Natural rubber: Difference between revisions
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:''This article is about the polymeric material '''natural rubber'''.'' |
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:''For man-made rubber materials, see [[Synthetic rubber]]. For other uses, see [[Rubber (disambiguation)]].'' |
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[[Image:Latex dripping.JPG|thumb|200px|right|Latex being collected from a [[rubber tapping|tapped]] [[Pará rubber tree|rubber tree]]]] |
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'''Natural rubber''' is an elastic [[hydrocarbon]] [[polymer]] that naturally occurs as a milky [[colloidal suspension]], or ''[[latex (rubber)|latex]]'', in the sap of some plants. It is used extensively in many applications and products. It can also be synthesized. The [[entropy]] model of rubber was developed in 1934 by [[Werner Kuhn]]. The scientific name for the [[Pará rubber tree|rubber tree]] commercially used to obtain natural rubber is ''Hevea brasiliensis''. |
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== Varieties == |
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The major commercial source of natural rubber latex is the [[Para rubber tree]], ''Hevea brasiliensis'' ([[Euphorbiaceae]]). This is largely because it responds to wounding by producing more latex. |
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Other plants containing latex include [[fig]]s (''[[Ficus elastica]]''), ''[[Castilla elastica|Castilla]]'' (Panama rubber tree), [[euphorbia]]s, [[lettuce]], the common [[dandelion]], [[Taraxacum kok-saghyz]] (Russian dandelion), [[Scorzonera tau-saghyz]], and [[Guayule]]. 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{{Fact|date=August 2008}}. These attempts were later supplanted by the development of [[synthetic rubber]]. To distinguish the tree-obtained version of natural rubber from the synthetic version, the term '''gum rubber''' is sometimes used. |
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==Discovery of commercial potential== |
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[[Charles Marie de La Condamine]] is credited with introducing samples of rubber to the [[Académie Royale des Sciences]] of [[France]] in 1736.<ref name=bouncingballs>[http://www.bouncing-balls.com/timeline/people/nr_condamine.htm Untitled Document<!-- Bot generated title -->]</ref> 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.<ref name=bouncingballs/> |
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The para rubber tree initially grew in [[South America]], and the first European to return to [[Portugal]] from Brazil with samples of water-repellent rubberized cloth so shocked people that he was brought to court on the charge of witchcraft. 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". |
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South America remained the main source of what limited amount of latex rubber was consumed during much of the 19th century. 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. |
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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, [[Kolkata]]. The first commercial ''Hevea'' plantations in India were established at Thattekadu in [[Kerala]] in 1902. |
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==Properties== |
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[[Image:20070104rubberlatex.jpg|thumb|200px|Rubber latex.]] |
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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 material|hyperelastic]]. Rubber [[Strain crystallization|strain crystallizes]]. |
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Owing to the presence of a [[double bond]] in each and every [[repeat unit]], natural rubber is sensitive to [[ozone cracking]] |
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== Chemical makeup == |
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Aside from a few natural product impurities, natural rubber is essentially a [[polymer]] of [[isoprene]] units, a [[hydrocarbon]] [[diene]] [[monomer]]. Synthetic rubber can be made as a polymer of isoprene or various other monomers. The material properties of natural rubber make it an [[elastomer]] and a [[thermoplastic]]. 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. |
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===Elasticity=== |
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In most elastic materials, such as [[metals]] used in [[Coil spring|springs]], the elastic behavior is caused by [[Chemical bond|bond]] distortions. When force is applied, bond lengths deviate from the (minimum energy) equilibrium and strain energy is stored [[electrostatic potential|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 [[thermal energy|thermally]]. |
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In its relaxed state rubber consists of long, coiled-up polymer chains that are [[Disulfide bond|interlinked]] at a few points. 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. |
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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 process|adiabatic cooling]] during contraction. This property of rubber can easily be verified by holding a stretched rubber band to your lips and relaxing it. |
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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 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. |
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[[Vulcanization]] of rubber creates more [[disulfide bonds]] between chains so it makes each free section of chain shorter. The result is that the chains tighten more quickly for a given length of [[Strain (materials science)|strain]]. This increases the elastic force constant and makes rubber harder and less extendable. |
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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. This critical temperature is the reason that [[winter tire]]s 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. |
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==Current sources== |
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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{{Fact|date=June 2008}}. 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. |
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=== Cultivation === |
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[[Image:Arun image7.jpg|thumb|left|Rubber is generally cultivated in large plantations. See the coconut shell used in collecting latex, in plantations in [[Kerala]], [[India]]]] |
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Rubber latex is extracted from Rubber trees. The economic life period of rubber trees in plantations is around 32 years – 7 years of immature phase and about 25 years of productive phase. |
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The soil requirement of the plant is generally well-drained weathered soil consisting of laterite, lateritic types, sedimentary types, nonlateritic red or alluvial soils. |
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The climatic conditions for optimum growth of Rubber tree consist of (a) Rainfall of around 250 cm evenly distributed without any marked dry season and with at least 100 rainy days per annum (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 annum at the rate of 6 hours per day throughout the year and (e) Absence of strong winds. |
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Many high-yielding clones have been developed for commercial planting. These clones yield more than 1,500 kilograms of dry Rubber per hectare (or, over 4 tons per acre), per annum, when grown under ideal conditions. |
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=== Collection === |
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[[Image:Ceylon rubber.jpg|thumb|A tree woman in [[Sri Lanka]] in the process of harvesting rubber.]] |
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In places like [[Kerala]], where coconuts are in abundance, the shell of half a coconut is used as the collection container for the latex. The shells are attached to the tree via a short sharp stick and the latex drips down into it overnight. This usually produces latex up to a level of half to three quarters of the shell. |
