Definitions

rubber

rubber

[ruhb-er]
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.

Chemistry and Properties

All rubberlike 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.

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 Pará 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 Pará 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 a 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.

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 calender (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 Pará 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 to 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.

Bibliography

See P. W. Allen, Natural Rubber and the Synthetics (1972); M. Morton, Rubber Technology (3d ed. 1987).

or India rubber plant

Tropical tree (Ficus elastica) of the mulberry family. The rubber plant is large in its native Southeast Asia and other warm areas; elsewhere it is commonly grown indoors as a potted plant. The plant has large, thick, oblong leaves and pairs of figlike fruits along its branches. The milky sap, or latex, was once an important source of an inferior natural rubber. Young plants available in the florist's trade are durable and grow well under less-than-ideal indoor conditions. Some cultivated varieties have broader, darker green leaves; others are variegated. Seealso rubber tree.

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Flexible material that can recover its shape after considerable deformation.The best-known rubber is natural rubber, made from the milky latex of the rubber tree (Hevea brasiliensis). Natural rubber is still important industrially, but it now competes with synthetic alternatives (e.g., neoprene, silicone) derived from petroleum, natural gas, and other source materials. Rubber's usefulness is based on the unique elasticity of its constituent polymer molecules (built of thousands of isoprene monomers; see isoprenoid), which are capable of returning to their original coiled shape after being stretched to great extents; it is made more durable by vulcanization with sulfur or another agent that establishes chemical cross-links between the polymers. Fillers and other additives allow tailoring of properties to the desired use (e.g., by foaming, shaping, and curing). More than half of all rubber goes into making tires; the rest is used principally in belts, hoses, gaskets, shoes, clothing, furniture, and toys.

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A rubber-tyred metro (or rubber-tired in non-British English) is a form of rapid transit system that uses a mix of road and rail technology. The vehicles have wheels with rubber tyres which run inside a guideway for traction, as well as traditional railway steel wheels with flanges on steel tracks for guidance. Most rubber-tyred trains are purpose-built and designed for the system on which they operate. Guided buses are sometimes referred to as 'trams on tyres', and compared to rubber-tyred metros. See also Rubber-tyred trams and Bombardier Guided Light Transit.

History

During the World War II German occupation of Paris, the Metro system was used to capacity, with relatively little maintenance performed. At the end of the war, the system was so worn out that thought was given as to how to renovate it. Rubber-tyred metro technology was first applied to the Paris Métro, developed by Michelin, who provided the tyres and guidance system, in collaboration with Renault, who provided the vehicles. Starting in 1951, an experimental vehicle, the MP 51, operated on a test track between Porte des Lilas and Pré Saint Gervais, a section of line not open to the public.

Line 11 Châtelet - Mairie des Lilas was the first line to be converted, in 1956, chosen because of its steep grades. This was followed by Line 1 Château de Vincennes - Pont de Neuilly in 1964, and Line 4 Porte d'Orléans - Porte de Clignancourt in 1967, converted because they had the heaviest traffic load of all Paris Métro lines. Finally, Line 6 Charles de Gaulle - Étoile - Nation was converted in 1974 to cut down noise on its many elevated sections. Because of the high cost of converting existing rail-based lines, this is no longer done in Paris, nor elsewhere; now rubber-tyred metros are used in new systems or lines only, including the new Paris Métro Line 14.

The first completely rubber-tyred metro system was built in Montreal, Canada; see Montreal Metro. A few more recent rubber-tyred systems have used automated, driverless trains; one of the first such systems, developed by Matra, opened in 1983 in Lille, and others have since been built in Toulouse and Rennes the first automated rubber-tyred system opened in Kobe (Japan) in February 1981. It is the Portliner linking Sanomiya railway station with Port Island.

Technology

Overview

The vehicle is in the form of electric multiple unit, with power supplied by one, or both, of the guide bars, which thus also serves as the third rail (the current is not picked up through the horizontal wheels, but through a separate lateral pickup shoe). The return current passes through a return shoe to one, or both of the rails, or to the other guide bar, depending on the type of system.

The type of guideway used on a system varies between networks. Two parallel rollways, each the width of a tyre are used, either of concrete (Montreal Metro, Lille Metro, Toulouse Metro, most part of Santiago Metro), H-Shape hot rolled steel (Paris Métro, Mexico City Metro, the non-underground section of Santiago Metro), or flat steel (Sapporo Municipal Subway). As on a railway, the driver does not have to steer, because the system relies on a redundant system of railway steel wheels with flanges on steel rail tracks. The Sapporo system is an exception as it uses a central guide rail only. The VAL system used in Lille and Toulouse has conventional track between the guide bars.

On some systems (e.g., Paris, Montreal, Mexico City) there is a regular railway track between the rollways and the vehicles also have railway wheels with larger (taller) than normal flanges, but these are normally at some distance above the rails and are used only in the case of a flat tyre and at switches/points and crossings. In Paris these rails were also used to enable mixed traffic with rubber-tyred and steel-wheeled trains using the same track, particularly during conversion from normal railway track. Other systems (e.g. Lille and Toulouse) have other sorts of flat tyre compensation and switching methods.

