Rubber

Natural rubber is an elastomer-an elastic hydrocarbon polymer-that was originally derived from a a milky colloidal suspension, or latex, found in the sap of some plants. The purefied form of natural rubber is the chemical polyisoprene which can also be produced synthetically. Natural rubber is used extensively in many applications and products. The entropy model of rubber was developed in 1934 by Werner Kuhn.

Rubber, any solid substance that upon vulcanization becomes elastic; the term includes natural rubber (caoutchouc) and synthetic rubber. The term elastomer is sometimes used to designate synthetic rubber only and is sometimes extended to include caoutchouc as well.

History

Pre-Columbian peoples of South and Central America used rubber for balls, containers, and shoes and for waterproofing fabrics. Mentioned by Spanish and Portuguese writers in the 16th cent., rubber did not attract the interest of Europeans until reports about it were made (1736-51) to the French Academy of Sciences by Charles de la Condamine and François Fresneau. Pioneer research in finding rubber solvents and in waterproofing fabrics was done before 1800, but rubber was used only for elastic bands and erasers, and these were made by cutting up pieces imported from Brazil. Joseph Priestley is credited with the discovery c.1770 of its use as an eraser, thus the name rubber.

The first rubber factory in the world was established near Paris in 1803, the first in England by Thomas Hancock in 1820. Hancock devised the forerunner of the masticator (the rollers through which the rubber is passed to partially break the polymer chains), and in 1835 Edwin Chaffee, an American, patented a mixing mill and a calendar (a press for rolling the rubber into sheets).

In 1823, Charles Macintosh found a practical process for waterproofing fabrics, and in 1839 Charles Goodyear discovered vulcanization, which revolutionized the rubber industry. In the latter half of the 19th cent. The demand for rubber insulation by the electrical industry and the invention of the pneumatic tire extended the demand for rubber. In the 19th cent. Wild rubber was harvested in South and Central America and in Africa; most of it came from the Para rubber tree of the Amazon basin.

Despite Brazil's legal restrictions, seeds of the tree were smuggled to England in 1876. The resultant seedlings were sent to Ceylon (Sri Lanka) and later too many tropical regions, especially the Malay area and Java and Sumatra, beginning the enormous East Asian rubber industry. Here the plantations were so carefully cultivated and managed that the relative importance of Amazon rubber diminished. American rubber companies, as a step toward diminishing foreign control of the supply, enlarged their plantation holdings in Liberia and in South and Central America.

During World War I, Germany made a synthetic rubber, but it was too expensive for peacetime use. In 1927 a less costly variety was invented, and in 1931 neoprene was made, both in the United States. German scientists developed Buna rubber just prior to World War II. When importation of natural rubber from the East Indies was cut off during World War II, the United States began large-scale manufacture of synthetic rubber, concentrating on Buna S. Today synthetic rubber accounts for about 60% of the world's rubber production.

Natural Rubber

Natural rubber is obtained from the milky secretion (latex) of various plants, but the only important commercial source of natural rubber (sometimes called Para rubber) is the tree Hevea brasiliensis. The only other plant under cultivation as a commercial rubber source is guayule (Parthenium argentatum), a shrub native to the arid regions of Mexico and the SW United States. To soften the rubber so that compounding ingredients can be added, the long polymer chains must be partially broken by mastication, mechanical shearing forces applied by passing the rubber between rollers or rotating blades. Thus, for most purposes, the rubber is ground, dissolved in a suitable solvent, and compounded with other ingredients, e.g., fillers and pigments such as carbon black for strength and whiting for stiffening; antioxidants; plasticizers, usually in the form of oils, waxes, or tars; accelerators; and vulcanizing agents. The compounded rubber is sheeted, extruded in special shapes, applied as coating or molded, then vulcanized. Most Para rubber is exported as crude rubber and prepared for market by rolling slabs of latex coagulated with acid into thin sheets of crepe rubber or into heavier, firmly pressed sheets that are usually ribbed and smoked.

An increasing quantity of latex, treated with alkali to prevent coagulation, is shipped for processing in manufacturing centers. Much of it is used to make foam rubber by beating air into it before pouring it into a vulcanizing mold. Other products are made by dipping a mold into latex (e.g., rubber gloves) or by casting latex. Sponge rubber is prepared by adding to ordinary rubber a powder that forms a gas during vulcanization. Most of the rubber imported into the United States is used in tires and tire products; other items that account for large quantities are belting, hose, tubing, insulators, valves, gaskets, and footwear. Uncoagulated latex, compounded with colloidal emulsions and dispersions, is extruded as thread, coated on other materials, or beaten to foam and used as sponge rubber. Used and waste rubber may be reclaimed by grinding followed by devulcanization with steam and chemicals, refining, and remanufacture.

