedia > Natural Rubber