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The latex can be either collected in its liquid state, in which case [[ammonia]] solution can be added to the collecting cup prior to tapping in order to prevent natural coagulation, or it can be left in the field to coagulate into a cup lump. |
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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 TSR3L or TSRCV or used to produce Ribbed Smoke Sheet grades. |
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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 in order to remove contamination and prepare the material for the final stage drying. |
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The dried material is then baled and palletized for shipment. |
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== Uses == |
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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, 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. |
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Other significant uses of rubber are door and window profiles, hoses, belts, matting, flooring and dampeners (anti-vibration mounts) for the [[automotive]] industry in what is known as the "under the bonnet" products. [[Gloves]] (medical, household and industrial) are also large consumers of rubber and toy balloons, although the type of rubber used is that of the concentrated latex. Significant tonnage of rubber is used as [[adhesive]]s 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 band]]s and pencil [[eraser]]s. |
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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 1900’s, 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. |
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Natural rubber is often [[Vulcanization|vulcanized]], a process by which the rubber is heated and [[sulfur]], peroxide or bisphenol are added to improve resilience and elasticity, and to prevent it from [[vulcanization#Reason for vulcanizing|perishing]]. Vulcanization greatly improved the durability and utility of rubber from the 1830s on. The successful development of vulcanization is most closely associated with [[Charles Goodyear]]. [[Carbon black]] is often used as an additive to rubber to improve its strength, especially in vehicle tires. |
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==Allergic reactions== |
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{{main|Latex allergy}} |
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Some people have a serious [[latex allergy]], and exposure to certain natural rubber latex products such as [[latex glove]]s can cause [[anaphylactic shock]]. [[Guayule]] latex is [[hypoallergenic]] and is being researched as a substitute to the allergy-inducing [[Hevea]] latexes. |
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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). |
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== See also == |
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* [[Akron, Ohio]], center of the rubber industry |
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* [[Charles Greville Williams]], researched natural rubber being a polymer of the monomer isoprene |
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* [[Elastomer]] |
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* [[Fordlândia]], failed attempt to establish a rubber plantation in Brazil |
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* [[Molded]] rubber |
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* [[Ozone cracking]] |
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* [[Rubber tapping]], the process of harvesting the rubber sap |
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* [[Stevenson Plan]], historical British plan to stabilise rubber prices |
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==References== |
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{{Reflist}} |
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* ''Rubbery Materials and their Compounds'' by J.A Brydson |
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* ''Rubber Technology'' by Maurice Morton |
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== External links == |
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{{Wiktionary}} |
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{{commonscat|Rubber}} |
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*[http://www.zrunek.at/download/Bestaendigkeitsliste.pdf Chemical Resistance Guide] |
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*[http://www.irrdb.com/IRRDB/NaturalRubber/Default.htm International Rubber Research & Development Board] |
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*[http://eh.net/encyclopedia/article/frank.international.rubber.market History of the International Rubber Industry from 1870-1930 from EH.NET] |
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*[http://www.lgm.gov.my Malaysian Rubber Board] |
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*[http://www.rubberboard.org Rubber Board Of India] |
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*[http://www.rubberdivision.org Rubber Division of ACS] |
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*[http://www.bouncing-balls.com/timeline/timeline3.htm Rubber timeline] |
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[[Category:Natural materials]] |
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[[Category:Organic polymers]] |
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[[Category:Terpenes and terpenoids]] |
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[[Category:Rubber]] |
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[[Category:Elastomers]] |
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[[Category:Adhesives]] |
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[[Category:Nonwoven fabrics]] |
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[[ar:مطاط]] |
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[[bg:Каучук]] |
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[[cs:Pryž]] |
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[[de:Gummi]] |
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[[el:Καουτσούκ]] |
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[[es:Hule]] |
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[[eo:Gumo]] |
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[[fa:کائوچو]] |
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[[fr:Caoutchouc (matériau)]] |
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[[gl:Caucho]] |
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[[ko:고무]] |
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[[id:Karet]] |
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[[it:Gomma (materiale)]] |
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[[he:גומי]] |
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[[lt:Guma]] |
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[[hu:Gumi]] |
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[[ml:റബ്ബര് മരം]] |
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[[nl:Rubber]] |
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[[ja:ゴム]] |
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[[no:Gummi]] |
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[[nn:Gummi]] |
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[[pl:Guma]] |
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[[pt:Borracha]] |
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[[ro:Gumă]] |
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[[ru:Резина]] |
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[[sl:Guma]] |
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[[fi:Kumi]] |
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[[sv:Gummi]] |
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[[th:ยาง]] |
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[[vi:Cao su]] |
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[[uk:Каучук]] |
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[[ur:ربڑ]] |
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[[yi:ראבער]] |
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[[zh:橡膠]] |
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Revision as of 15:25, 11 October 2008
APPLES ARE ROUND AND JUSICU