The essential difference between rubber-on-concrete and steel-on-steel is that rubber-on-concrete generates more friction. This results in various pros and cons.

Advantages

Advantages of rubber-tyred metro systems (compared to steel wheel on steel rail):

  • Smooth ride (with little "jostling" around)
  • Faster acceleration
  • Shorter braking distances, allowing trains to be signalled closer together
  • The ability to climb or descend steeper slopes (~gradient 13%) than would be feasible with conventional rail tracks.
  • Quiet ride in open air (for residents and those outside the train)

  1. Modern steel-on-steel rolling stock using distributed-traction with a high-proportion of powered axles, have narrowed the gap to the acceleration/performance found in rubber-tyre rolling stock.

Disadvantages

The higher friction causes disadvantages (compared to steel wheel on steel rail):

  • Higher energy consumption than steel-on-steel
  • A larger quantity of excess heat is generated
  • Weather variance. Losing the traction-advantage in inclement weather (snow and ice)
  • Heavier; steel rails remain for switching purposes, and as a safety backup
  • Tyre replacement cost

  1. To reduce weather disruption, the Montreal Metro runs 100% underground. On Paris Métro Line 6, upgrades of tyres (as used with cars) and special ribbed tracks have been trialled.
  2. In effect, there are two systems running in parallel. This is more expensive to build, install and maintain.
  3. Rubber tyres have higher wear rates and therefore need more frequent replacement. Although a steel wheels set is more expensive than a pair of tyres, the frequency of their respective replacements makes rubber tyres the more expensive option. And in addition many rubber tyres for guidance will be needed, too.

Although it is a more complex technology, most rubber-tyred metro systems use quite simple techniques, in contrary to guided buses. Heat dissipation is an issue as eventually all traction energy consumed by the train — except the electric energy regenerated back into the substation during electrodynamic braking — will end up in losses (mostly heat). In frequently operated tunnels (typical metro operation) the extra heat from rubber tyres is a widespread problem, necessitating ventilation of the tunnels.

Similar technologies

Automated driverless systems are not exclusively rubber-tyred; many have since been built using conventional rail technology, such as London's Docklands Light Railway and Vancouver's SkyTrain, as well as AirTrain JFK which is linking JFK Airport in New York City with local subway and commuter trains. Most monorail manufacturers prefer rubber tyres.

List of systems

Country/Region City/Region System Technology
Montreal Montreal Metro Michelin
Santiago Santiago Metro (Lines 1, 2 and 5) Michelin
Laon Poma 2000 Cable-driven
Lille Lille Metro VAL 206, 208
Lyon Lyon Metro (Lines A, B, and D) Michelin
Marseille Marseille Metro Michelin
Paris Paris Métro (Lines 1, 4, 6, 11, and 14) Michelin
Paris (Orly) Orlyval VAL 206
Paris (Charles-De-Gaulle Airport) CDGVAL VAL 208
Rennes Rennes Metro VAL 208
Toulouse Toulouse Metro VAL 206, 208
Turin Metrotorino VAL 208
Kobe Kobe New Transit
Port Island Line Kawasaki
Hiroshima Hiroshima Rapid Transit
Astram Line Kawasaki / Mitsubishi / Niigata Transys
Sapporo Sapporo Municipal Subway Kawasaki
Tokyo Yurikamome Mitsubishi / Niigata Transys / Nippon Sharyo / Tokyu
Nippori-Toneri Liner
Mexico City Mexico City Metro Michelin
N/A Light Rapid Transit Bombardier / Mitsubishi
Lausanne Lausanne Metro Line m2 (2008) Michelin
Taipei, Taiwan Taipei Rapid Transit System (Muzha Line) VAL 256
(will be replaced by Bombardier's CITYFLO 650 in 2009)
Chicago, Illinois (O'Hare) Airport Transit System VAL 256

Under construction

Country/Region City/Region System
Taipei, Taiwan Taipei Rapid Transit System (Neihu Line) (2009)

Planned

Country/Region City/Region System
Hong Kong Island MTR South Island Line
Uijeongbu one line, name not yet announced (2011)
N/A Macau Light Transit System
Istanbul three lines, name not yet announced
Ankara one line, name not yet announced

See also

  • VAL (Véhicule Automatique Léger)

References

  • Bindi, A. & Lefeuvre, D. (1990). Le Métro de Paris: Histoire d'hier à demain, Rennes: Ouest-France. ISBN 2-7373-0204-8.
  • Gaillard, M. (1991). Du Madeleine-Bastille à Météor: Histoire des transports Parisiens, Amiens: Martelle. ISBN 2-87890-013-8.
  • Marc Dufour's "The principle behind the rubber-tired metro".

External links

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