Synthetic rubber

The more than one dozen major classes of synthetic rubber are made of raw material derived from petroleum, coal, oil, natural gas, and acetylene. Many of them are copolymers, i.e., polymers consisting of more than one monomer. By changing the composition it is possible to achieve specific properties desired for special applications. The earliest synthetic rubbers were the styrene-butadiene copolymers, Buna S and SBR, whose properties are closest to those of natural rubber. SBR is the most commonly used elastomer because of its low cost and good properties; it is used mainly for tires. Other general purpose elastomers are cis-polybutadiene and cis-polyisoprene, whose properties are also close to that of natural rubber.

Among the specialty elastomers are copolymers of acrylonitrile and butadiene that were originally called Buna N and are now known as nitrile elastomers or NBR rubbers. They have excellent oil resistance and are widely used for flexible couplings, hoses, and washing machine parts. Butyl rubbers are copolymers of isobutylene and 1.3% isoprene; they are valuable because of their good resistance to abrasion, low gas permeability, and high dielectric strength. Neoprene (polychloroprene) is particularly useful at elevated temperatures and is used for heavy-duty applications. Ethylene-propylene rubbers (RPDM) with their high resistance to weathering and sunlight are used for automobile parts, hose, electrical insulation, and footwear. Urethane elastomers are called spandex and they consist of urethane blocks and polyether or

polyester blocks; the urethane blocks provide strength and heat resistance, the polyester and polyether blocks provide elasticity; they are the most versatile elastomer family because of their hardness, strength, oil resistance, and aging characteristics. They have replaced rubber in elasticized materials. Other uses range from airplane wheels to seat cushions. Other synthetics are highly oil-resistant, but their high cost limits their use. Silicone rubbers are organic derivatives of inorganic polymers, e.g., the polymer of dimethysilanediol. Very stable and flexible over a wide temperature range, they are used in wire and cable insulation.

Varieties

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.

Other plants containing latex include figs (Ficus elastica), Castilla (Panama rubber tree), euphorbias, 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. 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.

Chemistry and Properties

All rubber like materials are polymers, which are high molecular weight compounds consisting of long chains of one or more types of molecules, such as monomers. Vulcanization (or curing) produces chemical links between the loosely coiled polymeric chains; elasticity occurs because the chains can be stretched and the crosslinks cause them to spring back when the stress is released. Natural rubber is a polyterpene, i.e., it consists of isoprene molecules linked into loosely twisted chains. The monomer units along the backbone of the carbon chains are in a cis arrangement (see isomer) and it is this spatial configuration that gives rubber its highly elastic character. In gutta-percha, which is another natural polyterpene, the isoprene molecules are bonded in a trans configuration leading to a crystalline solid at room temperature. Unvulcanized rubber is soluble in a number of hydrocarbons, including benzene, toluene, gasoline, and lubricating oils.

Rubber is water repellent and resistant to alkalies and weak acids. Rubber's elasticity, toughness, impermeability, adhesiveness, and electrical resistance make it useful as an adhesive, a coating composition, a fiber, a molding compound, and an electrical insulator. In general, synthetic rubber has the following advantages over natural rubber: better aging and weathering, more resistance to oil, solvents, oxygen, ozone, and certain chemicals, and resilience over a wider temperature range. The advantages of natural rubber are less buildup of heat from flexing and greater resistance to tearing when hot.

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 and every repeat unit, natural rubber is sensitive to ozone cracking

Chemical makeup

Natural rubber is a polymer of isoprene - most often cis-1,4-polyisoprene - with a molecular weight of 100,000 to 1,000,000. Typically, a few percent of other materials, such as proteins, fatty acids, resins and inorganic materials 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 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.

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. 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 (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 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.

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

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 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

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 - 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 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.

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.

Collection

A tree woman in Sri Lanka in the process of harvesting rubber.

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.

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.

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.

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.

The dried material is then baled and palletized for shipment.

Uses

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.
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 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.

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.

Natural rubber is often 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 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.

Allergic reactions

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.

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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